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{{Dablink|This article is about evolution in biology. For other uses, see [[Evolution (disambiguation)]].}} |
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{{Redirect|Theory of evolution|more on how evolution is defined|Evolution as theory and fact}} |
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{{evolution3}} |
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In [[biology]], '''evolution''' is change in the [[genetic material]] of a population of [[organisms]] from one generation to the next. Though changes produced in any one generation are small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a process that can culminate in the [[speciation|emergence of new species]].<ref>{{wikiref |id=Gould-2002 |text=Gould 2002}}</ref> Indeed, the similarities amongst [[species]] suggest that all known species are [[common descent|descended from a common ancestor]] (or ancestral [[gene pool]]) through this process of gradual divergence.<ref name=Futuyma>{{cite book |last=Futuyma |first=Douglas J. |authorlink=Douglas J. Futuyma |year=2005 |title=Evolution |publisher=Sinauer Associates, Inc |location=Sunderland, Massachusetts |isbn=0-87893-187-2}}</ref> |
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The basis of evolution is the [[gene]]s that are passed on from generation to generation; these produce an organism's inherited [[trait (biology)|traits]]. These traits vary within populations, with organisms showing [[genetic variation|heritable differences]] (variation) in their traits. Evolution itself is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants either become more common or rare. New variation arises in two main ways: either from [[mutation]]s in genes, or from the transfer of genes between populations and between species. In species that [[sexual reproduction|reproduce sexually]], new combinations of genes are also produced by [[genetic recombination]], which can increase variation between organisms. |
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Two major mechanisms determine which variants will become more common or rare in a population. One is [[natural selection]], a process that causes helpful traits (those that increase the chance of survival and reproduction) to become more common in a population and causes harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, meaning that more individuals in the next generation will inherit these traits.<ref name=Futuyma/><ref name=Lande>{{cite journal |author=Lande R, Arnold SJ |year=1983 |title=The measurement of selection on correlated characters |journal=Evolution |volume=37 |pages=1210–26|doi=10.2307/2408842}}</ref> Over many generations, [[adaptation]]s occur through a combination of successive, small, random changes in traits, and natural selection of the variants best-suited for their environment.<ref name="Ayala">{{cite journal |author=Ayala FJ |title=Darwin's greatest discovery: design without designer |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 |issue=Suppl 1 |pages=8567–73 |year=2007 |pmid=17494753 |url=http://www.pnas.org/content/104/suppl.1/8567.full |doi=10.1073/pnas.0701072104}}</ref> The other major mechanism driving evolution is [[genetic drift]], an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that [[probability|chance]] plays in whether a given trait will be passed on as individuals survive and reproduce. |
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[[Evolutionary biology|Evolutionary biologists]] document the [[Fact#Fact in science|fact]] that evolution occurs, and also develop and test [[Theory#Science|theories]] that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when studies of the [[fossil|fossil record]] and the [[Biodiversity|diversity]] of living organisms convinced most scientists that species changed over time.<ref name=EarlyModernGeology>{{cite web |url=http://records.viu.ca/~johnstoi/darwin/sect2.htm |title=History of Science: Early Modern Geology |accessdate=2008-01-15 |author=Ian C. Johnston |year=1999 |work= |publisher=[[Malaspina University-College]] }}</ref><ref name=bowler>{{cite book|last=Bowler|first=Peter J.|authorlink=Peter J. Bowler|title=Evolution:The History of an Idea|publisher=University of California Press|year=2003|isbn=0-52023693-9}}</ref> However, the mechanism driving these changes remained unclear until the theories of [[natural selection]] were independently discovered by [[Charles Darwin]] and [[Alfred Russel Wallace|Alfred Wallace]]. Darwin's landmark work ''[[On the Origin of Species]]'' of 1859 brought the new theories of evolution by natural selection to a wide audience.<ref name=Darwin>{{cite book |last=Darwin |first=Charles |authorlink = Charles Darwin |year=1859 |title=On the Origin of Species |place=London |publisher=John Murray |edition=1st |page=1 |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16}}. Related earlier ideas were acknowledged in {{cite book |last=Darwin |first=Charles |authorlink = Charles Darwin |year=1861 |title=On the Origin of Species |place=London |publisher=John Murray |edition=3rd |pages=xiii |url=http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20 |nopp=true}}</ref> Darwin's work soon led to overwhelming acceptance of evolution among scientists.<ref name="AAAS1922Resolution">{{cite web | url=http://archives.aaas.org/docs/resolutions.php?doc_id=450 | title=AAAS Resolution: Present Scientific Status of the Theory of Evolution | date=December 26, 1922 | author=AAAS Council | publisher=American Association for the Advancement of Science }}</ref><ref name="IAP2006Statement">{{cite web | url=http://www.interacademies.net/Object.File/Master/6/150/Evolution%20statement.pdf |format=PDF| title=IAP Statement on the Teaching of Evolution |year=2006 |publisher=The Interacademy Panel on International Issues |accessdate=2007-04-25}} Joint statement issued by the national science academies of 67 countries, including the [[United Kingdom|United Kingdom's]] [[Royal Society]]</ref><ref name="AAAS2006Statement">{{cite web | url=http://www.aaas.org/news/releases/2006/pdf/0219boardstatement.pdf |format=PDF| title=Statement on the Teaching of Evolution | date=2006-02-16 | author=Board of Directors, American Association for the Advancement of Science | publisher=American Association for the Advancement of Science }} from the world's largest general scientific society</ref><ref name="NCSEStatementsFromScientificOrgs">{{cite web | url=http://ncseweb.org/media/voices/science | title=Statements from Scientific and Scholarly Organizations | publisher=National Center for Science Education }}</ref> In the 1930s, Darwinian natural selection was combined with [[Gregor Mendel|Mendelian]] [[Mendelian inheritance|inheritance]] to form the [[modern evolutionary synthesis]],<ref name=Kutschera/> which connected the ''units'' of evolution (genes) and the ''mechanism'' of evolution (natural selection). This powerful explanatory and [[predictive power|predictive]] theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the [[Biodiversity|diversity of life]] on [[Earth]].<ref name="IAP2006Statement" /><ref name="AAAS2006Statement" /><ref name="NewScientistJan2008SpecialReport">{{cite web | url=http://www.newscientist.com/topic/evolution | title=Special report on evolution | publisher=New Scientist | date=2008-01-19 }}</ref> |
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==Heredity== |
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{{details more|Introduction to genetics|Genetics|Heredity}} |
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[[File:ADN static.png|thumb|right|200px|DNA structure. [[nucleobase|Bases]] are in the center, surrounded by phosphate–sugar chains in a [[double helix]].]] |
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Evolution in organisms occurs through changes in heritable [[trait (biology)|traits]] – particular characteristics of an organism. In humans, for example, [[eye color]] is an inherited characteristic, which individuals can inherit from one of their parents.<ref>{{cite journal |author=Sturm RA, Frudakis TN |title=Eye colour: portals into pigmentation genes and ancestry |journal=Trends Genet. |volume=20 |issue=8 |pages=327–32 |year=2004 |pmid=15262401 |doi=10.1016/j.tig.2004.06.010}}</ref> Inherited traits are controlled by [[gene]]s and the complete set of genes within an organism's [[genome]] is called its [[genotype]].<ref name=Pearson_2006>{{cite journal |author=Pearson H |title=Genetics: what is a gene? |journal=Nature |volume=441 |issue=7092 |pages=398–401 |year=2006 |pmid=16724031 |doi=10.1038/441398a}}</ref> |
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The complete set of observable traits that make up the structure and behavior of an organism is called its [[phenotype]]. These traits come from the interaction of its genotype with the [[Environment (biophysical)|environment]].<ref>{{cite journal |author=Visscher PM, Hill WG, Wray NR |title=Heritability in the genomics era—concepts and misconceptions |journal=Nat. Rev. Genet. |volume=9 |issue=4 |pages=255–66 |year=2008 |month=April |pmid=18319743 |doi=10.1038/nrg2322}}</ref> As a result, not every aspect of an organism's phenotype is inherited. [[sun tanning|Suntanned]] skin results from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, people have different responses to sunlight, arising from differences in their genotype; a striking example is individuals with the inherited trait of [[albinism]], who do not tan and are highly sensitive to [[sunburn]].<ref>{{cite journal |author=Oetting WS, Brilliant MH, King RA |title=The clinical spectrum of albinism in humans |journal=Molecular medicine today |volume=2 |issue=8 |pages=330–5 |year=1996 |pmid=8796918 |doi=10.1016/1357-4310(96)81798-9}}</ref> |
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Heritable traits are passed from one generation to the next via [[DNA]], a [[molecule]] that encodes genetic information.<ref name=Pearson_2006/> DNA is a [[polymer]] composed of four types of [[nucleobase|bases]]. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters specifying a sentence. Portions of a DNA molecule that specify a single functional unit are called [[gene]]s; different genes have different sequences of bases. Within [[cell (biology)|cells]], the long strands of DNA form condensed structures called [[chromosome]]s. A specific location within a chromosome is known as a [[locus (genetics)|locus]]. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called [[allele]]s. DNA sequences can change through [[mutation]]s, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. |
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However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by [[quantitative trait locus|multiple interacting genes]].<ref>{{cite journal |author=Phillips PC |title=Epistasis—the essential role of gene interactions in the structure and evolution of genetic systems |journal=Nat. Rev. Genet. |volume=9 |issue=11 |pages=855–67 |year=2008 |month=November |pmid=18852697 |doi=10.1038/nrg2452}}</ref><ref name=Lin>{{cite journal |author=Wu R, Lin M |title=Functional mapping - how to map and study the genetic architecture of dynamic complex traits |journal=Nat. Rev. Genet. |volume=7 |issue=3 |pages=229–37 |year=2006 |pmid=16485021 |doi=10.1038/nrg1804}}</ref> The study of such complex traits is a major area of current genetic research. Another interesting but unsolved question in genetics is if [[epigenetics]] is important in evolution, this is where heritable changes occur in organisms without there being any changes to the sequences of their genes.<ref>{{cite journal |author=Richards EJ |title=Inherited epigenetic variation—revisiting soft inheritance |journal=Nat. Rev. Genet. |volume=7 |issue=5 |pages=395–401 |year=2006 |month=May |pmid=16534512 |doi=10.1038/nrg1834}}</ref> |
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==Variation== |
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{{details more|Genetic diversity|Population genetics}} |
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An individual organism's [[phenotype]] results from both its [[genotype]] and the influence from the [[Environment (biophysical)|environment]] it has lived in. A substantial part of the variation in phenotypes in a population is caused by the differences between their genotypes.<ref name=Lin/> The [[modern evolutionary synthesis]] defines evolution as the change over time in this genetic variation. The frequency of one particular allele will fluctuate, becoming more or less prevalent relative to other forms of that gene. Evolutionary [[force]]s act by driving these changes in allele frequency in one direction or another. Variation disappears when an allele reaches the point of [[fixation (population genetics)|fixation]] — when it either disappears from the population or replaces the ancestral allele entirely.<ref name=Amos>{{cite journal |author=Harwood AJ |title=Factors affecting levels of genetic diversity in natural populations |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=353 |issue=1366 |pages=177–86 |year=1998 |pmid=9533122 |pmc=1692205 |doi=10.1098/rstb.1998.0200}}</ref> |
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Variation comes from [[mutation]]s in [[genetic material]], migration between populations ([[gene flow]]), and the reshuffling of genes through [[sexual reproduction]]. Variation also comes from exchanges of genes between different species; for example, through [[horizontal gene transfer]] in [[bacteria]], and [[Hybrid (biology)|hybrid]]ization in plants.<ref>{{cite journal |author=Draghi J, Turner P |title=DNA secretion and gene-level selection in bacteria |journal=Microbiology (Reading, Engl.) |volume=152 |issue=Pt 9 |pages=2683–8 |year=2006 |pmid=16946263}}<br />*{{cite journal |author=Mallet J |title=Hybrid speciation |journal=Nature |volume=446 |issue=7133 |pages=279–83 |year=2007 |pmid=17361174 |doi=10.1038/nature05706}}</ref> Despite the constant introduction of variation through these processes, most of the [[genome]] of a species is identical in all individuals of that species.<ref>{{cite journal | author=Butlin RK, Tregenza T |title=Levels of genetic polymorphism: marker loci versus quantitative traits |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=353 |issue=1366 |pages=187–98 |year=1998 |pmid=9533123 |pmc=1692210 |doi=10.1098/rstb.1998.0201}}</ref> However, even relatively small changes in genotype can lead to dramatic changes in phenotype: chimpanzees and humans differ in only about 5% of their genomes.<ref>{{cite journal |author=Wetterbom A, Sevov M, Cavelier L, Bergström TF |title=Comparative genomic analysis of human and chimpanzee indicates a key role for indels in primate evolution |journal= J. Mol. Evol. |volume=63 |issue=5 |pages=682–90 |year=2006 |pmid=17075697 |doi=10.1007/s00239-006-0045-7}}</ref> |
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===Mutation=== |
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{{details more|Mutation|Molecular evolution}} |
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[[File:Gene-duplication.svg|thumb|100px|left|Duplication of part of a [[chromosome]]]] |
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Genetic variation comes from [[randomness|random]] mutations that occur in the genomes of organisms. Mutations are changes in the DNA sequence of a cell's genome and are caused by [[Radioactive decay|radiation]], [[virus]]es, [[transposon]]s and [[mutagen|mutagenic chemicals]], as well as errors that occur during [[meiosis]] or [[DNA replication]].<ref name=Bertram>{{cite journal |author=Bertram J |title=The molecular biology of cancer |journal=Mol. Aspects Med. |volume=21 |issue=6 |pages=167–223 |year=2000 |pmid=11173079 |doi=10.1016/S0098-2997(00)00007-8}}</ref><ref name="transposition764">{{cite journal |author=Aminetzach YT, Macpherson JM, Petrov DA |title=Pesticide resistance via transposition-mediated adaptive gene truncation in Drosophila |journal=Science |volume=309 |issue=5735 |pages=764–7 |year=2005 |pmid=16051794 |doi=10.1126/science.1112699}}</ref><ref name=Burrus>{{cite journal |author=Burrus V, Waldor M |title=Shaping bacterial genomes with integrative and conjugative elements |journal=Res. Microbiol. |volume=155 |issue=5 |pages=376–86 |year=2004 |pmid=15207870 |doi=10.1016/j.resmic.2004.01.012}}</ref> These mutagens produce several different types of change in DNA sequences; these can either have no effect, alter the [[gene product|product of a gene]], or prevent the gene from functioning. Studies in the fly ''[[Drosophila melanogaster]]'' suggest that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70 percent of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.<ref>{{cite journal |author=Sawyer SA, Parsch J, Zhang Z, Hartl DL |title=Prevalence of positive selection among nearly neutral amino acid replacements in Drosophila |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 |issue=16 |pages=6504–10 |year=2007 |pmid=17409186 |doi=10.1073/pnas.0701572104}}</ref> Due to the damaging effects that mutations can have on cells, organisms have evolved mechanisms such as [[DNA repair]] to remove mutations.<ref name=Bertram/> Therefore, the optimal mutation rate for a species is a trade-off between costs of a high mutation rate, such as deleterious mutations, and the [[metabolism|metabolic]] costs of maintaining systems to reduce the mutation rate, such as DNA repair enzymes.<ref name=Sniegowski>{{cite journal |author=Sniegowski P, Gerrish P, Johnson T, Shaver A |title=The evolution of mutation rates: separating causes from consequences |journal=Bioessays |volume=22 |issue=12 |pages=1057–66 |year=2000 |pmid=11084621 |doi=10.1002/1521-1878(200012)22:12<1057::AID-BIES3>3.0.CO;2-W}}</ref> Some species such as [[retrovirus]]es have such high mutation rates that most of their offspring will possess a mutated gene.<ref>{{cite journal |author=Drake JW, Charlesworth B, Charlesworth D, Crow JF |title=Rates of spontaneous mutation |journal=Genetics |volume=148 |issue=4 |pages=1667–86 |year=1998 |pmid=9560386 |pmc=1460098 |month=April |day=01}}</ref> Such rapid mutation may be an advantage since these viruses will evolve constantly and rapidly, and thus evade the responses of the human [[immune system]].<ref>{{cite journal |author=Holland J, Spindler K, Horodyski F, Grabau E, Nichol S, VandePol S |title=Rapid evolution of RNA genomes |journal=Science |volume=215 |issue=4540 |pages=1577–85 |year=1982 |pmid=7041255 |doi=10.1126/science.7041255}}</ref> |
