Difference between revisions of "Evolution"

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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 [[bacterium|bacteria]].<ref>{{cite journal |author=Boucher Y, Douady CJ, Papke RT, Walsh DA, Boudreau ME, Nesbo CL, Case RJ, Doolittle WF [http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.genet.37.050503.084247] 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. 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. [[Virus]]es can also carry DNA between organisms, allowing transfer of genes even across [[domain (biology)|biological domains]]. Gene transfer has also occurred within [[eukaryote|eukaryotic cells]], from the [[chloroplast]] and [[mitochondria]]l genomes to [[cell nucleus|nuclear genomes]].
 
[[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 [[bacterium|bacteria]].<ref>{{cite journal |author=Boucher Y, Douady CJ, Papke RT, Walsh DA, Boudreau ME, Nesbo CL, Case RJ, Doolittle WF [http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.genet.37.050503.084247] 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. 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. [[Virus]]es can also carry DNA between organisms, allowing transfer of genes even across [[domain (biology)|biological domains]]. Gene transfer has also occurred within [[eukaryote|eukaryotic cells]], from the [[chloroplast]] and [[mitochondria]]l genomes to [[cell nucleus|nuclear genomes]].
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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 are unable to breed with one another.
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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 [[speciation]], and [[microevolution]], which is smaller evolutionary changes, such as adaptations, within a species or population. In general, macroevolution is 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}}</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}}</ref> However, in macroevolution, the traits of the entire species are 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 can sometimes be separate.<ref>{{wikiref |id=Gould-2002 |text=Gould 2002, pp. 657–658}}</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>[http://www.sciam.com/askexpert_question.cfm?articleID=00071863-683B-1C72-9EB7809EC588F2D7 Scientific American; Biology: Is the human race evolving or devolving?], see also [[biological devolution]].</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=1102-09 |year=2001 |pmid=11234024}}</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 |url=http://www.pnas.org/cgi/content/full/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 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=15590780#r6 |journal=Microbiol Mol Biol Rev |volume=68 |issue=4 |pages=686–91 |year=2004 |pmid=15590780}}</ref> Simple organisms therefore remain the dominant form of life on Earth, and complex life appears more diverse only 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}}</ref>
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===Adaptation===
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{{details|Adaptation}}
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Adaptations are structures or behaviors that enhance a specific function, causing organisms to become better at surviving and reproducing.<ref name=Darwin/> They are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment.<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}}</ref> This process can 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 mutations causing [[antibiotic resistance]] by either modifying the target of the drug, or removing the transporters that allow the drug into 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=391–95 |year=2002 |pmid=12153116}}</ref> 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. 1235–1236}} </ref> One example is the African lizard ''Holapsis 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/>
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[[Image: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 [[vestigial structure|vestigial]] leg bones.<ref>{{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}}</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 [[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/cgi/content/full/103/2/379 |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}}</ref>
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During adaption, 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 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 |url=http://jhered.oxfordjournals.org/cgi/content/full/96/3/185 |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}}</ref> and the presence of hip bones in whales and snakes.<ref>{{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}}</ref> 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=450–55 |year=2003 |pmid=12733778}}</ref> the [[coccyx]],<ref name=Fong/> and the [[vermiform appendix]].<ref name=Fong/>
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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}}</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=Gilbert SF |title=The morphogenesis of evolutionary developmental biology |journal=Int. J. Dev. Biol. |volume=47 |issue=7–8 |pages=467–77 |year=2003 |pmid=14756322}}</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}}</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=371–77 |year=2006 |pmid=16488870}}</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 [[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}}</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}}<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}}<br />*{{cite journal |author=Nowak M |title=Five rules for the evolution of cooperation |journal=Science |volume=314 |issue=5805 |pages=1560–63 |year=2006 |pmid=17158317}}</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}}</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}}</ref>