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Mutations can involve large sections of DNA becoming [[gene duplication|duplicated]], which is a major source of raw material for evolving new genes, with tens to hundreds of genes duplicated in animal genomes every million years.<ref>{{cite book|last=Carroll SB, Grenier J, Weatherbee SD |title=From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Second Edition |publisher=Blackwell Publishing |year=2005 |location=Oxford |isbn=1-4051-1950-0}}</ref> Most genes belong to larger [[gene family|families of genes]] of [[homology (biology)|shared ancestry]].<ref>{{cite journal |author=Harrison P, Gerstein M |title=Studying genomes through the aeons: protein families, pseudogenes and proteome evolution |journal=J Mol Biol |volume=318 |issue=5 |pages=1155–74 |year=2002 |pmid=12083509 |doi=10.1016/S0022-2836(02)00109-2}}</ref> Novel genes are produced by several methods, commonly through the duplication and mutation of an ancestral gene, or by recombining parts of different genes to form new combinations with new functions.<ref>{{cite journal |author=Orengo CA, Thornton JM |title=Protein families and their evolution-a structural perspective |journal=Annu. Rev. Biochem. |volume=74 |issue= |pages=867–900 |year=2005 |pmid=15954844 |doi=10.1146/annurev.biochem.74.082803.133029}}</ref><ref>{{cite journal |author=Long M, Betrán E, Thornton K, Wang W |title=The origin of new genes: glimpses from the young and old |journal=Nat. Rev. Genet. |volume=4 |issue=11 |pages=865–75 |year=2003 |month=November |pmid=14634634 |doi=10.1038/nrg1204}}</ref> Here, [[protein domain|domains]] act as modules, each with a particular and independent function, that can be mixed together to produce genes encoding new proteins with novel properties.<ref>{{cite journal |author=Wang M, Caetano-Anollés G |title=The evolutionary mechanics of domain organization in proteomes and the rise of modularity in the protein world |journal=Structure |volume=17 |issue=1 |pages=66–78 |year=2009 |doi=10.1016/j.str.2008.11.008}}</ref> For example, the human eye uses four genes to make structures that sense light: three for [[Cone cell|color vision]] and one for [[Rod cell|night vision]]; all four arose from a single ancestral gene.<ref>{{cite journal |author=Bowmaker JK |title=Evolution of colour vision in vertebrates |journal=Eye (London, England) |volume=12 |issue=Pt 3b |pages=541–7 |year=1998 |pmid=9775215}}</ref> Another advantage of duplicating a gene (or even an [[Polyploidy|entire genome]]) is that this increases [[Redundancy (engineering)|redundancy]]; this allows one gene in the pair to acquire a new function while the other copy performs the original function.<ref>{{cite journal |author=Gregory TR, Hebert PD |title=The modulation of DNA content: proximate causes and ultimate consequences |url=http://genome.cshlp.org/content/9/4/317.full |journal=Genome Res. |volume=9 |issue=4 |pages=317–24 |year=1999 |pmid=10207154}}</ref><ref>{{cite journal |author=Hurles M |title=Gene duplication: the genomic trade in spare parts |journal=PLoS Biol. |volume=2 |issue=7 |pages=E206 |year=2004 |month=July |pmid=15252449 |pmc=449868 |doi=10.1371/journal.pbio.0020206}}</ref> |
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Changes in chromosome number may involve even larger mutations, where segments of the DNA within chromosomes break and then rearrange. For example, two chromosomes in the [[Homo (genus)|''Homo'']] [[genus]] fused to produce human [[chromosome 2 (human)|chromosome 2]]; this fusion did not occur in the [[Lineage (evolution)|lineage]] of the other apes, and they retain these separate chromosomes.<ref>{{cite journal |author=Zhang J, Wang X, Podlaha O |title=Testing the chromosomal speciation hypothesis for humans and chimpanzees |doi= 10.1101/gr.1891104 |journal=Genome Res. |volume=14 |issue=5 |pages=845–51 |year=2004 |pmid=15123584}}</ref> In evolution, the most important role of such chromosomal rearrangements may be to accelerate the divergence of a population into new species by making populations less likely to interbreed, and thereby preserving genetic differences between these populations.<ref>{{cite journal |author=Ayala FJ, Coluzzi M |title=Chromosome speciation: humans, Drosophila, and mosquitoes |url=http://www.pnas.org/content/102/suppl.1/6535.full |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=102 |issue=Suppl 1 |pages=6535–42 |year=2005 |pmid=15851677 |doi=10.1073/pnas.0501847102}}</ref> |
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Sequences of DNA that can move about the genome, such as [[transposon]]s, make up a major fraction of the genetic material of plants and animals, and may have been important in the evolution of genomes.<ref>{{cite journal |author=Hurst GD, Werren JH |title=The role of selfish genetic elements in eukaryotic evolution |journal=Nat. Rev. Genet. |volume=2 |issue=8 |pages=597–606 |year=2001 |pmid=11483984 |doi=10.1038/35084545}}</ref> For example, more than a million copies of the [[Alu sequence]] are present in the [[human genome]], and these sequences have now been recruited to perform functions such as regulating [[gene expression]].<ref>{{cite journal |author=Häsler J, Strub K |title=Alu elements as regulators of gene expression |journal=Nucleic Acids Res. |volume=34 |issue=19 |pages=5491–7 |year=2006 |pmid=17020921 |doi=10.1093/nar/gkl706}}</ref> Another effect of these mobile DNA sequences is that when they move within a genome, they can mutate or delete existing genes and thereby produce genetic diversity.<ref name="transposition764"/> |
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===Sex and recombination=== |
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{{details more|Genetic recombination|Sexual reproduction}} |
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In asexual organisms, genes are inherited together, or ''linked'', as they cannot mix with genes in other organisms during reproduction. However, the offspring of [[sex]]ual organisms contain random mixtures of their parents' chromosomes that are produced through [[independent assortment]]. In the related process of [[genetic recombination]], sexual organisms can also exchange DNA between two matching chromosomes.<ref>{{cite journal |author=Radding C |title=Homologous pairing and strand exchange in genetic recombination |journal=Annu. Rev. Genet. |volume=16 |pages=405–37 |year=1982 |pmid=6297377 |doi=10.1146/annurev.ge.16.120182.002201}}</ref> Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.<ref name=Agrawal>{{cite journal |author=Agrawal AF |title=Evolution of sex: why do organisms shuffle their genotypes? |journal=Curr. Biol. |volume=16 |issue=17 |page=R696 |year=2006 |pmid=16950096 |doi=10.1016/j.cub.2006.07.063}}</ref> Sex usually increases genetic variation and may increase the rate of evolution.<ref>{{cite journal |author=Peters AD, Otto SP |title=Liberating genetic variance through sex |journal=Bioessays |volume=25 |issue=6 |pages=533–7 |year=2003 |pmid=12766942 |doi=10.1002/bies.10291}}</ref><ref>{{cite journal |author=Goddard MR, Godfray HC, Burt A |title=Sex increases the efficacy of natural selection in experimental yeast populations |journal=Nature |volume=434 |issue=7033 |pages=636–40 |year=2005 |pmid=15800622 |doi=10.1038/nature03405}}</ref> However, asexuality is advantageous in some environments as it can evolve in previously-sexual animals.<ref>{{cite journal |author=Fontaneto D, Herniou EA, Boschetti C, ''et al.'' |title=Independently evolving species in asexual bdelloid rotifers |journal=PLoS Biol. |volume=5 |issue=4 |pages=e87 |year=2007 |month=April |pmid=17373857 |pmc=1828144 |doi=10.1371/journal.pbio.0050087 |laysummary=http://www.physorg.com/news93597385.html}}</ref> Here, asexuality might allow the two sets of alleles in their genome to diverge and gain different functions.<ref>{{cite journal |author=Pouchkina-Stantcheva NN, McGee BM, Boschetti C, ''et al.'' |title=Functional divergence of former alleles in an ancient asexual invertebrate |journal=Science |volume=318 |issue=5848 |pages=268–71 |year=2007 |month=October |pmid=17932297 |doi=10.1126/science.1144363 |laysummary=http://news.bbc.co.uk/2/hi/science/nature/7039478.stm}}</ref> |
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Recombination allows even alleles that are close together in a strand of DNA to be [[Mendelian inheritance#Mendel.27s law of segregation|inherited independently]]. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other, and genes that are close together tend to be inherited together, a phenomenon known as [[genetic linkage|linkage]].<ref>{{cite journal |author=Lien S, Szyda J, Schechinger B, Rappold G, Arnheim N |title=Evidence for heterogeneity in recombination in the human pseudoautosomal region: high resolution analysis by sperm typing and radiation-hybrid mapping |journal=Am. J. Hum. Genet. |volume=66 |issue=2 |pages=557–66 |year=2000 |month=February |pmid=10677316 |pmc=1288109 |doi=10.1086/302754}}</ref> This tendency is measured by finding how often two alleles occur together on a single chromosome, which is called their [[linkage disequilibrium]]. A set of alleles that is usually inherited in a group is called a [[haplotype]]. |
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When alleles cannot be separated by recombination – such as in mammalian [[Y chromosome]]s, which pass intact from fathers to sons – harmful [[Muller's ratchet|mutations accumulate]].<ref>{{cite journal |author=Muller H |title=The relation of recombination to mutational advance |journal=Mutat. Res. |volume=106 |issue= |pages=2–9 |year=1964 |pmid=14195748}}</ref><ref>{{cite journal |author=Charlesworth B, Charlesworth D |title=The degeneration of Y chromosomes |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=355 |issue=1403 |pages=1563–72 |year=2000 |month=November |pmid=11127901 |pmc=1692900 |doi=10.1098/rstb.2000.0717}}</ref> By breaking up allele combinations, sexual reproduction allows the removal of harmful mutations and the retention of beneficial mutations.<ref name=Otto>{{cite journal |author=Otto S |title=The advantages of segregation and the evolution of sex |journal=Genetics |volume=164 |issue=3 |pages=1099–118 |year=2003 |pmid=12871918 |pmc=1462613 |month=July |day=01}}</ref> In addition, recombination and reassortment can produce individuals with new and advantageous gene combinations. These positive effects are balanced by the fact that [[Evolution_of_sex#The_two-fold_cost_of_sex|sex reduces an organism's reproductive rate]], can cause mutations and may separate beneficial combinations of genes.<ref name=Otto/> The reasons for the [[evolution of sexual reproduction]] are therefore unclear and this question is still an active area of research in evolutionary biology,<ref>{{cite journal |author=Doncaster CP, Pound GE, Cox SJ |title=The ecological cost of sex |journal=Nature |volume=404 |issue=6775 |pages=281–5 |year=2000 |month=March |pmid=10749210 |doi=10.1038/35005078}}</ref><ref>{{cite journal |author=Butlin R |title=Evolution of sex: The costs and benefits of sex: new insights from old asexual lineages |journal=Nat. Rev. Genet. |volume=3 |issue=4 |pages=311–7 |year=2002 |month=April |pmid=11967555 |doi=10.1038/nrg749}}</ref> that has prompted ideas such as the [[Red Queen]] hypothesis.<ref>{{cite journal |author=Salathé M, Kouyos RD, Bonhoeffer S |title=The state of affairs in the kingdom of the Red Queen |journal=Trends Ecol. Evol. (Amst.) |volume=23 |issue=8 |pages=439–45 |year=2008 |month=August |pmid=18597889 |doi=10.1016/j.tree.2008.04.010}}</ref> |
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===Population genetics=== |
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{{Double image stack |right|Biston.betularia.7200.jpg |Biston.betularia.f.carbonaria.7209.jpg|200| White [[peppered moth]] |Black morph in [[peppered moth evolution]]}} |
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From a genetic viewpoint, evolution is a ''generation-to-generation change in the frequencies of alleles within a population that shares a common gene pool''.<ref>{{cite journal |author=Stoltzfus A |title=Mutationism and the dual causation of evolutionary change |journal=Evol. Dev. |volume=8 |issue=3 |pages=304–17 |year=2006 |pmid=16686641 |doi=10.1111/j.1525-142X.2006.00101.x}}</ref> A [[population]] is a localized group of individuals belonging to the same species. For example, all of the moths of the same species living in an isolated forest represent a population. A single gene in this population may have several alternate forms, which account for variations between the phenotypes of the organisms. An example might be a gene for coloration in moths that has two alleles: black and white. A [[gene pool]] is the complete set of alleles for a gene in a single population; the [[allele frequency]] measures the fraction of the gene pool comprised of a single allele (for example, what fraction of moth coloration genes are the black allele). Evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms; for example, the allele for black color in a population of moths becoming more common. |
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To understand the mechanisms that cause a population to evolve, it is useful to consider what conditions are required for a population not to evolve. The ''[[Hardy-Weinberg principle]]'' states that the frequencies of alleles (variations in a gene) in a sufficiently large population will remain constant if the only forces acting on that population are the random reshuffling of alleles during the formation of the sperm or egg, and the random combination of the alleles in these sex cells during [[Fertilisation|fertilization]].<ref name=oneil>{{cite web |url=http://anthro.palomar.edu/synthetic/synth_2.htm|title= Hardy-Weinberg Equilibrium Model|accessdate=2008-01-06 |last= O'Neil |first=Dennis |year=2008 |work= The synthetic theory of evolution: An introduction to modern evolutionary concepts and theories|publisher=Behavioral Sciences Department, Palomar College }}</ref> Such a population is said to be in ''Hardy-Weinberg equilibrium''; it is not evolving.<ref name= Teach2>{{cite web |url=http://www.evoled.org/lessons/speciation.htm|title= Causes of evolution|accessdate=2007-12-30 |last= Bright |first=Kerry |year=2006 |work= Teach Evolution and Make It Relevant |publisher=National Science Foundation}}</ref>{{Dead link|date=July 2009}} |
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===Gene flow=== |
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{{details more|Gene flow|Hybrid (biology)|Horizontal gene transfer}} |
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[[File:Lion waiting in Nambia.jpg|250px|thumb|left|When they mature, male [[lion]]s leave the pride where they were born and take over a new pride to mate, causing [[gene flow]] between prides.<ref>{{cite journal |author= Packer C, Gilbert DA, Pusey AE, O'Brieni SJ. |title=A molecular genetic analysis of kinship and cooperation in African lions |journal=Nature |volume=351 |pages=562-65 |year=1991 |month=June |doi=10.1038/351562a0}}</ref>]] |
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[[Gene flow]] is the exchange of genes between populations, which are usually of the same species.<ref>{{cite journal |author=Morjan C, Rieseberg L |title=How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles |journal=Mol. Ecol. |volume=13 |issue=6 |pages=1341–56 |year=2004 |pmid=15140081 |doi=10.1111/j.1365-294X.2004.02164.x}}</ref> Examples of gene flow within a species include the migration and then breeding of organisms, or the exchange of [[pollen]]. Gene transfer between species includes the formation of [[Hybrid (biology)|hybrid]] organisms and [[horizontal gene transfer]]. |