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Coalitions between organisms of the same species have also evolved. An extreme case is the [[Eusociality]] found in [[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 are limited in their capacity to reproduce in order to 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 to either [[growth factor|grow]] or [[Apoptosis|kill themselves]]. If cells ignore these signals and attempt to multiply inappropriately, their uncontrolled growth 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 |url=http://www.pnas.org/cgi/content/full/104/23/9736 |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=1390–96 |year=1981 |pmid=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 |url=http://www.pnas.org/cgi/content/full/102/38/13367 |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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[[Image:Speciation modes edit.svg|right|thumb|300px|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}}</ref> It has been observed multiple times under both controlled laboratory conditions and in nature.<ref>{{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}}<br />*{{cite web|author=Boxhorn, J|date=1995|url=http://www.talkorigins.org/faqs/faq-speciation.html|title=Observed Instances of Speciation|publisher=The TalkOrigins Archive|accessdate=2007-05-10}}<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. As selection and drift act independently in isolated populations, separation will eventually produce organisms that cannot interbreed.<ref>{{cite journal|author=Hoskin CJ, Higgle M, McDonald KR, Moritz C |date=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}}</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=33–37 |year=2006 |pmid=16639420 |url=http://www.nature.com/hdy/journal/v97/n1/full/6800835a.html}}</ref> Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produces a change in flowering time of the metal-resistant plants, causing 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}}</ref>
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[[Image:Darwin's finches cropped.jpeg|frame|left|[[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 |date=2006 |title=Sympatric speciation in palms on an oceanic island |journal=Nature |volume=441 |pages=210–13 | pmid=16467788}}<br />*{{cite journal| author=Barluenga M, Stölting KN, Salzburger W, Muschick M, Meyer A |date=2006 |title=Sympatric speciation in Nicaraguan crater lake cichlid fish |journal=Nature |volume=439 |pages=719–723 |pmid=16467837}}</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}}</ref>
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One type of sympatric speciation involves cross-breeding of two related species to produce a new [[hybrid]] species. This is not common in animals as animal hybrids are usually sterile, because during [[meiosis]] the [[homologous chromosome]]s from each parent, being from different species cannot successfully pair. It is more common in plants, however 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, as each parent's chromosomes is represented by a pair already.<ref>Belderok, Bob & Hans Mesdag & Dingena A. Donner. (2000) ''Bread-Making Quality of Wheat''. Springer. p.3. ISBN 0-7923-6383-3.<br />*Hancock, James F. (2004) ''Planti Evolution and the Origin of Crop Species''. CABI Publishing. ISBN 0-85199-685-X.</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}}</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}}</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=500-504 |year=2007 |pmid=18006296}}</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 |url=http://www.pnas.org/cgi/reprint/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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[[Image:Tarbosaurus museum Muenster.jpg|thumb|right|225px|A ''[[Tarbosaurus]]'' skeleton. 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=52–58 |year=1995 |pmid=7701342}}</ref> Indeed, virtually 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}}</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 |url=http://www.pnas.org/cgi/reprint/91/15/6758.pdf |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=91 |issue=15 |pages=6758–63 |year=1994 |pmid=8041694}}</ref> The [[Cretaceous–Tertiary extinction event]], during which the 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 |url=http://www.pnas.org/cgi/content/full/98/10/5466 |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 |url=http://www.pnas.org/cgi/content/full/103/29/10941 |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=70–05 |year=2004 |pmid=15459379}}</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 |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=361 |issue=1465 |pages=163–71 |year=2006 |pmid=16553315}}</ref>
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The role of extinction in evolution depends on which type is considered. The causes of the continuous "low-level" extinction events, which form the majority of extinctions, are not well understood and may be the result of competition between species for shared resources.<ref name=Kutschera/> If competition from other species does alter the probability that a species will become extinct, this could produce [[Unit of selection#Species selection and selection at higher taxonomic levels|species selection]] as a level of natural selection.<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 name=Raup/>
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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|Origin of life|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=2007-05-13}}</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 |journal=Int. Microbiol. |volume=8 |issue=1 |pages=23&ndash;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&ndash;13 |year=2006 |pmid=16292523}}</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&ndash;39 |year=2004 |pmid=15563395}}{{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&ndash;32 |year=1992 |pmid=1337989}}</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&ndash;21 |year=2002 |pmid=12110897}}</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&ndash;82 |year=2001 |pmid=11742692}}</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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[[Image:Ape skeletons.png|right|320px|thumbnail|The [[ape|hominoids]] are descendants of a [[common descent|common ancestor]].]]
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All [[organisms]] 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&ndash;77 |year=1999 |pmid=10607605}}</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&ndash;61 |year=2005 |pmid=15936656}}</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.<ref name=Darwin/>