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Migration into or out of a population can change allele frequencies, as well as introducing genetic variation into a population. Immigration may add new genetic material to the established [[gene pool]] of a population. Conversely, emigration may remove genetic material. As [[reproductive isolation|barriers to reproduction]] between two diverging populations are required for the populations to [[speciation|become new species]], gene flow may slow this process by spreading genetic differences between the populations. Gene flow is hindered by mountain ranges, oceans and deserts or even man-made structures such as the [[Great Wall of China]], which has hindered the flow of plant genes.<ref>{{cite journal |author=Su H, Qu L, He K, Zhang Z, Wang J, Chen Z, Gu H |title=The Great Wall of China: a physical barrier to gene flow? |journal=Heredity |volume=90 |issue=3 |pages=212–9 |year=2003 |pmid=12634804 |doi=10.1038/sj.hdy.6800237}}</ref> |
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Depending on how far two species have diverged since their [[most recent common ancestor]], it may still be possible for them to produce offspring, as with [[horse]]s and [[donkey]]s mating to produce [[mule]]s.<ref>{{cite journal |author=Short RV |title=The contribution of the mule to scientific thought |journal=J. Reprod. Fertil. Suppl. |issue=23 |pages=359–64 |year=1975 |pmid=1107543}}</ref> Such [[Hybrid (biology)|hybrid]]s are generally [[infertility|infertile]], due to the two different sets of chromosomes being unable to pair up during [[meiosis]]. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype.<ref>{{cite journal |author=Gross B, Rieseberg L |title=The ecological genetics of homoploid hybrid speciation |doi= 10.1093/jhered/esi026 |journal=J. Hered. |volume=96 |issue=3 |pages=241–52 |year=2005 |pmid=15618301}}</ref> The importance of hybridization in creating [[hybrid speciation|new species]] of animals is unclear, although cases have been seen in many types of animals,<ref>{{cite journal |author=Burke JM, Arnold ML |title=Genetics and the fitness of hybrids |journal=Annu. Rev. Genet. |volume=35 |issue= |pages=31–52 |year=2001 |pmid=11700276 |doi=10.1146/annurev.genet.35.102401.085719 }}</ref> with the [[gray tree frog]] being a particularly well-studied example.<ref>{{cite journal |author=Vrijenhoek RC |title=Polyploid hybrids: multiple origins of a treefrog species |journal=Curr. Biol. |volume=16 |issue=7 | page = R245 |year=2006 |pmid=16581499 |doi=10.1016/j.cub.2006.03.005 }}</ref> |
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Hybridization is, however, an important means of speciation in plants, since [[polyploidy]] (having more than two copies of each chromosome) is tolerated in plants more readily than in animals.<ref name=Wendel>{{cite journal |author=Wendel J |title=Genome evolution in polyploids |journal=Plant Mol. Biol. |volume=42 |issue=1 |pages=225–49 |year=2000 |pmid=10688139 |doi=10.1023/A:1006392424384 }}</ref><ref name=Semon>{{cite journal |author=Sémon M, Wolfe KH |title=Consequences of genome duplication |journal=Curr Opin Genet Dev |volume=17 |issue=6 |pages=505–12 |year=2007 |pmid=18006297 |doi=10.1016/j.gde.2007.09.007 }}</ref> Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.<ref>{{cite journal |author=Comai L |title=The advantages and disadvantages of being polyploid |journal=Nat. Rev. Genet. |volume=6 |issue=11 |pages=836–46 |year=2005 |pmid=16304599 |doi=10.1038/nrg1711 }}</ref> Polyploids also have more genetic diversity, which allows them to avoid [[inbreeding depression]] in small populations.<ref>{{cite journal |author=Soltis P, Soltis D |title=The role of genetic and genomic attributes in the success of polyploids |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=13 |pages=7051–7 |year=2000 |month=June |pmid=10860970 |pmc=34383 |doi=10.1073/pnas.97.13.7051 }}</ref> |
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[[Horizontal gene transfer]] is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among [[bacteria]].<ref>{{cite journal |author=Boucher Y, Douady CJ, Papke RT, Walsh DA, Boudreau ME, Nesbo CL, Case RJ, Doolittle WF |title=Lateral gene transfer and the origins of prokaryotic groups |doi=10.1146/annurev.genet.37.050503.084247 |journal=Annu Rev Genet |volume=37 |pages=283–328 |year=2003 |pmid=14616063}}</ref> In medicine, this contributes to the spread of [[antibiotic resistance]], as when one bacteria acquires resistance genes it can rapidly transfer them to other species.<ref>{{cite journal |author=Walsh T |title=Combinatorial genetic evolution of multiresistance |journal=Curr. Opin. Microbiol. |volume=9 |issue=5 |pages=476–82 |year=2006 |pmid=16942901 |doi=10.1016/j.mib.2006.08.009 }}</ref> Horizontal transfer of genes from bacteria to eukaryotes such as the yeast ''[[Saccharomyces cerevisiae]]'' and the adzuki bean beetle ''Callosobruchus chinensis'' may also have occurred.<ref>{{cite journal |author=Kondo N, Nikoh N, Ijichi N, Shimada M, Fukatsu T |title=Genome fragment of Wolbachia endosymbiont transferred to X chromosome of host insect |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=99 |issue=22 |pages=14280–5 |year=2002 |pmid=12386340 |doi=10.1073/pnas.222228199 }}</ref><ref>{{cite journal |author=Sprague G |title=Genetic exchange between kingdoms |journal=Curr. Opin. Genet. Dev. |volume=1 |issue=4 |pages=530–3 |year=1991 |pmid=1822285 |doi=10.1016/S0959-437X(05)80203-5}}</ref> An example of larger-scale transfers are the eukaryotic [[Bdelloidea|bdelloid rotifers]], which appear to have received a range of genes from bacteria, fungi, and plants.<ref>{{cite journal |author=Gladyshev EA, Meselson M, Arkhipova IR |title=Massive horizontal gene transfer in bdelloid rotifers |journal=Science |volume=320 |issue=5880 |pages=1210–3 |year=2008 |month=May |pmid=18511688 |doi=10.1126/science.1156407}}</ref> [[Virus]]es can also carry DNA between organisms, allowing transfer of genes even across [[domain (biology)|biological domains]].<ref>{{cite journal |author=Baldo A, McClure M |title=Evolution and horizontal transfer of dUTPase-encoding genes in viruses and their hosts |journal=J. Virol. |volume=73 |issue=9 |pages=7710–21 |year=1999 |pmid=10438861 |pmc=104298 |month=September |day=01}}</ref> Large-scale gene transfer has also occurred between the ancestors of [[eukaryote|eukaryotic cells]] and prokaryotes, during the acquisition of [[chloroplast]]s and [[Mitochondrion|mitochondria]].<ref name = "rgruqh">{{cite journal |author=Poole A, Penny D |title=Evaluating hypotheses for the origin of eukaryotes |journal=Bioessays |volume=29 |issue=1 |pages=74–84 |year=2007 |pmid=17187354 |doi=10.1002/bies.20516 }}</ref> |
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==Mechanisms== |
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The two main mechanisms that produce evolution are [[natural selection]] and [[genetic drift]]. Natural selection favors genes that improve capacity for survival and reproduction. Genetic drift is random change in the frequency of alleles, caused by the random sampling of a generation's genes during reproduction. The relative importance of natural selection and genetic drift in a population varies depending on the strength of the selection and the [[effective population size]], which is the number of individuals capable of breeding.<ref name=Whitlock>{{cite journal |author=Whitlock M |title=Fixation probability and time in subdivided populations |journal=Genetics |volume=164 |issue=2 |pages=767–79 |year=2003 |pmid=12807795 |pmc=1462574 |month=June |day=01}}</ref> Natural selection usually predominates in large populations, while genetic drift dominates in small populations. The dominance of genetic drift in small populations can even lead to the fixation of slightly deleterious mutations.<ref name=Ohta>{{cite journal |author=Ohta T |title=Near-neutrality in evolution of genes and gene regulation |url=http://www.pnas.org/cgi/content/abstract/252626899v1 |journal=[[Proceedings of the National Academy of Sciences|Proc. Natl. Acad. Sci. U.S.A.]] |volume=99 |issue=25 |pages=16134–7 |year=2002 |doi=10.1073/pnas.252626899 |pmid=12461171}}</ref> As a result, changing population size can dramatically influence the course of evolution. [[Population bottleneck]]s, where the population shrinks temporarily and therefore loses genetic variation, result in a more uniform population.<ref name=Amos/> |
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===Natural selection=== |
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{{details more|Natural selection|Fitness (biology)}} |
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[[File:Mutation and selection diagram.svg|thumb|right|300px|[[Natural selection]] of a population for dark coloration.]] |
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[[Natural selection]] is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts: |
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* Heritable variation exists within populations of organisms. |
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* Organisms produce more offspring than can survive. |
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* These offspring vary in their ability to survive and reproduce. |
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These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation.<ref name=Hurst>{{cite journal |author=Hurst LD |title=Fundamental concepts in genetics: genetics and the understanding of selection |journal=Nat. Rev. Genet. |volume=10 |issue=2 |pages=83–93 |year=2009 |month=February |pmid=19119264 |doi=10.1038/nrg2506}}</ref> |
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The central concept of natural selection is the [[fitness (biology)|evolutionary fitness]] of an organism.<ref name=Orr>{{cite journal |author=Orr HA |title=Fitness and its role in evolutionary genetics |journal=Nat. Rev. Genet. |volume=10 |issue=8 |pages=531–9 |year=2009 |month=August |pmid=19546856 |doi=10.1038/nrg2603}}</ref> This measures an organism's ability to survive and reproduce, which decides the size of its genetic contribution to the next generation.<ref name=Orr/> However, fitness is not the same as the total number of offspring: instead fitness measures the proportion of subsequent generations that carry an organism's genes.<ref name=Haldane>{{cite journal |author=Haldane J |title=The theory of natural selection today |journal=Nature |volume=183 |issue=4663 |pages=710–3 |year=1959 |pmid=13644170 | doi=10.1038/183710a0}}</ref> For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.<ref name=Orr/> |
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If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected ''for''". Examples of traits that can increase fitness are enhanced survival, and increased [[fecundity]]. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer — they are "selected ''against''".<ref name=Lande/> Importantly, the fitness of an allele is not a fixed characteristic, if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.<ref name="Futuyma"/> |
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Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorized into three different types. The first is [[directional selection]], which is a shift in the average value of a trait over time — for example organisms slowly getting taller.<ref>{{cite journal |author=Hoekstra H, Hoekstra J, Berrigan D, Vignieri S, Hoang A, Hill C, Beerli P, Kingsolver J |title=Strength and tempo of directional selection in the wild |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=98 |issue=16 |pages=9157–60 |year=2001 |month=July |pmid=11470913 |pmc=55389 |doi=10.1073/pnas.161281098}}</ref> Secondly, [[disruptive selection]] is selection for extreme trait values and often results in [[bimodal distribution|two different values]] becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in [[stabilizing selection]] there is selection against extreme trait values on both ends, which causes a decrease in [[variance]] around the average value and less diversity.<ref name=Hurst/><ref>{{cite journal |author=Felsenstein |title=Excursions along the Interface between Disruptive and Stabilizing Selection |journal=Genetics |volume=93 |issue=3 |pages=773–95 |year=1979 |pmid=17248980 |pmc=1214112 |month=November |day=01}}</ref> This would, for example, cause organisms to slowly become all the same height. |
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A special case of natural selection is [[sexual selection]], which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.<ref>{{cite journal |author=Andersson M, Simmons L |title=Sexual selection and mate choice |journal=Trends Ecol. Evol. (Amst.) |volume=21 |issue=6 |pages=296–302 |year=2006 |pmid=16769428 |doi=10.1016/j.tree.2006.03.015}}</ref> Traits that evolved through sexual selection are particularly prominent in males of some animal species, despite traits such as cumbersome antlers, mating calls or bright colors that attract predators, decreasing the survival of individual males.<ref>{{cite journal |author=Kokko H, Brooks R, McNamara J, Houston A |title=The sexual selection continuum |pmc=1691039 |journal=Proc. Biol. Sci. |volume=269 |issue=1498 |pages=1331–40 |year=2002 |pmid=12079655 |doi=10.1098/rspb.2002.2020}}</ref> This survival disadvantage is balanced by higher reproductive success in males that show these [[Handicap principle|hard to fake]], sexually selected traits.<ref>{{cite journal |author=Hunt J, Brooks R, Jennions M, Smith M, Bentsen C, Bussière L |title=High-quality male field crickets invest heavily in sexual display but die young |journal=Nature |volume=432 |issue=7020 |pages=1024–7 |year=2004 |pmid=15616562 | doi=10.1038/nature03084}}</ref> |
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Natural selection most generally makes nature the measure against which individuals, and individual traits, are more or less likely to survive. "Nature" in this sense refers to an [[ecosystem]], that is, a system in which organisms interact with every other element, [[abiotic|physical]] as well as [[biotic|biological]], in their local [[environment (biophysical)|environment]]. Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (ie: exchange of materials between living and nonliving parts) within the system."<ref name="Odum1971">Odum, EP (1971) Fundamentals of ecology, third edition, Saunders New York</ref> Each population within an ecosystem occupies a distinct [[Ecological niche|niche]], or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the [[food chain]], and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection. |
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An active area of research is the [[unit of selection]], with natural selection being proposed to work at the level of genes, cells, individual organisms, groups of organisms and even species.<ref name=Gould>{{cite journal |author=Gould SJ |title=Gulliver's further travels: the necessity and difficulty of a hierarchical theory of selection |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=353 |issue=1366 |pages=307–14 |year=1998 |month=February |pmid=9533127 |pmc=1692213 |doi=10.1098/rstb.1998.0211}}</ref><ref>{{cite journal |author=Mayr E |title=The objects of selection |doi= 10.1073/pnas.94.6.2091 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=94 |issue=6 |pages=2091–4 |year=1997 |pmid=9122151}}</ref> None of these are mutually exclusive and selection may act on multiple levels simultaneously.<ref>{{cite journal |author=Maynard Smith J |title=The units of selection |journal=Novartis Found. Symp. |volume=213 |pages=203–11; discussion 211–7 |year=1998 |pmid=9653725}}</ref> An example of selection occurring below the level of the individual organism are genes called [[transposon]]s, which can replicate and spread throughout a [[genome]].<ref>{{cite journal |author=Hickey DA |title=Evolutionary dynamics of transposable elements in prokaryotes and eukaryotes |journal=Genetica |volume=86 |issue=1–3 |pages=269–74 |year=1992 |pmid=1334911 | doi=10.1007/BF00133725}}</ref> Selection at a level above the individual, such as [[group selection]], may allow the evolution of co-operation, as discussed below.<ref>{{cite journal |author=Gould SJ, Lloyd EA |title=Individuality and adaptation across levels of selection: how shall we name and generalize the unit of Darwinism? |doi= 10.1073/pnas.96.21.11904 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=96 |issue=21 |pages=11904–9 |year=1999 |pmid=10518549}}</ref> |
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===Genetic drift=== |
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{{details more|Genetic drift|Effective population size}} |
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[[File:Allele-frequency.png|thumb|right|250px| Simulation of [[genetic drift]] of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift to [[Fixation (population genetics)|fixation]] is more rapid in the smaller population.]] |
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Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles in offspring are a [[sampling (statistics)|random sample]] of those in the parents, as well as from the role that chance plays in determining whether a given individual will survive and reproduce.<ref name=Amos/> In mathematical terms, alleles are subject to [[sampling error]]. As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a [[random walk]]). This drift halts when an allele eventually becomes [[Fixation (population genetics)|fixed]], either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.<ref>{{cite journal |author=Lande R |title=Fisherian and Wrightian theories of speciation |journal=Genome |volume=31 |issue=1 |pages=221–7 |year=1989 |pmid=2687093}}</ref> |
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The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.<ref>{{cite journal |author=Otto S, Whitlock M |title=The probability of fixation in populations of changing size |journal=Genetics |volume=146 |issue=2 |pages=723–33 |year=1997 |pmid=9178020 |pmc=1208011 |month=June |day=01}}</ref> The precise measure of population that is important is called the [[effective population size]]. The effective population is always smaller than the total population since it takes into account factors such as the level of inbreeding, the number of animals that are too old or young to breed, and the lower probability of animals that live far apart managing to mate with each other.<ref>{{cite journal |author=Charlesworth B |title=Fundamental concepts in genetics: Effective population size and patterns of molecular evolution and variation |journal=Nat. Rev. Genet. |volume=10 |pages=195–205 |year=2009 |month=March |pmid=19204717 |doi=10.1038/nrg2526}}</ref> |
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Although natural selection is responsible for adaptation, the relative importance of the two forces of natural selection and genetic drift in driving evolutionary change in general is an area of current research in evolutionary biology.<ref>{{cite journal |author=Nei M |title=Selectionism and neutralism in molecular evolution |doi= 10.1093/molbev/msi242 |journal=Mol. Biol. Evol. |volume=22 |issue=12 |pages=2318–42 |year=2005 |pmid=16120807}}</ref> These investigations were prompted by the [[neutral theory of molecular evolution]], which proposed that most evolutionary changes are the result of the fixation of [[neutral mutation]]s that do not have any immediate effects on the fitness of an organism.<ref>{{cite journal |author=Kimura M |title=The neutral theory of molecular evolution: a review of recent evidence |url=http://www.jstage.jst.go.jp/article/jjg/66/4/66_367/_article |journal=Jpn. J. Genet. |volume=66 |issue=4 |pages=367–86 |year=1991 |pmid=1954033 |doi=10.1266/jjg.66.367}}</ref> Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.<ref>{{cite journal |author=Kimura M |title=The neutral theory of molecular evolution and the world view of the neutralists |journal=Genome |volume=31 |issue=1 |pages=24–31 |year=1989 |pmid=2687096}}</ref> This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature.<ref>{{cite journal |author=Kreitman M |title=The neutral theory is dead. Long live the neutral theory |journal=Bioessays |volume=18 |issue=8 |pages=678–83; discussion 683 |year=1996 |month=August |pmid=8760341 |doi=10.1002/bies.950180812 |url=http://www.cs.ucsb.edu/~ambuj/Courses/bioinformatics/neutral-theory}}</ref><ref>{{cite journal|author=Leigh E.G. (Jr) | year=2007| title=Neutral theory: a historical perspective.| journal=[[Journal of Evolutionary Biology]] |volume=20 |pages=2075–91| doi=10.1111/j.1420-9101.2007.01410.x}}</ref> However, a more recent and better-supported version of this model is the [[nearly neutral theory of molecular evolution|nearly neutral theory]], where most mutations only have small effects on fitness.<ref name=Hurst/> |
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==Outcomes== |
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Evolution influences every aspect of the form and behavior of organisms. Most prominent are the specific behavioral and physical [[adaptation]]s that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by [[Co-operation (evolution)|co-operating]] with each other, usually by aiding their relatives or engaging in mutually beneficial [[symbiosis]]. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. |
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These outcomes of evolution are sometimes divided into [[macroevolution]], which is evolution that occurs at or above the level of species, such as [[extinction]] and [[speciation]], and [[microevolution]], which is smaller evolutionary changes, such as adaptations, within a species or population.<ref name=ScottEC>{{cite journal |author=Scott EC, Matzke NJ |title=Biological design in science classrooms |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 Suppl 1 |issue= |pages=8669–76 |year=2007 |month=May |pmid=17494747 |pmc=1876445 |doi=10.1073/pnas.0701505104}}</ref> In general, macroevolution is regarded as the outcome of long periods of microevolution.<ref>{{cite journal |author=Hendry AP, Kinnison MT |title=An introduction to microevolution: rate, pattern, process |journal=Genetica |volume=112–113 |issue= |pages=1–8 |year=2001 |pmid=11838760 |doi=10.1023/A:1013368628607}}</ref> Thus, the distinction between micro- and macroevolution is not a fundamental one - the difference is simply the time involved.<ref>{{cite journal |author=Leroi AM |title=The scale independence of evolution |journal=Evol. Dev. |volume=2 |issue=2 |pages=67–77 |year=2000 |pmid=11258392 |doi=10.1046/j.1525-142x.2000.00044.x }}</ref> However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels - with microevolution acting on genes and organisms, versus macroevolutionary processes acting on entire species and affecting the rate of speciation and extinction.<ref>{{wikiref |id=Gould-2002 |text=Gould 2002, pp. 657–8}}</ref><ref>{{cite journal |author=Gould SJ |title=Tempo and mode in the macroevolutionary reconstruction of Darwinism |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=91 |issue=15 |pages=6764–71 |year=1994 |month=July |pmid=8041695 |pmc=44281 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=8041695 |doi=10.1073/pnas.91.15.6764}}</ref><ref name=Jablonski2000>{{cite journal | author = Jablonski, D. | year = 2000 | title = Micro- and macroevolution: scale and hierarchy in evolutionary biology and paleobiology | journal = Paleobiology | volume = 26 | issue = sp4 | pages = 15–52 | doi = 10.1666/0094-8373(2000)26[15:MAMSAH]2.0.CO;2 | url = http://www.bioone.org/perlserv/?request=get-abstract}}</ref> |
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A common misconception is that evolution is "progressive," but natural selection has no long-term goal and does not necessarily produce greater complexity.<ref name=sciam>Michael J. Dougherty. [http://www.sciam.com/article.cfm?id=is-the-human-race-evolvin Is the human race evolving or devolving?] ''[[Scientific American]]'' July 20, 1998.</ref><ref>[[TalkOrigins Archive]] response to [[Creationist]] claims - [http://www.talkorigins.org/indexcc/CB/CB932.html Claim CB932: Evolution of degenerate forms]</ref> Although [[evolution of complexity|complex species]] have evolved, this occurs as a side effect of the overall number of organisms increasing, and simple forms of life remain more common.<ref name=Carroll>{{cite journal |author=Carroll SB |title=Chance and necessity: the evolution of morphological complexity and diversity |journal=Nature |volume=409 |issue=6823 |pages=1102–9 |year=2001 |pmid=11234024 |doi=10.1038/35059227 }}</ref> For example, the overwhelming majority of species are microscopic [[prokaryote]]s, which form about half the world's [[biomass]] despite their small size,<ref>{{cite journal |author=Whitman W, Coleman D, Wiebe W |title=Prokaryotes: the unseen majority |doi= 10.1073/pnas.95.12.6578 |journal=Proc Natl Acad Sci U S a |volume=95 |issue=12 |pages=6578–83 |year=1998|pmid=9618454}}</ref> and constitute the vast majority of Earth's biodiversity.<ref name=Schloss>{{cite journal |author=Schloss P, Handelsman J |title=Status of the microbial census |journal=Microbiol Mol Biol Rev |volume=68 |issue=4 |pages=686–91 |year=2004 |month=December |pmid=15590780 |pmc=539005 |doi=10.1128/MMBR.68.4.686-691.2004 }}</ref> Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is [[biased sample|more noticeable]].<ref>{{cite journal |author=Nealson K |title=Post-Viking microbiology: new approaches, new data, new insights |journal=Orig Life Evol Biosph |volume=29 |issue=1 |pages=73–93 |year=1999 |pmid=11536899 |doi=10.1023/A:1006515817767 }}</ref> Indeed, the evolution of [[microorganism]]s is particularly important to modern evolutionary research, since their rapid reproduction allows the study of [[experimental evolution]] and the observation of evolution and adaptation in real time.<ref name=Buckling>{{cite journal |author=Buckling A, Craig Maclean R, Brockhurst MA, Colegrave N |title=The Beagle in a bottle |journal=Nature |volume=457 |issue=7231 |pages=824–9 |year=2009 |month=February |pmid=19212400 |doi=10.1038/nature07892}}</ref><ref>{{cite journal |author=Elena SF, Lenski RE |title=Evolution experiments with microorganisms: the dynamics and genetic bases of adaptation |journal=Nat. Rev. Genet. |volume=4 |issue=6 |pages=457–69 |year=2003 |month=June |pmid= |doi=10.1038/nrg1088}}</ref> |
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===Adaptation=== |
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{{details|Adaptation}} |
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Adaptation is one of the basic phenomena of biology,<ref>Williams, George C. 1966. ''Adaptation and natural selection: a critique of some current evolutionary thought''. Princeton. "Evolutionary adaptation is a phenomenon of pervasive importance in biology." p5</ref> and is the ''process'' whereby an organism becomes better suited to its [[habitat]].<ref>Mayr, Ernst 1982. ''The growth of biological thought''. Harvard. p483: "Adaptation... could no longer be considered a static condition, a product of a creative past, and became instead a continuing dynamic process."</ref><ref>The ''Oxford Dictionary of Science'' defines ''adaptation'' as "Any change in the structure or functioning of an organism that makes it better suited to its environment".</ref> Also, the term adaptation may refer to a [[Trait (biology)|trait]] that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass, or the ability of horses to run fast and escape predators. By using the term ''adaptation'' for the evolutionary process, and ''adaptive trait'' for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by [[natural selection]].<ref>{{cite journal |author=Orr H |title=The genetic theory of adaptation: a brief history |journal=Nat. Rev. Genet. |volume=6 |issue=2 |pages=119–27 |year=2005 |pmid=15716908 |doi=10.1038/nrg1523 }}</ref> The following definitions are due to [[Theodosius Dobzhansky]]. |
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:1. ''Adaptation'' is the evolutionary process whereby an organism becomes better able to live in its [[habitat]] or habitats.<ref>Dobzhansky T. 1968. On some fundamental concepts of evolutionary biology. ''Evolutionary biology'' '''2''', 1–34.</ref> |
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:2. ''Adaptedness'' is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.<ref>Dobzhansky T. 1970. ''Genetics of the evolutionary process''. Columbia, N.Y. p4–6, 79–82, 84–87</ref> |
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:3. An ''adaptive trait'' is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.<ref>Dobzhansky T. 1956. Genetics of natural populations XXV. Genetic changes in populations of ''Drosophila pseudoobscura'' and ''Drosphila persimilis'' in some locations in California. ''Evolution'' '''10''', 82–92.</ref> |
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Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to [[antibiotic]] selection, with genetic changes causing [[antibiotic resistance]] by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell.<ref>{{cite journal |author=Nakajima A, Sugimoto Y, Yoneyama H, Nakae T |title=High-level fluoroquinolone resistance in Pseudomonas aeruginosa due to interplay of the MexAB-OprM efflux pump and the DNA gyrase mutation |url=http://www.jstage.jst.go.jp/article/mandi/46/6/46_391/_article/-char/en |journal=Microbiol. Immunol. |volume=46 |issue=6 |pages=391–5 |year=2002 |pmid=12153116}}</ref> Other striking examples are the bacteria ''[[Escherichia coli]]'' evolving the ability to use [[citric acid]] as a nutrient in a [[E. coli long-term evolution experiment|long-term laboratory experiment]],<ref>{{cite journal |author=Blount ZD, Borland CZ, Lenski RE |title=Inaugural Article: Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=23 |pages=7899–906 |year=2008 |month=June |pmid=18524956 |doi=10.1073/pnas.0803151105 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=18524956}}</ref> ''[[Flavobacterium]]'' evolving a novel enzyme that allows these bacteria to grow on the by-products of [[nylon]] manufacturing,<ref>{{cite journal |author=Okada H, Negoro S, Kimura H, Nakamura S |title=Evolutionary adaptation of plasmid-encoded enzymes for degrading nylon oligomers |journal=Nature |volume=306 |issue=5939 |pages=203–6 |year=1983 |pmid=6646204 |doi=10.1038/306203a0}}</ref><ref>{{cite journal |author=Ohno S |title=Birth of a unique enzyme from an alternative reading frame of the preexisted, internally repetitious coding sequence |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=81 |issue=8 |pages=2421–5 |year=1984 |month=April |pmid=6585807 |pmc=345072 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=6585807 |doi=10.1073/pnas.81.8.2421}}</ref> and the soil bacterium ''[[Sphingobium]]'' evolving an entirely new [[metabolic pathway]] that degrades the synthetic [[pesticide]] [[pentachlorophenol]].<ref>{{cite journal |author=Copley SD |title=Evolution of a metabolic pathway for degradation of a toxic xenobiotic: the patchwork approach |journal=Trends Biochem. Sci. |volume=25 |issue=6 |pages=261–5 |year=2000 |month=June |pmid=10838562 |doi=10.1016/S0968-0004(00)01562-0}}</ref><ref>{{cite journal |author=Crawford RL, Jung CM, Strap JL |title=The recent evolution of pentachlorophenol (PCP)-4-monooxygenase (PcpB) and associated pathways for bacterial degradation of PCP |journal=Biodegradation |volume=18 |issue=5 |pages=525–39 |year=2007 |month=October |pmid=17123025 |doi=10.1007/s10532-006-9090-6}}</ref> An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' [[evolvability]]).<ref>{{cite journal |author=Colegrave N, Collins S |title=Experimental evolution: experimental evolution and evolvability |journal=Heredity |volume=100 |issue=5 |pages=464–70 |year=2008 |month=May |pmid=18212804 |doi=10.1038/sj.hdy.6801095 |url=http://www.nature.com/hdy/journal/v100/n5/full/6801095a.html}}</ref><ref>{{cite journal |author=Kirschner M, Gerhart J |title=Evolvability |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=95 |issue=15 |pages=8420–7 |year=1998 |month=July |pmid=9671692 |pmc=33871}}</ref> |
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However, many traits that appear to be simple adaptations are in fact [[exaptation]]s: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process.<ref name=GouldStructP1235>{{wikiref |id=Gould-2002 |text=Gould 2002, pp. 1235–6}} </ref> One example is the African lizard ''Holaspis guentheri'', which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an [[exaptation]].<ref name=GouldStructP1235/> Another is the recruitment of enzymes from [[glycolysis]] and [[xenobiotic metabolism]] to serve as structural proteins called [[crystallin]]s within the lenses of organisms' [[eye]]s.<ref>{{cite journal |author=Piatigorsky J, Kantorow M, Gopal-Srivastava R, Tomarev SI |title=Recruitment of enzymes and stress proteins as lens crystallins |journal=EXS |volume=71 |pages=241–50 |year=1994 |pmid=8032155}}</ref><ref>{{cite journal |author=Wistow G |title=Lens crystallins: gene recruitment and evolutionary dynamism |journal=Trends Biochem. Sci. |volume=18 |issue=8 |pages=301–6 |year=1993 |month=August |pmid=8236445 |doi=10.1016/0968-0004(93)90041-K}}</ref> |
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[[File:Whale skeleton.png|350px|thumb|right|A [[baleen whale]] skeleton, ''a'' and ''b'' label [[flipper (anatomy)|flipper]] bones, which were [[adaptation|adapted]] from front [[leg]] bones: while ''c'' indicates [[Vestigiality|vestigial]] leg bones.<ref name="transformation445">{{cite journal |author=Bejder L, Hall BK |title=Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss |journal=Evol. Dev. |volume=4 |issue=6 |pages=445–58 |year=2002 |pmid=12492145 |doi=10.1046/j.1525-142X.2002.02033.x }}</ref>]] |
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As adaptation occurs through the gradual modification of existing structures, structures with similar internal organization may have very different functions in related organisms. This is the result of a single [[homology (biology)|ancestral structure]] being adapted to function in different ways. The bones within bat wings, for example, are structurally similar to both human hands and seal flippers, due to the common descent of these structures from an ancestor that also had five digits at the end of each forelimb. Other idiosyncratic anatomical features, such as [[sesamoid bone|bones in the wrist]] of the [[Giant Panda|panda]] being formed into a false "thumb," indicate that an organism's evolutionary lineage can limit what adaptations are possible.<ref>{{cite journal |author=Salesa MJ, Antón M, Peigné S, Morales J |title=Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas |url=http://www.pnas.org/content/103/2/379.full |journal=[[Proceedings of the National Academy of Sciences|Proc. Natl. Acad. Sci. U.S.A.]] |volume=103 |issue=2 |pages=379–82 |year=2006 |pmid=16387860 |doi=10.1073/pnas.0504899102}}</ref> |