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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&ndash;16 |year=1999 |pmid=10381868}}</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 [[nucleic acid]]s and [[amino acid]]s.<ref>{{cite journal |author=Mason SF |title=Origins of biomolecular handedness |journal=Nature |volume=311 |issue=5981 |pages=19&ndash;23 |year=1984 |pmid=6472461}}</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&ndash;79 |year=2002 |pmid=12175808}}</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&ndash;58 |year=2005 |pmid=16339373}}</ref>
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===Evolution of life===
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{{details|Timeline of evolution}}
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[[Image:Collapsed tree labels simplified.png|thumb|400px|right|[[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&ndash;87 |year=2006 |pmid=16513982}}</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 clear that [[prokaryote]]s were the first organisms to inhabit Earth,<ref name=Cavalier-Smith>{{cite journal |author=Cavalier-Smith T |title=Cell evolution and Earth history: stasis and revolution |url=http://www.journals.royalsoc.ac.uk/content/0164755512w92302/fulltext.pdf |journal=Philos Trans R Soc Lond B Biol Sci |volume=361 |issue=1470 |pages=969&ndash;1006 |year=2006 |pmid=16754610}}</ref> approximately 3&ndash;4 billion years ago.<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&ndash;85 |year=2006 |pmid=16754604}}<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&ndash;17 |year=2003 |pmid=14596897}}</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. |url=http://www.pnas.org/cgi/reprint/91/15/6735 |journal=Proc Natl Acad Sci U S A |volume=91 |issue=15 |pages=6735&ndash;42 |year=1994 |pmid=8041691}}</ref>
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The [[eukaryote]]s were the next major innovation in evolution. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in a cooperative association called [[endosymbiont|endosymbiosis]].<ref>{{cite journal |author=Poole A, Penny D |title=Evaluating hypotheses for the origin of eukaryotes |journal=Bioessays |volume=29 |issue=1 |pages=74&ndash;84 |year=2007 |pmid=17187354}}</ref><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&ndash;57 |year=2004 |pmid=15073369}}</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&ndash;59 |year=2005 |pmid=16109488}}</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&ndash;97 |year=1999 |pmid=10690412}}<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&ndash;19 |year=1999 |pmid=10607659}}</ref>
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The history of life was that of the unicellular eukaryotes, prokaryotes, and archaea until about a billion years ago when multicellular organisms began to appear in the oceans in the [[Ediacaran 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&ndash;8 | year = 2001|id = PMID 12116647}}</ref> The [[evolution of multicellularity]] occurred in multiple independent events, in organisms as diverse as [[sponge]]s, [[brown algae]], [[cyanobacteria]], [[slime mould]]s and [[myxobacteria]].<ref>{{cite journal |author=Kaiser D |title=Building a multicellular organism |journal=Annu. Rev. Genet. |volume=35 |issue= |pages=103&ndash;23 |year=2001 |pmid=11700279}}</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 (biology)|types]] of modern animals evolved, 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&ndash;9 |year=1999 |pmid=9927587}}</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&ndash;7 |year=1997 |pmid=9071008}}<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&ndash;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&ndash;63 |year=2003 |pmid=14615186}}</ref> [[Amphibian]]s first appeared around 300 million years ago, followed by early [[amniotes]], 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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[[Image:Charles Darwin aged 51 crop.jpg |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|date=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> Evolutionary thought was further developed by other early thinkers, including the Greek philosopher [[Empedocles]], the [[History of Western philosophy|Roman philosopher]] [[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&ndash;123 |year=1941}}</ref> the [[Early Islamic philosophy|Persian philosopher]] [[Ibn Miskawayh]], and 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> Also in the Far East, the philosopher [[Zhuang Zi|Zhuangzi]] discussed a transformative power of species to adapt to their surroundings.<ref> "A Source Book In Chinese Philosophy", Chan, Wing-Tsit, p. 204, 1962. </ref>  As biological knowledge grew in the 18th century, a variety of such ideas developed, beginning with [[Pierre Louis Maupertuis|Pierre Maupertuis]] in 1745, and with contributions from natural philosophers such as [[Erasmus Darwin]] and [[Jean-Baptiste Lamarck]].<ref>{{cite book|author=Terrall, M|date=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> In 1858, [[Charles Darwin]] and [[Alfred Russel Wallace]] jointly proposed the theory of evolution by natural selection 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|date=1858|pages=53–62|accessdate=2007-05-13}}</ref> Shortly after, Darwin's publication of ''[[On the Origin of Species]]'' provided detailed support for the theory and led to increasingly wide acceptance of the occurrence of evolution.
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[[Image:Mendel.png|thumb|left|[[Gregor Mendel]], who laid the foundation for [[genetics]].]]
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Nonetheless, Darwin's specific ideas about evolution, such as [[gradualism]] and the mechanisms of natural selection, were strongly contested at first. [[Lamarckism|Lamarckists]] argued that [[transmutation of species]] occurred as parents [[inheritance of acquired characters|passed on adaptations acquired]] during their lifetimes.<ref>{{cite journal |author=Stafleu F |title=Lamarck: The birth of biology |journal=Taxon |volume=20 |issue= |pages=397&ndash;442 |year=1971 |pmid=11636092}}</ref> Eventually, when experiments failed to support it, this idea was abandoned in favor of Darwinism.<ref>{{cite book|author=Magner, LN|date=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. A mechanism was provided in 1865 by [[Gregor Mendel]], who 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&ndash;25; discussion 26 |year=1991 |pmid=1887835}}</ref> When Mendel's work was rediscovered in 1900, 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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This contradiction was reconciled in the 1930s by biologists such as [[Ronald Fisher]]. 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&ndash;76 |year=2004 |pmid=15241603}}</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]].