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During adaptation, some structures may lose their original function and become [[vestigial structure]]s.<ref name=Fong>{{cite journal |author=Fong D, Kane T, Culver D |title=Vestigialization and Loss of Nonfunctional Characters |url=http://links.jstor.org/sici?sici=0066-4162%281995%2926%3C249%3AVALONC%3E2.0.CO%3B2-2 |journal=Ann. Rev. Ecol. Syst. |volume=26 |pages=249–68 |year=1995 |doi=10.1146/annurev.es.26.110195.001341}}</ref> Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include [[pseudogene]]s,<ref>{{cite journal |author=Zhang Z, Gerstein M |title=Large-scale analysis of pseudogenes in the human genome |journal=Curr. Opin. Genet. Dev. |volume=14 |issue=4 |pages=328–35 |year=2004 |month=August |pmid=15261647 |doi=10.1016/j.gde.2004.06.003}}</ref> the non-functional remains of eyes in blind cave-dwelling fish,<ref>{{cite journal |author=Jeffery WR |title=Adaptive evolution of eye degeneration in the Mexican blind cavefish |doi= 10.1093/jhered/esi028 |journal=J. Hered. |volume=96 |issue=3 |pages=185–96 |year=2005 |pmid=15653557}}</ref> wings in flightless birds,<ref>{{cite journal |author=Maxwell EE, Larsson HC |title=Osteology and myology of the wing of the Emu (Dromaius novaehollandiae), and its bearing on the evolution of vestigial structures |journal=J. Morphol. |volume=268 |issue=5 |pages=423–41 |year=2007 |pmid=17390336 |doi=10.1002/jmor.10527 }}</ref> and the presence of hip bones in whales and snakes.<ref name="transformation445"/> Examples of vestigial structures in humans include [[wisdom teeth]],<ref>{{cite journal |author=Silvestri AR, Singh I |title=The unresolved problem of the third molar: would people be better off without it? |url=http://jada.ada.org/cgi/content/full/134/4/450 |journal=Journal of the American Dental Association (1939) |volume=134 |issue=4 |pages=450–5 |year=2003 |pmid=12733778 |doi=10.1146/annurev.es.26.110195.001341}}</ref> the [[coccyx]],<ref name=Fong/> and the [[vermiform appendix]].<ref name=Fong/> |
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A critical principle of [[ecology]] is that of [[competitive exclusion principle|competitive exclusion]]: no two species can occupy the same niche in the same environment for a long time.<ref>{{cite journal |author=Hardin G |authorlink=Garrett Hardin |title=The competitive exclusion principle |journal=Science |volume=131 |pages=1292–7 |year=1960 |month=April |pmid=14399717 |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=14399717}}</ref> Consequently, natural selection will tend to force species to adapt to different [[ecological niche]]s. This may mean that, for example, two species of [[cichlid]] fish adapt to live in different [[habitat]]s, which will minimize the competition between them for food.<ref>{{cite journal |author=Kocher TD |title=Adaptive evolution and explosive speciation: the cichlid fish model |journal=Nat. Rev. Genet. |volume=5 |issue=4 |pages=288–98 |year=2004 |month=April |pmid=15131652 |doi=10.1038/nrg1316 |url=http://hcgs.unh.edu/staff/kocher/pdfs/Kocher2004.pdf}}</ref> |
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An area of current investigation in [[evolutionary developmental biology]] is the [[Developmental biology|developmental]] basis of adaptations and exaptations.<ref>{{cite journal |author=Johnson NA, Porter AH |title=Toward a new synthesis: population genetics and evolutionary developmental biology |journal=Genetica |volume=112–113 |issue= |pages=45–58 |year=2001 |pmid=11838782 |doi=10.1023/A:1013371201773}}</ref> This research addresses the origin and evolution of [[Embryogenesis|embryonic development]] and how modifications of development and developmental processes produce novel features.<ref>{{cite journal |author=Baguñà J, Garcia-Fernàndez J |title=Evo-Devo: the long and winding road |url=http://www.ijdb.ehu.es/web/paper.php?doi=14756346 |journal=Int. J. Dev. Biol. |volume=47 |issue=7–8 |pages=705–13 |year=2003 |pmid=14756346}}<br />*{{cite journal | author = Love AC. | year = 2003 | title = Evolutionary Morphology, Innovation, and the Synthesis of Evolutionary and Developmental Biology | journal = Biology and Philosophy | volume = 18 | issue = 2 | pages = 309–345 | doi = 10.1023/A:1023940220348 | url = http://philsci-archive.pitt.edu/archive/00000375/00/LondonPaper.doc}}</ref> These studies have shown that evolution can alter development to create new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals.<ref>{{cite journal |author=Allin EF |title=Evolution of the mammalian middle ear |journal=J. Morphol. |volume=147 |issue=4 |pages=403–37 |year=1975 |pmid=1202224 |doi=10.1002/jmor.1051470404 }}</ref> It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in [[chicken]]s causing embryos to grow teeth similar to those of [[crocodile]]s.<ref>{{cite journal |author=Harris MP, Hasso SM, Ferguson MW, Fallon JF |title=The development of archosaurian first-generation teeth in a chicken mutant |journal=Curr. Biol. |volume=16 |issue=4 |pages=371–7 |year=2006 |pmid=16488870 |doi=10.1016/j.cub.2005.12.047 }}</ref> It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.<ref>{{cite journal |author=Carroll SB |title=Evo-devo and an expanding evolutionary synthesis: a genetic theory of morphological evolution |journal=Cell |volume=134 |issue=1 |pages=25–36 |year=2008 |month=July |pmid=18614008 |doi=10.1016/j.cell.2008.06.030}}</ref> |
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===Co-evolution=== |
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{{details more|Co-evolution}} |
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Interactions between organisms can produce both conflict and co-operation. When the interaction is between pairs of species, such as a [[pathogen]] and a [[host (biology)|host]], or a [[Predation|predator]] and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called [[co-evolution]].<ref>{{cite journal |author=Wade MJ |title=The co-evolutionary genetics of ecological communities |journal=Nat. Rev. Genet. |volume=8 |issue=3 |pages=185–95 |year=2007 |pmid=17279094 |doi=10.1038/nrg2031 }}</ref> An example is the production of [[tetrodotoxin]] in the [[rough-skinned newt]] and the evolution of tetrodotoxin resistance in its predator, the [[Common Garter Snake|common garter snake]]. In this predator-prey pair, an [[evolutionary arms race]] has produced high levels of toxin in the newt and correspondingly high levels of resistance in the snake.<ref>{{cite journal |author=Geffeney S, Brodie ED, Ruben PC, Brodie ED |title=Mechanisms of adaptation in a predator-prey arms race: TTX-resistant sodium channels |journal=Science |volume=297 |issue=5585 |pages=1336–9 |year=2002 |pmid=12193784 |doi=10.1126/science.1074310 }}<br />*{{cite journal |author=Brodie ED, Ridenhour BJ, Brodie ED |title=The evolutionary response of predators to dangerous prey: hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts |journal=Evolution |volume=56 |issue=10 |pages=2067–82 |year=2002 |pmid=12449493}}</ref> |
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===Co-operation=== |
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{{details more|Co-operation (evolution)}} |
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However, not all interactions between species involve conflict.<ref>{{cite journal |author=Sachs J |title=Cooperation within and among species |journal=J. Evol. Biol. |volume=19 |issue=5 |pages=1415–8; discussion 1426–36 |year=2006 |pmid=16910971 |doi=10.1111/j.1420-9101.2006.01152.x }}<br />*{{cite journal |author=Nowak M |title=Five rules for the evolution of cooperation |journal=Science |volume=314 |issue=5805 |pages=1560–3 |year=2006 |pmid=17158317 |doi=10.1126/science.1133755 }}</ref> Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the [[Mycorrhiza|mycorrhizal fungi]] that grow on their roots and aid the plant in absorbing nutrients from the soil.<ref>{{cite journal |author=Paszkowski U |title=Mutualism and parasitism: the yin and yang of plant symbioses |journal=Curr. Opin. Plant Biol. |volume=9 |issue=4 |pages=364–70 |year=2006 |pmid=16713732 |doi=10.1016/j.pbi.2006.05.008 }}</ref> This is a [[Reciprocity (evolution)|reciprocal]] relationship as the plants provide the fungi with sugars from photosynthesis. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending [[signal transduction|signals]] that suppress the plant [[immune system]].<ref>{{cite journal |author=Hause B, Fester T |title=Molecular and cell biology of arbuscular mycorrhizal symbiosis |journal=Planta |volume=221 |issue=2 |pages=184–96 |year=2005 |pmid=15871030 |doi=10.1007/s00425-004-1436-x }}</ref> |
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Coalitions between organisms of the same species have also evolved. An extreme case is the [[eusociality]] found in [[Eusociality|social insect]]s, such as [[bee]]s, [[termite]]s and [[ant]]s, where sterile insects feed and guard the small number of organisms in a [[Colony (biology)|colony]] that are able to reproduce. On an even smaller scale, the [[somatic cell]]s that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's [[germ cell]]s to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth [[carcinogenesis|causes cancer]].<ref name=Bertram/> |
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These examples of cooperation within species are thought to have evolved through the process of [[kin selection]], which is where one organism acts to help raise a relative's offspring.<ref>{{cite journal |author=Reeve HK, Hölldobler B |title=The emergence of a superorganism through intergroup competition |doi= 10.1073/pnas.0703466104 |journal=Proc Natl Acad Sci U S A. |volume=104 |issue=23 |pages=9736–40 |year=2007 |pmid=17517608}}</ref> This activity is selected for because if the ''helping'' individual contains alleles which promote the helping activity, it is likely that its kin will ''also'' contain these alleles and thus those alleles will be passed on.<ref>{{cite journal |author=Axelrod R, Hamilton W |title=The evolution of cooperation |journal=Science |volume=211 |issue=4489 |pages=1390–6 | year = 2005 |pmid=7466396 |doi=10.1126/science.7466396 }}</ref> Other processes that may promote cooperation include [[group selection]], where cooperation provides benefits to a group of organisms.<ref>{{cite journal |author=Wilson EO, Hölldobler B |title=Eusociality: origin and consequences |doi= 10.1073/pnas.0505858102 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=102 |issue=38 |pages=13367–71 |year=2005 |pmid=16157878}}</ref> |
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===Speciation=== |
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{{details more|Speciation}} |
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[[File:Speciation modes edit.svg|left|thumb|350px|The four mechanisms of [[speciation]].]] |
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[[Speciation]] is the process where a species diverges into two or more descendant species.<ref name=Gavrilets>{{cite journal |author=Gavrilets S |title=Perspective: models of speciation: what have we learned in 40 years? |journal=Evolution |volume=57 |issue=10 |pages=2197–215 |year=2003 |pmid=14628909 |doi=10.1554/02-727}}</ref> Evolutionary biologists view species as statistical phenomena and not categories or types. This view is counterintuitive since the classical idea of species is still widely held, with a species seen as a class of organisms exemplified by a "[[Biological type|type specimen]]" that bears all the traits common to this species. Instead, a species is now defined as a separately evolving lineage that forms a single [[gene pool]]. Although properties such as genetics and morphology are used to help separate closely related lineages, this definition has fuzzy boundaries.<ref>{{cite journal |author=De Queiroz K |title=Species concepts and species delimitation |journal=Syst. Biol. |volume=56 |issue=6 |pages=879–86 |year=2007 |month=December |pmid=18027281 |doi=10.1080/10635150701701083}}</ref> Indeed, the exact definition of the term "species" is still controversial, particularly in prokaryotes,<ref>{{cite journal |author=Fraser C, Alm EJ, Polz MF, Spratt BG, Hanage WP |title=The bacterial species challenge: making sense of genetic and ecological diversity |journal=Science |volume=323 |issue=5915 |pages=741–6 |year=2009 |month=February |pmid=19197054 |doi=10.1126/science.1159388}}</ref> and this is called the [[species problem]].<ref name=Queiroz>{{cite journal |author=de Queiroz K |title=Ernst Mayr and the modern concept of species |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=102 |issue=Suppl 1 |pages=6600–7 |year=2005 |month=May |pmid=15851674 |pmc=1131873 |doi=10.1073/pnas.0502030102 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=15851674}}</ref> Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species concerned.<ref name=Queiroz/> Typically the actual focus on biological study is the [[population]], an observable ''interacting'' group of organisms, rather than a [[species]], an observable ''similar'' group of individuals. |
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Speciation has been observed multiple times under both controlled laboratory conditions and in nature.<ref>{{cite journal | author = Rice, W.R. | coauthors = Hostert, E.E. | year = 1993 | title = Laboratory experiments on speciation: what have we learned in 40 years | journal = Evolution | volume = 47 | issue = 6 | pages = 1637–1653 |
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| url = http://links.jstor.org/sici?sici=0014-3820(199312)47%3A6%3C1637%3ALEOSWH%3E2.0.CO%3B2-T | accessdate = 2008-05-19 | doi = 10.2307/2410209 |
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}}<br />*{{cite journal |author=Jiggins CD, Bridle JR |title=Speciation in the apple maggot fly: a blend of vintages? |journal=Trends Ecol. Evol. (Amst.) |volume=19 |issue=3 |pages=111–4 |year=2004 |pmid=16701238 |doi=10.1016/j.tree.2003.12.008}}<br />*{{cite web|author=Boxhorn, J|year=1995|url=http://www.talkorigins.org/faqs/faq-speciation.html|title=Observed Instances of Speciation|publisher=[[TalkOrigins Archive]]|accessdate=2008-12-26}}<br />*{{cite journal |author=Weinberg JR, Starczak VR, Jorg, D |title=Evidence for Rapid Speciation Following a Founder Event in the Laboratory |journal=Evolution |volume=46 |issue=4 |pages=1214–20 |year=1992 |doi=10.2307/2409766}}</ref> In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is [[allopatric speciation]], which occurs in populations initially isolated geographically, such as by [[habitat fragmentation]] or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.<ref>{{cite journal |year=2008 |title=Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=12 |pages=4792–5 |pmid=18344323 |doi=10.1073/pnas.0711998105 | author= Herrel, A.; Huyghe, K.; Vanhooydonck, B.; Backeljau, T.; Breugelmans, K.; Grbac, I.; Van Damme, R.; Irschick, D.J.}}</ref><ref name=Losos1997>{{cite journal |year=1997 |title=Adaptive differentiation following experimental island colonization in Anolis lizards| journal=Nature |volume=387 |issue=6628 |pages=70–3 |doi=10.1038/387070a0 |author=Losos, J.B. Warhelt, K.I. Schoener, T.W.}}</ref> As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.<ref>{{cite journal|author=Hoskin CJ, Higgle M, McDonald KR, Moritz C |year=2005 |title=Reinforcement drives rapid allopatric speciation |journal=Nature |volume=437 |pages =1353–356|doi=10.1038/nature04004}}</ref> |
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The second mechanism of speciation is [[peripatric speciation]], which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the [[founder effect]] causes rapid speciation through both rapid genetic drift and selection on a small gene pool.<ref>{{cite journal |author=Templeton AR |title=The theory of speciation via the founder principle |url=http://www.genetics.org/cgi/reprint/94/4/1011 |journal=Genetics |volume=94 |issue=4 |pages=1011–38 |year=1980 |pmid=6777243 |month=Apr |day=01}}</ref> |
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The third mechanism of speciation is [[parapatric speciation]]. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.<ref name=Gavrilets/> Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass ''[[Anthoxanthum|Anthoxanthum odoratum]]'', which can undergo parapatric speciation in response to localized metal pollution from mines.<ref>{{cite journal |author=Antonovics J |title=Evolution in closely adjacent plant populations X: long-term persistence of prereproductive isolation at a mine boundary |journal=Heredity |volume=97 |issue=1 |pages=33–7 |year=2006 |pmid=16639420 |url=http://www.nature.com/hdy/journal/v97/n1/full/6800835a.html |doi=10.1038/sj.hdy.6800835 }}</ref> Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause ''reinforcement'', which is the evolution of traits that promote mating within a species, as well as [[character displacement]], which is when two species become more distinct in appearance.<ref>{{cite journal |author=Nosil P, Crespi B, Gries R, Gries G |title=Natural selection and divergence in mate preference during speciation |journal=Genetica |volume=129 |issue=3 |pages=309–27 |year=2007 |pmid=16900317 |doi=10.1007/s10709-006-0013-6 }}</ref> |
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[[File:Darwin's finches cropped.jpeg|frame|right|[[Geographical isolation]] of [[Darwin's finches|finches]] on the [[Galápagos Islands]] produced over a dozen new species.]] |
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Finally, in [[sympatric speciation]] species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of [[gene flow]] may remove genetic differences between parts of a population.<ref>{{cite journal|author=Savolainen V, Anstett M-C, Lexer C, Hutton I, Clarkson JJ, Norup MV, Powell MP, Springate D, Salamin N, Baker WJr |year=2006 |title=Sympatric speciation in palms on an oceanic island |journal=Nature |volume=441 |pages=210–3 | pmid=16467788 |doi=10.1038/nature04566}}<br />*{{cite journal| author=Barluenga M, Stölting KN, Salzburger W, Muschick M, Meyer A |year=2006 |title=Sympatric speciation in Nicaraguan crater lake cichlid fish |journal=Nature |volume=439 |pages=719–23 |pmid=16467837 |doi=10.1038/nature04325}}</ref> Generally, sympatric speciation in animals requires the evolution of both [[Polymorphism (biology)|genetic differences]] and [[assortative mating|non-random mating]], to allow reproductive isolation to evolve.<ref>{{cite journal |author=Gavrilets S |title=The Maynard Smith model of sympatric speciation |journal=J. Theor. Biol. |volume=239 |issue=2 |pages=172–82 |year=2006 |pmid=16242727 |doi=10.1016/j.jtbi.2005.08.041 }}</ref> |
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One type of sympatric speciation involves cross-breeding of two related species to produce a new [[Hybrid (biology)|hybrid]] species. This is not common in animals as animal hybrids are usually sterile. This is because during [[meiosis]] the [[homologous chromosome]]s from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form [[polyploidy|polyploids]]. This allows the chromosomes from each parental species to form a matching pair during meiosis, since as each parent's chromosomes is represented by a pair already.<ref>{{cite journal |author=Hegarty Mf, Hiscock SJ |title=Genomic clues to the evolutionary success of polyploid plants |journal=Current Biology |volume=18 |issue=10 |pages=435–44 |year=2008 |doi=10.1016/j.cub.2008.03.043}}</ref> An example of such a speciation event is when the plant species ''[[Arabidopsis thaliana]]'' and ''Arabidopsis arenosa'' cross-bred to give the new species ''Arabidopsis suecica''.<ref>{{cite journal |author=Jakobsson M, Hagenblad J, Tavaré S, ''et al.'' |title=A unique recent origin of the allotetraploid species Arabidopsis suecica: Evidence from nuclear DNA markers |journal=Mol. Biol. Evol. |volume=23 |issue=6 |pages=1217–31 |year=2006 |pmid=16549398 |doi=10.1093/molbev/msk006 }}</ref> This happened about 20,000 years ago,<ref>{{cite journal |author=Säll T, Jakobsson M, Lind-Halldén C, Halldén C |title=Chloroplast DNA indicates a single origin of the allotetraploid Arabidopsis suecica |journal=J. Evol. Biol. |volume=16 |issue=5 |pages=1019–29 |year=2003 |pmid=14635917 |doi=10.1046/j.1420-9101.2003.00554.x }}</ref> and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process.<ref>{{cite journal |author=Bomblies K, Weigel D |title=Arabidopsis-a model genus for speciation |journal=Curr Opin Genet Dev |volume=17 |issue=6 |pages=500–4 |year=2007 |pmid=18006296 |doi=10.1016/j.gde.2007.09.006 }}</ref> Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.<ref name=Semon/> |
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Speciation events are important in the theory of [[punctuated equilibrium]], which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged.<ref name=pe1972>Niles Eldredge and Stephen Jay Gould, 1972. [http://www.blackwellpublishing.com/ridley/classictexts/eldredge.asp "Punctuated equilibria: an alternative to phyletic gradualism"] In T.J.M. Schopf, ed., ''Models in Paleobiology''. San Francisco: Freeman Cooper. pp. 82–115. Reprinted in N. Eldredge ''Time frames''. Princeton: Princeton Univ. Press. 1985</ref> In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population, and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats, and therefore rarely being preserved as fossils.<ref>{{cite journal |author=Gould SJ |title=Tempo and mode in the macroevolutionary reconstruction of Darwinism |doi= 10.1073/pnas.91.15.6764 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=91 |issue=15 |pages=6764–71 |year=1994 |pmid=8041695}}</ref> |
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===Extinction=== |
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{{details more|Extinction}} |
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[[File:Tarbosaurus museum Muenster.jpg|thumb|left|225px|''[[Tarbosaurus]]'' fossil. Non-[[bird|avian]] [[dinosaur]]s died out in the [[Cretaceous–Tertiary extinction event]] at the end of the [[Cretaceous]] period.]] |
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[[Extinction]] is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation, and disappear through extinction.<ref>{{cite journal |author=Benton MJ |title=Diversification and extinction in the history of life |journal=Science |volume=268 |issue=5207 |pages=52–8 |year=1995 |pmid=7701342 |doi=10.1126/science.7701342 }}</ref> Nearly all animal and plant species that have lived on earth are now extinct,<ref>{{cite journal |author=Raup DM |title=Biological extinction in earth history |journal=Science |volume=231 |issue= |pages=1528–33 |year=1986 |pmid=11542058 |doi=10.1126/science.11542058 }}</ref> and extinction appears to be the ultimate fate of all species.<ref>{{cite journal |author=Avise JC, Hubbell SP, Ayala FJ. |title=In the light of evolution II: Biodiversity and extinction |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=Suppl 1 |pages=11453–7 |year=2008 |month=August |pmid=18695213 |pmc=2556414 |doi=10.1073/pnas.0802504105 |url=http://www.pnas.org/content/105/suppl.1/11453.full}}</ref> These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass [[extinction event]]s.<ref name=Raup>{{cite journal |author=Raup DM |title=The role of extinction in evolution |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=91 |issue=15 |pages=6758–63 |year=1994 |pmid=8041694 |doi=10.1073/pnas.91.15.6758 |pmc=44280 }}</ref> The [[Cretaceous–Tertiary extinction event]], during which the non-avian dinosaurs went extinct, is the most well-known, but the earlier [[Permian–Triassic extinction event]] was even more severe, with approximately 96 percent of species driven to extinction.<ref name=Raup/> The [[Holocene extinction event]] is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100-1000 times greater than the background rate, and up to 30 percent of species may be extinct by the mid 21st century.<ref>{{cite journal |author=Novacek MJ, Cleland EE |title=The current biodiversity extinction event: scenarios for mitigation and recovery |doi= 10.1073/pnas.091093698 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=98 |issue=10 |pages=5466–70 |year=2001 |pmid=11344295}}</ref> Human activities are now the primary cause of the ongoing extinction event;<ref>{{cite journal |author=Pimm S, Raven P, Peterson A, Sekercioglu CH, Ehrlich PR |title=Human impacts on the rates of recent, present, and future bird extinctions |doi= 10.1073/pnas.0604181103 |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=103 |issue=29 |pages=10941–6 |year=2006 |pmid=16829570}}<br />*{{cite journal |author=Barnosky AD, Koch PL, Feranec RS, Wing SL, Shabel AB |title=Assessing the causes of late Pleistocene extinctions on the continents |journal=Science |volume=306 |issue=5693 |pages=70–5 |year=2004 |pmid=15459379 |doi=10.1126/science.1101476 }}</ref> [[global warming]] may further accelerate it in the future.<ref>{{cite journal |author=Lewis OT |title=Climate change, species-area curves and the extinction crisis |url=http://www.journals.royalsoc.ac.uk/content/711761513317h856/fulltext.pdf |format=PDF|journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=361 |issue=1465 |pages=163–71 |year=2006 |pmid=16553315 |doi=10.1098/rstb.2005.1712}}</ref> |
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The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered.<ref name=Raup/> The causes of the continuous "low-level" extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources ([[competitive exclusion]]).<ref name=Kutschera/> If one species can out-compete another, this could produce [[Unit of selection#Species selection and selection at higher taxonomic levels|species selection]], with the fitter species surviving and the other species being driven to extinction.<ref name=Gould/> The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of [[Adaptive radiation|rapid evolution]] and speciation in survivors.<ref>{{cite journal |author=Jablonski D |title=Lessons from the past: evolutionary impacts of mass extinctions |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=98 |issue=10 |pages=5393–8 |year=2001 |month=May |pmid=11344284 |pmc=33224 |doi=10.1073/pnas.101092598}}</ref> |
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{{-}} |
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==Evolutionary history of life== |
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{{Main|Evolutionary history of life}} |
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===Origin of life=== |
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{{details more|Abiogenesis|RNA world hypothesis}} |
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The origin of [[life]] is a necessary precursor for biological evolution, but understanding that evolution occurred once organisms appeared and investigating how this happens does not depend on understanding exactly how life began.<ref>{{cite web |last=Isaak |first=Mark |year=2005 |title=Claim CB090: Evolution without abiogenesis |publisher=[[TalkOrigins Archive]] |url=http://www.talkorigins.org/indexcc/CB/CB090.html |accessdate=2008-12-26}}</ref> The current [[scientific consensus]] is that the complex [[biochemistry]] that makes up life came from simpler chemical reactions, but it is unclear how this occurred.<ref>{{cite journal |author=Peretó J |title=Controversies on the origin of life |url=http://www.im.microbios.org/0801/0801023.pdf |format=PDF|journal=Int. Microbiol. |volume=8 |issue=1 |pages=23–31 |year=2005 |pmid=15906258}}</ref> Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any [[last universal ancestor|last universal common ancestor]] or ancestral gene pool.<ref>{{cite journal |author=Luisi PL, Ferri F, Stano P |title=Approaches to semi-synthetic minimal cells: a review |journal=Naturwissenschaften |volume=93 |issue=1 |pages=1–13 |year=2006 |pmid=16292523 |doi=10.1007/s00114-005-0056-z }}</ref><ref>{{cite journal |author=Trevors JT, Abel DL |title=Chance and necessity do not explain the origin of life |journal=Cell Biol. Int. |volume=28 |issue=11 |pages=729–39 |year=2004 |pmid=15563395 |doi=10.1016/j.cellbi.2004.06.006 }}{{cite journal |author=Forterre P, Benachenhou-Lahfa N, Confalonieri F, Duguet M, Elie C, Labedan B |title=The nature of the last universal ancestor and the root of the tree of life, still open questions |journal=BioSystems |volume=28 |issue=1–3 |pages=15–32 |year=1992 |pmid=1337989 |doi=10.1016/0303-2647(92)90004-I }}</ref> Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as [[RNA]],<ref>{{cite journal |author=Joyce GF |title=The antiquity of RNA-based evolution |journal=Nature |volume=418 |issue=6894 |pages=214–21 |year=2002 |pmid=12110897 |doi=10.1038/418214a }}</ref> and the assembly of simple cells.<ref>{{cite journal |author=Trevors JT, Psenner R |title=From self-assembly of life to present-day bacteria: a possible role for nanocells |journal=FEMS Microbiol. Rev. |volume=25 |issue=5 |pages=573–82 |year=2001 |pmid=11742692 |doi=10.1111/j.1574-6976.2001.tb00592.x }}</ref> |
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===Common descent=== |
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{{details more|Evidence of common descent|Common descent|Homology (biology)}} |
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[[File:Ape skeletons.png|right|320px|thumbnail|The [[Ape|hominoids]] are descendants of a [[common descent|common ancestor]].]] |
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All [[organism]]s on [[Earth]] are descended from a common ancestor or ancestral gene pool.<ref>{{cite journal |author=Penny D, Poole A |title=The nature of the last universal common ancestor |journal=Curr. Opin. Genet. Dev. |volume=9 |issue=6 |pages=672–77 |year=1999 |pmid=10607605 |doi=10.1016/S0959-437X(99)00020-9}}</ref> Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events.<ref>{{cite journal |author=Bapteste E, Walsh DA |title=Does the 'Ring of Life' ring true? |journal=Trends Microbiol. |volume=13 |issue=6 |pages=256–61 |year=2005 |pmid=15936656 |doi=10.1016/j.tim.2005.03.012 }}</ref> The [[common descent]] of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits, and finally, that organisms can be classified using these similarities into a hierarchy of nested groups - similar to a family tree.<ref name=Darwin/> However, modern research has suggested that, due to horizontal gene transfer, this "[[Tree of life (science)|tree of life]]" may be more complicated than a simple branching tree since some genes have spread independently between distantly related species.<ref>{{cite journal |author=Doolittle WF, Bapteste E |title=Pattern pluralism and the Tree of Life hypothesis |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 |issue=7 |pages=2043–9 |year=2007 |month=February |pmid=17261804 |pmc=1892968 |doi=10.1073/pnas.0610699104 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=17261804}}</ref><ref>{{cite journal |author=Kunin V, Goldovsky L, Darzentas N, Ouzounis CA |title=The net of life: reconstructing the microbial phylogenetic network |journal=Genome Res. |volume=15 |issue=7 |pages=954–9 |year=2005 |pmid=15965028 |url=http://www.genome.org/cgi/pmidlookup?view=long&pmid=15965028 |doi =10.1101/gr.3666505}}</ref> |
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Past species have also left records of their evolutionary history. [[Fossil]]s, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.<ref name=Jablonski>{{cite journal |author=Jablonski D |title=The future of the fossil record |journal=Science |volume=284 |issue=5423 |pages=2114–16 |year=1999 |pmid=10381868 |doi=10.1126/science.284.5423.2114 }}</ref> By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as [[bacteria]] and [[archaea]] share a limited set of common morphologies, their fossils do not provide information on their ancestry. |
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More recently, evidence for common descent has come from the study of [[biochemistry|biochemical]] similarities between organisms. For example, all living cells use the same basic set of [[nucleotide]]s and [[amino acid]]s.<ref>{{cite journal |author=Mason SF |title=Origins of biomolecular handedness |journal=Nature |volume=311 |issue=5981 |pages=19–23 |year=1984 |pmid=6472461 |doi=10.1038/311019a0 }}</ref> The development of [[molecular genetics]] has revealed the record of evolution left in organisms' [[genome]]s: dating when species diverged through the [[molecular clock]] produced by mutations.<ref>{{cite journal |author=Wolf YI, Rogozin IB, Grishin NV, Koonin EV |title=Genome trees and the tree of life |journal=Trends Genet. |volume=18 |issue=9 |pages=472–79 |year=2002 |pmid=12175808 |doi=10.1016/S0168-9525(02)02744-0}}</ref> For example, these DNA sequence comparisons have revealed the close genetic similarity between humans and chimpanzees and shed light on when the common ancestor of these species existed.<ref>{{cite journal |author=Varki A, Altheide TK |title=Comparing the human and chimpanzee genomes: searching for needles in a haystack |journal=Genome Res. |volume=15 |issue=12 |pages=1746–58 |year=2005 |pmid=16339373 |doi=10.1101/gr.3737405 }}</ref> |
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===Evolution of life=== |
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{{details|Timeline of evolution}} |
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{{PhylomapA|size=400px|align=left|caption=[[Phylogenetic tree|Evolutionary tree]] showing the divergence of modern species from their common ancestor in the center.<ref name=Ciccarelli>{{cite journal |author=Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P |title=Toward automatic reconstruction of a highly resolved tree of life |journal=Science |volume=311 |issue=5765 |pages=1283–87 |year=2006 |pmid=16513982 |doi=10.1126/science.1123061 }}</ref> The three [[Domain (biology)|domains]] are colored, with [[bacteria]] blue, [[archaea]] green, and [[eukaryote]]s red.}} |
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Despite the uncertainty on how life began, it is generally accepted that [[prokaryote]]s inhabited the Earth from approximately 3–4 billion years ago.<ref name=Cavalier-Smith>{{cite journal |author=Cavalier-Smith T |title=Cell evolution and Earth history: stasis and revolution |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=969–1006 |year=2006 |pmid=16754610 |doi=10.1098/rstb.2006.1842 |pmc=1578732}}</ref><ref>{{cite journal |author=Schopf J |title=Fossil evidence of Archaean life |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=869–85 |year=2006 |pmid=16754604 |doi=10.1098/rstb.2006.1834}}<br />*{{cite journal |author=Altermann W, Kazmierczak J |title=Archean microfossils: a reappraisal of early life on Earth |journal=Res Microbiol |volume=154 |issue=9 |pages=611–17 |year=2003 |pmid=14596897 |doi=10.1016/j.resmic.2003.08.006 }}</ref> No obvious changes in [[morphology (biology)|morphology]] or cellular organization occurred in these organisms over the next few billion years.<ref>{{cite journal |author=Schopf J |title=Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic |doi= 10.1073/pnas.91.15.6735 |journal=Proc Natl Acad Sci U S a |volume=91 |issue=15 |pages=6735–42 |year=1994 |pmid=8041691}}</ref> |