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==Social and cultural views==
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{{details more|Social effect of evolutionary theory}}
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[[Image:Darwin ape.jpg|right|150px|thumb|Caricature of [[Charles Darwin]] as a quadrupedal [[ape]], reflecting the cultural backlash against evolution.]]
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Even before the publication of ''[[On the Origin of Species]]'', the idea that life had evolved was a source of debate and evolution is still a contentious concept. Debate has generally centered on the philosophical, social and religious implications of evolution, not on the science itself; the proposition that biological evolution occurs through the mechanism of natural selection is standard in the [[scientific literature]].<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|date=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://www.mala.bc.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|date=2003}}<br />*{{cite journal |author=Zuckerkandl E |title=Intelligent design and biological complexity |journal=Gene |volume=385 |issue= |pages=2&ndash;18 |year=2006 |pmid=17011142}}</ref>
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Although [[Level of support for evolution#Support for evolution by religious bodies|many religions and denominations]] have reconciled their beliefs with evolution through various concepts of [[theistic evolution]], there are many [[creationism|creationists]] who believe that evolution is contradicted by the [[origin beliefs|creation stories]] found in their respective religions.<ref>{{cite web|url=http://www.answersingenesis.org/home/area/re1/chapter1.asp|title=Evolution & creation, science & religion, facts & bias|last=Sarfati|first=J|publisher=[http://www.answersingenesis.org/ Answers in Genesis]|accessdate=2007-04-16}}</ref> As Darwin recognized early on, the most controversial aspect of evolutionary thought is its [[human evolution|implications for human origins]]. In some countries &ndash; notably the United States &ndash; these tensions between scientific and religious teachings have fueled the ongoing [[creation-evolution controversy|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=Miller JD, Scott EC, Okamoto S |title=Science communication. Public acceptance of evolution |journal=Science |volume=313 |issue=5788 |pages=765&mdash;66 |year=2006 |pmid=16902112}}</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&ndash;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&ndash;78 |year=2001 |pmid=11196637}}</ref> also conflict with literal interpretations of many religious texts, evolutionary biology is strongly opposed by many religious believers.
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Evolution has been used to support philosophical positions that promote [[discrimination]] and [[racism]]. For example, the [[eugenics|eugenic]] ideas of [[Francis Galton]] were developed to argue that the human gene pool should be improved by [[selective breeding]] policies, including incentives for those considered "good stock" to reproduce, and the [[compulsory sterilization]], [[prenatal testing]], [[birth control]], and even [[Action T4|killing]], of those considered ''bad stock''.<ref>{{cite journal |author=Kevles DJ |title=Eugenics and human rights |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=10445929 |journal=[[British Medical Journal|BMJ]] |volume=319 |issue=7207 |pages=435&ndash;8 |year=1999 |pmid=10445929}}</ref> Another example of an extension of evolutionary theory that is now widely regarded as unwarranted is "[[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]], racism, and [[imperialism]] were justified.<ref>On the history of eugenics and evolution, see {{cite book|authorlink=Daniel Kevles |first=D |last=Kevles |date=1998 |title=In the Name of Eugenics: Genetics and the Uses of Human Heredity |publisher=Harvard University Press|isbn=978-0674445574}}</ref> However, contemporary scientists and philosophers consider these ideas to have been 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|date=2004|title=The Myths we Live By|publisher=Routledge|pages=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&ndash;79 |year=2003 |pmid=15293515}}</ref>
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==Applications in technology==
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{{details more|Artificial selection|Evolutionary computation}}
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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}}</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]].
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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}}</ref> [[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 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 algorithms]] 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. 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}}</ref>
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==Further reading==
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'''Introductory reading'''
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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=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=[[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=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=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=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=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=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=Mayr, E. |authorlink=Ernst Mayr |title=What Evolution Is |publisher=Basic Books |location=New York |year=2001 |isbn=0-465-04426-3}}
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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=[[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=Barton, N.H., 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}}
  