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The [[eukaryote]]s were the next major change in cell structure. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in a cooperative association called [[endosymbiont|endosymbiosis]].<ref name = "rgruqh"/><ref name=Dyall>{{cite journal |author=Dyall S, Brown M, Johnson P |title= Ancient invasions: from endosymbionts to organelles |journal=Science |volume=304 |issue=5668 |pages=253–57 |year=2004 |pmid=15073369 |doi=10.1126/science.1094884 }}</ref> The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either [[mitochondrion|mitochondria]] or [[hydrogenosome]]s.<ref>{{cite journal |author=Martin W |title=The missing link between hydrogenosomes and mitochondria |journal=Trends Microbiol. |volume=13 |issue=10 |pages=457–59 |year=2005 |pmid=16109488 |doi=10.1016/j.tim.2005.08.005 }}</ref> An independent second engulfment of [[cyanobacteria]]l-like organisms led to the formation of [[chloroplast]]s in algae and plants.<ref>{{cite journal |author=Lang B, Gray M, Burger G |title=Mitochondrial genome evolution and the origin of eukaryotes |journal=Annu Rev Genet |volume=33 |pages=351–97 |year=1999 |pmid=10690412 |doi=10.1146/annurev.genet.33.1.351 }}<br />*{{cite journal |author=McFadden G |title=Endosymbiosis and evolution of the plant cell |journal=Curr Opin Plant Biol |volume=2 |issue=6 |pages= 513–19 |year=1999 |pmid=10607659 |doi=10.1016/S1369-5266(99)00025-4}}</ref> It is unknown when the first eukaryotic cells appeared though they first emerged between 1.6 - 2.7 billion years ago. |
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The history of life was that of the unicellular eukaryotes, prokaryotes, and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the [[Ediacara biota|Ediacaran]] period.<ref name=Cavalier-Smith/><ref>{{cite journal |author=DeLong E, Pace N |title=Environmental diversity of bacteria and archaea |journal=Syst Biol |volume=50 |issue=4 |pages=470–8 |year=2001|pmid=12116647 |doi=10.1080/106351501750435040}}</ref> The [[evolution of multicellularity]] occurred in multiple independent events, in organisms as diverse as [[sponge]]s, [[brown algae]], [[cyanobacteria]], [[slime mold|slime mould]]s and [[myxobacteria]].<ref>{{cite journal |author=Kaiser D |title=Building a multicellular organism |journal=Annu. Rev. Genet. |volume=35 |pages=103–23 |year=2001 |pmid=11700279 |doi=10.1146/annurev.genet.35.102401.090145 }}</ref> |
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Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the [[Cambrian explosion]]. Here, the majority of [[Phylum|types]] of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.<ref name=Valentine>{{cite journal |author=Valentine JW, Jablonski D, Erwin DH |title=Fossils, molecules and embryos: new perspectives on the Cambrian explosion |url=http://dev.biologists.org/cgi/reprint/126/5/851 |journal=Development |volume=126 |issue=5 |pages=851–9 |year=1999 |pmid=9927587 |month=Mar |day=01}}</ref> Various triggers for the Cambrian explosion have been proposed, including the accumulation of [[oxygen]] in the [[atmosphere]] from [[photosynthesis]].<ref>{{cite journal |author=Ohno S |title=The reason for as well as the consequence of the Cambrian explosion in animal evolution |journal=J. Mol. Evol. |volume=44 Suppl 1 |issue= |pages=S23–7 |year=1997 |pmid=9071008 |doi=10.1007/PL00000055}}<br />*{{cite journal |author=Valentine J, Jablonski D |title=Morphological and developmental macroevolution: a paleontological perspective |url=http://www.ijdb.ehu.es/web/paper.php?doi=14756327 |journal=Int. J. Dev. Biol. |volume=47 |issue=7–8 |pages=517–22 |year=2003 |pmid=14756327}}</ref> About 500 million years ago, [[plant]]s and [[fungus|fungi]] colonized the land, and were soon followed by [[arthropod]]s and other animals.<ref>{{cite journal |author=Waters ER |title=Molecular adaptation and the origin of land plants |journal=Mol. Phylogenet. Evol. |volume=29 |issue=3 |pages=456–63 |year=2003 |pmid=14615186 |doi=10.1016/j.ympev.2003.07.018 }}</ref> [[Amphibian]]s first appeared around 300 million years ago, followed by early [[amniote]]s, then [[mammal]]s around 200 million years ago and [[bird]]s around 100 million years ago (both from "[[reptile]]"-like lineages). However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both [[Biomass (ecology)|biomass]] and species being prokaryotes.<ref name=Schloss/> |
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==History of evolutionary thought== |
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{{details|History of evolutionary thought}} |
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[[File:Charles Darwin aged 51 crop.jpg|right|thumb|150px|[[Charles Darwin]] at age 51, just after publishing ''[[On the Origin of Species]]''.]] |
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Evolutionary ideas such as [[common descent]] and the [[transmutation of species]] have existed since at least the 6th century BC, when they were expounded by the [[Greek philosophy|Greek philosopher]] [[Anaximander]].<ref>{{cite book|author=Wright, S|year=1984|title=Evolution and the Genetics of Populations, Volume 1: Genetic and Biometric Foundations|publisher=The University of Chicago Press|isbn=0-226-91038-5}}</ref> Others who considered such ideas included the Greek philosopher [[Empedocles]], the [[History of Western philosophy|Roman philosopher-poet]] [[Lucretius]], the [[Islamic science|Arab biologist]] [[Al-Jahiz]],<ref>{{cite journal |author=Zirkle C |title=Natural Selection before the "Origin of Species" |journal=Proceedings of the American Philosophical Society |volume=84 |issue=1 |pages=71–123 |year=1941}}</ref> the [[Early Islamic philosophy|Persian philosopher]] [[Ibn Miskawayh]], the [[Brethren of Purity]],<ref>[[Muhammad Hamidullah]] and Afzal Iqbal (1993), ''The Emergence of Islam: Lectures on the Development of Islamic World-view, Intellectual Tradition and Polity'', p. 143-144. Islamic Research Institute, Islamabad.</ref> and the Chinese philosopher [[Zhuangzi]].<ref> "A Source Book In Chinese Philosophy", Chan, Wing-Tsit, p. 204, 1962. </ref> As biological knowledge grew in the 18th century, evolutionary ideas were set out by a few natural philosophers including [[Pierre Louis Maupertuis|Pierre Maupertuis]] in 1745 and [[Erasmus Darwin]] in 1796.<ref>{{cite book|author=Terrall, M|year=2002|title=The Man Who Flattened the Earth: Maupertuis and the Sciences in the Enlightenment|publisher=The University of Chicago Press|isbn=978-0226793610}}</ref> The ideas of the biologist [[Jean-Baptiste Lamarck]] about [[transmutation of species]] had wide influence. [[Charles Darwin]] formulated his idea of [[natural selection]] in 1838 and was still developing his theory in 1858 when [[Alfred Russel Wallace]] sent him a similar theory, and both were presented to the [[Linnean Society of London]] in [[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection|separate papers]].<ref>{{cite journal|author=Wallace, A|coauthors= Darwin, C|url=http://darwin-online.org.uk/content/frameset?itemID=F350&viewtype=text&pageseq=1|title=On the Tendency of Species to form Varieties, and on the Perpetuation of Varieties and Species by Natural Means of Selection|journal=Journal of the Proceedings of the Linnean Society of London. Zoology|volume=3|year=1858|pages=53–62|accessdate=2007-05-13|doi=10.1098/rsnr.2006.0171}}</ref> At the end of 1859 Darwin's publication of ''[[On the Origin of Species]]'' explained natural selection in detail and presented evidence leading to increasingly wide acceptance of the occurrence of evolution. |
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Debate about the mechanisms of evolution continued, and Darwin could not explain the source of the heritable variations which would be acted on by natural selection. Like Lamarck, he thought that parents [[inheritance of acquired characters|passed on adaptations acquired]] during their lifetimes,<ref>{{cite web|url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F391&pageseq=136 |title=Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection |accessdate=2007-12-28 |author=Darwin, Charles |authorlink=Charles Darwin |year=1872 |work=[[On the Origin of Species|The Origin of Species]]. 6th edition, p. 108 |publisher=John Murray }}</ref> a theory which was subsequently dubbed [[Lamarckism]].<ref>{{cite book |author=Leakey, Richard E.; Darwin, Charles |title=The illustrated origin of species |publisher=Faber |location=London |year=1979 |pages= |isbn=0-571-14586-8 |oclc= |doi=}} p. 17-18 <!--superseded source {{cite journal |author=Stafleu F |title=Lamarck: The birth of biology |journal=Taxon |volume=20 |issue= |pages=397–442 |year=1971 |pmid=11636092 |doi=10.2307/1218244 }}--></ref> In the 1880s [[August Weismann|August Weismann's]] experiments indicated that changes from use and disuse were not heritable, and Lamarckism gradually fell from favour.<ref name= ImaginaryLamarck>{{citation |last =Ghiselin | first = Michael T.|authorlink=Michael Ghiselin | publication-date = September/October 1994| contribution =Nonsense in schoolbooks: 'The Imaginary Lamarck' | contribution-url =http://www.textbookleague.org/54marck.htm| title =The Textbook Letter | publisher =The Textbook League | url =http://www.textbookleague.org/|accessdate=2008-01-23 }}</ref><ref>{{cite book|author=Magner, LN|year=2002|title=A History of the Life Sciences, Third Edition, Revised and Expanded|publisher=CRC|isbn=978-0824708245}}</ref> More significantly, Darwin could not account for how traits were passed down from generation to generation. In 1865 [[Gregor Mendel]] found that traits were [[Mendelian inheritance|inherited]] in a predictable manner.<ref name=Weiling>{{cite journal |author=Weiling F |title=Historical study: Johann Gregor Mendel 1822–1884 |journal=Am. J. Med. Genet. |volume=40 |issue=1 |pages=1–25; discussion 26 |year=1991 |pmid=1887835 |doi=10.1002/ajmg.1320400103 }}</ref> When Mendel's work was rediscovered in 1900s, disagreements over the rate of evolution predicted by early geneticists and [[biostatistics|biometricians]] led to a rift between the Mendelian and Darwinian models of evolution. |
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Yet it was the rediscovery of Gregor Mendel’s pioneering work on the fundamentals of genetics (of which Darwin and Wallace were unaware) by [[Hugo de Vries]] and others in the early 1900s that provided the impetus for a better understanding of how variation occurs in plant and animal traits. That variation is the main fuel used by natural selection to shape the wide variety of adaptive traits observed in organic life. Even though [[Hugo de Vries]] and other early geneticists were very critical of the theory of evolution, their rediscovery of and subsequent work on genetics eventually provided a solid basis on which the theory of evolution stood even more convincingly than when it was originally proposed.<ref>Quammen, D. (2006). [http://www.nytimes.com/2006/08/27/books/review/Desmond.t.html?n=Top/Reference/Times%20Topics/People/D/Darwin,%20Charles%20Robert ''The reluctant Mr. Darwin: An intimate portrait of Charles Darwin and the making of his theory of evolution.''] New York, NY: W.W. Norton & Company.</ref> |
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The apparent contradiction between Darwin’s theory of evolution by natural selection and Mendel’s work was reconciled in the 1920s and 1930s by evolutionary biologists such as [[J.B.S. Haldane]], [[Sewall Wright]], and particularly [[Ronald Fisher]], who set the foundations for the establishment of the field of [[population genetics]]. The end result was a combination of evolution by natural selection and Mendelian inheritance, the [[modern evolutionary synthesis]].<ref>{{cite book | last = Bowler | first = Peter J. | authorlink = Peter J. Bowler | year = 1989 | title = The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society | publisher = Johns Hopkins University Press | location = Baltimore|isbn=978-0801838880}}</ref> In the 1940s, the identification of [[DNA]] as the genetic material by [[Oswald Avery]] and colleagues and the subsequent publication of the structure of DNA by [[James D. Watson|James Watson]] and [[Francis Crick]] in 1953, demonstrated the physical basis for inheritance. Since then, [[genetics]] and [[molecular biology]] have become core parts of [[evolutionary biology]] and have revolutionized the field of [[phylogenetics]].<ref name=Kutschera>{{cite journal |author=Kutschera U, Niklas K |title=The modern theory of biological evolution: an expanded synthesis |journal=Naturwissenschaften |volume=91 |issue=6 |pages=255–76 |year=2004 |pmid=15241603 |doi=10.1007/s00114-004-0515-y }}</ref> |
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In its early history, evolutionary biology primarily drew in scientists from traditional taxonomically oriented disciplines, whose specialist training in particular organisms addressed general questions in evolution. As evolutionary biology expanded as an academic discipline, particularly after the development of the modern evolutionary synthesis, it began to draw more widely from the biological sciences.<ref name=Kutschera/> Currently the study of evolutionary biology involves scientists from fields as diverse as [[biochemistry]], [[ecology]], [[genetics]] and [[physiology]], and evolutionary concepts are used in even more distant disciplines such as [[psychology]], [[medicine]], [[philosophy]] and [[computer science]]. In the 21st century, [[current research in evolutionary biology]] deals with several areas where the modern evolutionary synthesis may need modification or extension, such as assessing the relative importance of various ideas on the [[unit of selection]] and [[evolvability]] and how to fully incorporate the findings of [[evolutionary developmental biology]].<ref>{{cite journal |author=Pigliucci M |title=Do we need an extended evolutionary synthesis? |journal=Evolution |volume=61 |issue=12 |pages=2743–9 |year=2007 |month=December |pmid=17924956 |doi=10.1111/j.1558-5646.2007.00246.x}}</ref><ref>{{cite journal |author=Winther RG |title=Systemic darwinism |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=33 |pages=11833–8 |year=2008 |month=August |pmid=18697926 |pmc=2575274 |doi=10.1073/pnas.0711445105}}</ref> |
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==Social and cultural responses== |
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{{details more|Social effect of evolutionary theory}} |
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[[File:Darwin ape.jpg|right|150px|thumb|As "[[Darwinism]]" became widely accepted in the 1870s, [[caricature]]s of [[Charles Darwin]] with a [[quadrupedal]] [[ape]] or [[monkey]] body symbolised evolution.<ref name=Browne2003e>{{cite book|author=Browne, Janet |title=Charles Darwin: The Power of Place |publisher=Pimlico |location=London |year=2003 |pages=376–379 |isbn=0-7126-6837-3 }}</ref>]] |
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In the 19th century, particularly after the publication of ''[[On the Origin of Species]]'' in 1859, the idea that life had evolved was an active source of academic debate centered on the philosophical, social and religious implications of evolution. Nowadays, the fact that organisms evolve is uncontested in the [[scientific literature]] and the modern evolutionary synthesis is widely accepted by scientists.<ref name=Kutschera/> However, evolution remains a contentious concept for some religious groups.<ref>For an overview of the philosophical, religious, and cosmological controversies, see: {{cite book|authorlink=Daniel Dennett|last=Dennett|first=D|title=[[Darwin's Dangerous Idea|Darwin's Dangerous Idea: Evolution and the Meanings of Life]]|publisher=Simon & Schuster|year=1995|isbn=978-0684824710}}<br />*For the scientific and social reception of evolution in the 19th and early 20th centuries, see: {{cite web | last = Johnston | first = Ian C. | title = History of Science: Origins of Evolutionary Theory | work = And Still We Evolve | publisher = Liberal Studies Department, Malaspina University College | url =http://records.viu.ca/~johnstoi/darwin/sect3.htm| accessdate =2007-05-24}}<br />*{{cite book|authorlink=Peter J. Bowler|last=Bowler|first=PJ|title=Evolution: The History of an Idea, Third Edition, Completely Revised and Expanded|publisher=University of California Press|isbn=978-0520236936|year=2003}}<br />*{{cite journal |author=Zuckerkandl E |title=Intelligent design and biological complexity |journal=Gene |volume=385 |issue= |pages=2–18 |year=2006 |pmid=17011142 |doi=10.1016/j.gene.2006.03.025 }}</ref> |
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While [[Level of support for evolution#Support for evolution by religious bodies|various religions and denominations]] have reconciled their beliefs with evolution through concepts such as [[theistic evolution]], there are [[creationism|creationists]] who believe that evolution is contradicted by the [[creation myth]]s found in their respective religions and who raise various [[objections to evolution]].<ref name=ScottEC/><ref name=Ross2005>{{cite journal | author = Ross, M.R. | year = 2005 | title = Who Believes What? Clearing up Confusion over Intelligent Design and Young-Earth Creationism | journal = Journal of Geoscience Education | volume = 53 | issue = 3 | page = 319 | url = http://www.nagt.org/files/nagt/jge/abstracts/Ross_v53n3p319.pdf |format=PDF| accessdate = 2008-04-28}}</ref> As had been demonstrated by responses to the publication of ''[[Vestiges of the Natural History of Creation]]'' in 1844, the most controversial aspect of evolutionary biology is the implication of [[human evolution]] that human mental and moral faculties, which had been thought purely spiritual, are not distinctly separated from those of other animals.<ref name=bowler/> In some countries—notably the United States—these tensions between science and religion have fueled the current [[creation-evolution controversy]], a religious conflict focusing on [[politics of creationism|politics]] and [[creation and evolution in public education|public education]].<ref>{{cite journal | author = Spergel D. N. |title=Science communication. Public acceptance of evolution |journal=Science |volume=313 |issue=5788 |pages=765–66 |year=2006 |pmid=16902112 |doi=10.1126/science.1126746 }}</ref> While other scientific fields such as [[physical cosmology|cosmology]]<ref name="wmap">{{cite journal | doi=10.1086/377226 | title = First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters | first = D. N. | last = Spergel | coauthors = et al. | journal = The Astrophysical Journal Supplement Series | volume = 148 | year = 2003 | pages = 175–94}}</ref> and [[earth science]]<ref name="zircon">{{cite journal |author=Wilde SA, Valley JW, Peck WH, Graham CM |title=Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago |journal=Nature |volume=409 |issue=6817 |pages=175–78 |year=2001 |pmid=11196637 |doi=10.1038/35051550 }}</ref> also conflict with literal interpretations of many religious texts, evolutionary biology experiences significantly more opposition from religious literalists. |
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Another example associated with evolutionary theory that is now widely regarded as unwarranted is misnamed "[[Social Darwinism]]," a term given to the 19th century [[British Whig Party|Whig]] [[Malthusianism|Malthusian]] theory developed by [[Herbert Spencer]] into ideas about "[[survival of the fittest]]" in commerce and human societies as a whole, and by others into claims that [[social inequality]], [[sexism]], [[racism]], and [[imperialism]] were justified.<ref>On the history of [[eugenics]] and evolution, see {{cite book|authorlink=Daniel Kevles |first=D |last=Kevles |year=1998 |title=In the Name of Eugenics: Genetics and the Uses of Human Heredity |publisher=Harvard University Press|isbn=978-0674445574}}</ref> However, these ideas contradict [[Charles Darwin|Darwin]]'s own views, and contemporary scientists and philosophers consider these ideas to be neither mandated by evolutionary theory nor supported by data.<ref>[[Charles Darwin|Darwin]] strongly disagreed with attempts by Herbert Spencer and others to extrapolate evolutionary ideas to all possible subjects; see {{cite book|authorlink=Mary Midgley|first=M|last=Midgley|year=2004|title=The Myths we Live By|publisher=Routledge|page=62|isbn=978-0415340779}}</ref><ref>{{cite journal |author=Allhoff F |title=Evolutionary ethics from Darwin to Moore |journal=History and philosophy of the life sciences |volume=25 |issue=1 |pages=51–79 |year=2003 |pmid=15293515 |doi=10.1080/03919710312331272945}}</ref><ref>{{cite book |author=Gowaty, Patricia Adair |title=Feminism and evolutionary biology: boundaries, intersections, and frontiers |publisher=Chapman & Hall |location=London |year=1997 |isbn=0-412-07361-7}}</ref> |
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The teaching of evolution in American secondary school biology classes was uncommon in most of the first half of the 20th century. The [[Scopes Trial]] decision of 1925 caused the subject to become very rare in American secondary biology textbooks for a generation, but it was gradually re-introduced about a generation later and legally protected with the 1968 [[Epperson v. Arkansas]] decision. Since then, the competing religious belief of [[creationism]] was legally disallowed in secondary school curricula in various decisions in the 1970's and 1980's, but it returned in the form of [[intelligent design]], to be excluded once again in the 2005 [[Kitzmiller v. Dover Area School District]] case.<ref>[http://www.bioone.org/doi/full/10.1641/B570313 Understanding Creationism after Kitzmiller] 2007</ref> |
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==Applications== |
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{{details more|Artificial selection|Evolutionary computation}} |
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Evolutionary biology, and in particular the understanding of how organisms evolve through natural selection, is an area of science with many practical applications.<ref name=Bull>{{cite journal |author=Bull JJ, Wichman HA |title=Applied evolution |journal=Annu Rev Ecol Syst |volume=32 |pages=183–217 |year=2001 |doi=10.1146/annurev.ecolsys.32.081501.114020}}</ref> A major technological application of evolution is [[artificial selection]], which is the intentional selection of certain traits in a population of organisms. Humans have used artificial selection for thousands of years in the [[domestication]] of plants and animals.<ref>{{cite journal |author=Doebley JF, Gaut BS, Smith BD |title=The molecular genetics of crop domestication |journal=Cell |volume=127 |issue=7 |pages=1309–21 |year=2006 |pmid=17190597 |doi=10.1016/j.cell.2006.12.006 }}</ref> More recently, such selection has become a vital part of [[genetic engineering]], with [[selectable marker]]s such as antibiotic resistance genes being used to manipulate DNA in [[molecular biology]]. It is also possible to use repeated rounds of mutation and selection to evolve proteins with particular properties, such as modified [[enzyme]]s or new [[antibody|antibodies]], in a process called [[directed evolution]].<ref>{{cite journal |author=Jäckel C, Kast P, Hilvert D |title=Protein design by directed evolution |journal=Annu Rev Biophys |volume=37 |issue= |pages=153–73 |year=2008 |pmid=18573077 |doi=10.1146/annurev.biophys.37.032807.125832}}</ref> |
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Understanding the changes that have occurred during organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human [[genetic disorder]]s.<ref>{{cite journal |author=Maher B. |title=Evolution: Biology's next top model? |journal=Nature |volume=458 |issue=7239 |pages=695–8 |year=2009 |month=April |doi=10.1038/458695a}}</ref> For example, the [[Mexican tetra]] is an [[albino]] cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves.<ref>{{cite journal |author=Borowsky R |title=Restoring sight in blind cavefish |journal=Curr. Biol. |volume=18 |issue=1 |pages=R23–4 |year=2008 |month=January |pmid=18177707 |doi=10.1016/j.cub.2007.11.023}}</ref> This helped identify genes required for vision and pigmentation, such as [[crystallin]]s and the [[melanocortin 1 receptor]].<ref>{{cite journal |author=Gross JB, Borowsky R, Tabin CJ |title=A novel role for Mc1r in the parallel evolution of depigmentation in independent populations of the cavefish Astyanax mexicanus |journal=PLoS Genet. |volume=5 |issue=1 |pages=e1000326 |year=2009 |month=January |pmid=19119422 |pmc=2603666 |doi=10.1371/journal.pgen.1000326}}</ref> Similarly, comparing the genome of the [[Notothenioidei|Antarctic icefish]], which lacks [[red blood cell]]s, to close relatives such as the [[zebrafish]] revealed genes needed to make these blood cells.<ref>{{cite journal |author=Yergeau DA, Cornell CN, Parker SK, Zhou Y, Detrich HW |title=bloodthirsty, an RBCC/TRIM gene required for erythropoiesis in zebrafish |journal=Dev. Biol. |volume=283 |issue=1 |pages=97–112 |year=2005 |month=July |pmid=15890331 |doi=10.1016/j.ydbio.2005.04.006}}</ref> |
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As evolution can produce highly optimized processes and networks, it has many applications in [[computer science]]. Here, simulations of evolution using [[evolutionary algorithm]]s and [[artificial life]] started with the work of Nils Aall Barricelli in the 1960s, and was extended by [[Alex Fraser (scientist)|Alex Fraser]], who published a series of papers on simulation of [[artificial selection]].<ref>{{cite journal |author=Fraser AS |title=Monte Carlo analyses of genetic models |journal=Nature |volume=181 |issue=4603 |pages=208–9 |year=1958 |pmid=13504138 |doi=10.1038/181208a0 }}</ref> [[Evolutionary algorithm|Artificial evolution]] became a widely recognized optimization method as a result of the work of [[Ingo Rechenberg]] in the 1960s and early 1970s, who used [[Evolution strategy|evolution strategies]] to solve complex engineering problems.<ref>{{cite book |last=Rechenberg |first=Ingo |year=1973 |title=Evolutionsstrategie - Optimierung technischer Systeme nach Prinzipien der biologischen Evolution (PhD thesis) |publisher=Fromman-Holzboog | language = German}}</ref> [[Genetic algorithm]]s in particular became popular through the writing of [[John Henry Holland|John Holland]].<ref>{{cite book |last=Holland |first=John H. |year=1975 |title=Adaptation in Natural and Artificial Systems | publisher=University of Michigan Press | isbn = 0262581116}}</ref> As academic interest grew, dramatic increases in the power of computers allowed practical applications, including the automatic evolution of computer programs.<ref>{{cite book |last=Koza|first=John R. |year=1992 |title=Genetic Programming| subtitle=On the Programming of Computers by Means of Natural Selection | publisher=MIT Press |isbn=0262111705}}</ref> Evolutionary algorithms are now used to solve multi-dimensional problems more efficiently than software produced by human designers, and also to optimize the design of systems.<ref>{{cite journal |author=Jamshidi M |title=Tools for intelligent control: fuzzy controllers, neural networks and genetic algorithms |journal=Philosophical transactions. Series A, Mathematical, physical, and engineering sciences |volume=361 |issue=1809 |pages=1781–808 |year=2003 |pmid=12952685 |doi=10.1098/rsta.2003.1225}}</ref> |
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== See also == |
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* [[Portal:Evolutionary biology|Evolutionary biology portal]] |
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* [[Fossils of the Burgess Shale]] |
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==References== |
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{{reflist|2}} |
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==Further reading== |
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;Introductory reading |
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* {{cite book |author=Carroll, S. |authorlink=Sean B. Carroll |title=Endless Forms Most Beautiful |publisher=W.W. Norton |location=New York |year=2005 |isbn=0-393-06016-0}} |
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* {{cite book |author=[[Brian Charlesworth|Charlesworth, C.B.]] and [[Deborah Charlesworth|Charlesworth, D.]] |title=Evolution |publisher=Oxford University Press |location=Oxfordshire |year=2003 |isbn=0-192-80251-8}} |
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* {{cite book |author=Dawkins, R. |authorlink=Richard Dawkins |title=[[The Selfish Gene|The Selfish Gene: 30th Anniversary Edition]] |publisher=Oxford University Press |year=2006 |isbn=0199291152 }} |
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* {{cite book |author=Gould, S.J. |authorlink=Stephen Jay Gould |title=[[Wonderful Life (book)|Wonderful Life: The Burgess Shale and the Nature of History]] |publisher=W.W. Norton |location=New York |year=1989 |isbn=0-393-30700-X}} |
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* {{cite book |author=Jones, S. |authorlink = Steve Jones (biologist) |title=[[Almost Like a Whale|Almost Like a Whale: The Origin of Species Updated]]. (''American title:'' ''Darwin's Ghost'') |publisher=Ballantine Books |location=New York |year=2001 |isbn=0-345-42277-5}} |
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* {{cite book |author=Maynard Smith, J. |authorlink=John Maynard Smith |title=[[The Theory of Evolution|The Theory of Evolution: Canto Edition]] |publisher=[[Cambridge University Press]] |year=1993 |isbn=0-521-45128-0}} |
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* {{cite book |author=Pallen, M.J. |title=The Rough Guide to Evolution |publisher=[[Rough Guides]] |year=2009 |isbn=978-1-85828-946-5}} |
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* {{cite book |author=Smith, C.B. and Sullivan, C. |title=The Top 10 Myths about Evolution |publisher=[[Prometheus Books]] |year=2007 |isbn=978-1-59102-479-8}} |
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;History of evolutionary thought |
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* {{cite book |author=Larson, E.J. |authorlink=Edward Larson |title=Evolution: The Remarkable History of a Scientific Theory |publisher=Modern Library |location=New York |year=2004 |isbn=0-679-64288-9}} |
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* {{cite book |author=Zimmer, C. |authorlink=Carl Zimmer |title=Evolution: The Triumph of an Idea |publisher=HarperCollins |location=London |year=2001 |isbn=0-060-19906-7}} |
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;Advanced reading |
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* {{cite book |author=[[Nick Barton|Barton, N.H.]], [[Derek Briggs|Briggs, D.E.G.]], Eisen, J.A., Goldstein, D.B. and Patel, N.H. |title=Evolution |publisher=[[Cold Spring Harbor Laboratory Press]] |year=2007 |isbn=0-879-69684-2}} |
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* {{cite book |author=[[Jerry Coyne|Coyne, J.A.]] and [[H. Allen Orr|Orr, H.A.]] |title=Speciation |publisher=Sinauer Associates |location=Sunderland |year=2004 |isbn=0-878-93089-2}} |
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* {{cite book |author=Futuyma, D.J. |authorlink=Douglas J. Futuyma |title=Evolution |publisher=Sinauer Associates |location=Sunderland |year=2005 |isbn=0-878-93187-2}} |
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* {{cite book | author=Gould, S.J. |authorlink=Stephen Jay Gould |title=[[The Structure of Evolutionary Theory]] |publisher=Belknap Press (Harvard University Press) |location=Cambridge |year=2002 |isbn=0-674-00613-5}} |
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* {{cite book |author=[[John Maynard Smith|Maynard Smith, J.]] and [[Eörs Szathmáry|Szathmáry, E.]] |title=[[The Major Transitions in Evolution]] |publisher=Oxford University Press |location=Oxfordshire |year=1997 |isbn=0-198-50294-X}} |
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* {{cite book |author=Mayr, E. |authorlink=Ernst W. Mayr |title=What Evolution Is |publisher=Basic Books |location=New York |year=2001 |isbn=0-465-04426-3}} |
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==External links== |
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<!-- IMPORTANT! Please do not add any links before discussing them on the talk page. --> |
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{{Spoken Wikipedia|Evolution.ogg|2005-04-18}} <!-- updated changed sections 2005-04-18 --> |
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{{Sisterlinks|evolution}} |
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;General information |
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* [http://www.newscientist.com/topic/evolution Everything you wanted to know about evolution by ''New Scientist''] |
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* [http://science.howstuffworks.com/evolution/evolution.htm Howstuffworks.com — How Evolution Works] |
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* [http://nationalacademies.org/evolution/ National Academies Evolution Resources] |
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* [http://anthro.palomar.edu/synthetic/ Synthetic Theory Of Evolution: An Introduction to Modern Evolutionary Concepts and Theories] |
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* [http://evolution.berkeley.edu/ Understanding Evolution from University of California, Berkeley] |
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;History of evolutionary thought |
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* [http://darwin-online.org.uk/ The Complete Work of Charles Darwin Online] |
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* [http://www.rationalrevolution.net/articles/understanding_evolution.htm Understanding Evolution: History, Theory, Evidence, and Implications] |
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;On-line lectures |
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*[http://ascb.org/ibioseminars/brenner/brenner1.cfm What Genomes Can Tell Us About the Past] - lecture by [[Sydney Brenner]] |
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*[http://ascb.org/ibioseminars/kirschner/kirschner1.cfm The Origin of Vertebrates] - lecture by [[Marc Kirschner]] |
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{{clear}} |
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==Related topics==<!--navbox heading--> |
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