 
==Outcomes==
 
==Outcomes==

Revision as of 05:53, 31 December 2007

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"Darwin's Tree of Life"

In biology, evolution is the change in the inherited traits of a population from generation to generation. These traits are the expression of genes that are copied and passed on to offspring during reproduction. Mutations in these genes can produce new or altered traits, resulting in heritable differences (genetic variation) between organisms. New traits can also come from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. Evolution occurs when these heritable differences become more common or rare in a population, either non-randomly through natural selection or randomly through genetic drift.

Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits to become rarer. This occurs because organisms with advantageous traits pass on more copies of these traits to the next generation. The measurement of selection on correlated characters (Evolution, volume 37 Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment.) [1] , Mechanisms: the processes of evolution Understanding Evolution, In contrast, genetic drift produces random changes in the frequency of traits in a population. Genetic drift arises from the element of chance involved in which individuals survive and reproduce.

One definition of a species is a group of organisms that can reproduce with one another and produce fertile offspring. However, when a species is separated into populations that are prevented from interbreeding, mutations, genetic drift, and the selection of novel traits cause the accumulation of differences over generations and the emergence of new species. Stephen Gould, The Structure of Evolutionary Theory, Belknap Press, ISBN 0-674-00613-5 . The similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence. {Douglas J. Futuyma, Evolution, Sinauer Associates, Sunderland, Massachusetts, ISBN 0-87893-187-2 )

The theory of evolution by natural selection was first proposed by Charles Darwin and Alfred Russel Wallace and set out in detail in Darwin's 1859 book On the Origin of Species[2]. Related earlier ideas were acknowledged in [3] In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, "understanding evolution" in which the connection between the units of evolution (genes) and the mechanism of evolution (natural selection) was made. This powerful explanatory and predictive theory has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth. [4] Statement on the Teaching of Evolution, The Interacademy Panel on International Issues, [5] Statement on the Teaching of Evolution, American Association for the Advancement of Science.

Heredity

Inheritance in organisms occurs through discrete 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. (Sturm RA, Frudakis TN, Eye colour: portals into pigmentation genes and ancestry). Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype. Genetics: what is a gene? (Nature, v. 441, 2006)

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.Template:Cite journal (Epigenetics and phenotypic variation in mammals, Mamm. Genome

External links

General information

History of evolutionary thought

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