Evolution Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! === Genetic drift === {{further|Genetic drift|Effective population size}} [[File:Allele-frequency.png|thumb|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.{{imagefact|date=December 2022}}]] Genetic drift is the random fluctuation of [[allele frequency|allele frequencies]] within a population from one generation to the next.<ref name="Futuyma2017b">{{harvnb|Futuyma|Kirkpatrick|2017|pp=55β66|loc=Chapter 3: Natural Selection and Adaptation}}</ref> When selective forces are absent or relatively weak, allele frequencies are equally likely to ''drift'' upward or downward{{clarify|date=November 2022}} in each successive generation because the alleles are subject to [[sampling error]].<ref name="Masel 2011">{{cite journal |last=Masel |first=Joanna |s2cid=17619958 |date=25 October 2011 |title=Genetic drift |journal=Current Biology |volume=21 |issue=20 |pages=R837βR838 |doi=10.1016/j.cub.2011.08.007 |issn=0960-9822 |pmid=22032182|doi-access=free |bibcode=2011CBio...21.R837M }}</ref> This drift halts when an allele eventually becomes fixed, either by disappearing from the population or by 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 begin with the same genetic structure to drift apart into two divergent populations with different sets of alleles.<ref>{{cite journal |last=Lande |first=Russell |year=1989 |title=Fisherian and Wrightian theories of speciation |url=https://archive.org/details/sim_genome_1989_31_1/page/221 |journal=[[Genome (journal)|Genome]] |volume=31 |issue=1 |pages=221β227 |doi=10.1139/g89-037 |issn=0831-2796 |pmid=2687093}}</ref> According to the [[neutral theory of molecular evolution]] most evolutionary changes are the result of the fixation of [[neutral mutation]]s by genetic drift.<ref name="Kimura M 1991 367β86">{{cite journal |last=Kimura |first=Motoo |author-link=Motoo Kimura |year=1991 |title=The neutral theory of molecular evolution: a review of recent evidence |journal=[[Journal of Human Genetics|Japanese Journal of Human Genetics]] |volume=66 |issue=4 |pages=367β386 |doi=10.1266/jjg.66.367 |pmid=1954033 |url=https://www.jstage.jst.go.jp/article/jjg/66/4/66_4_367/_pdf |doi-access=free |archive-date=5 June 2022 |archive-url=https://web.archive.org/web/20220605101314/https://www.jstage.jst.go.jp/article/jjg/66/4/66_4_367/_pdf |url-status=live }}</ref> In this model, most genetic changes in a population are thus the result of constant mutation pressure and genetic drift.<ref>{{cite journal |last=Kimura |first=Motoo |year=1989 |title=The neutral theory of molecular evolution and the world view of the neutralists |journal=Genome |volume=31 |issue=1 |pages=24β31 |doi=10.1139/g89-009 |issn=0831-2796 |pmid=2687096}}</ref> This form of the neutral theory has been debated since it does not seem to fit some genetic variation seen in nature.<ref>{{cite journal |last=Kreitman |first=Martin |author-link=Martin Kreitman |date=August 1996 |title=The neutral theory is dead. Long live the neutral theory |url=https://archive.org/details/sim_bioessays_1996-08_18_8/page/678 |journal=BioEssays |volume=18 |issue=8 |pages=678β683; discussion 683 |doi=10.1002/bies.950180812 |issn=0265-9247 |pmid=8760341}}</ref><ref>{{cite journal |last=Leigh | first=E. G. Jr. |date=November 2007 |title=Neutral theory: a historical perspective |journal=[[Journal of Evolutionary Biology]] |volume=20 |issue=6 |pages=2075β2091 |doi=10.1111/j.1420-9101.2007.01410.x |issn=1010-061X |pmid=17956380 |s2cid=2081042 |doi-access=free }}</ref> A better-supported version of this model is the [[nearly neutral theory of molecular evolution|nearly neutral theory]], according to which a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population.<ref name="Hurst" /> Other theories propose that genetic drift is dwarfed by other [[stochastic]] forces in evolution, such as genetic hitchhiking, also known as genetic draft.<ref name="Masel 2011"/><ref name="gillespie 2001">{{cite journal |last=Gillespie |first=John H. |author-link=John H. Gillespie |date=November 2001 |title=Is the population size of a species relevant to its evolution? |journal=Evolution |volume=55 |issue=11 |pages=2161β2169 |doi=10.1111/j.0014-3820.2001.tb00732.x |issn=0014-3820 |pmid=11794777|s2cid=221735887 |doi-access=free }}</ref><ref>{{Cite journal |last1=Neher |first1=Richard A. |last2=Shraiman |first2=Boris I. |date=August 2011 |title=Genetic Draft and Quasi-Neutrality in Large Facultatively Sexual Populations |journal=Genetics |volume=188 |issue=4 |pages=975β996 |doi=10.1534/genetics.111.128876 |pmc=3176096 |pmid=21625002 |arxiv=1108.1635 |bibcode=2011arXiv1108.1635N }}</ref> Another concept is [[constructive neutral evolution]] (CNE), which explains that complex systems can emerge and spread into a population through neutral transitions due to the principles of excess capacity, presuppression, and ratcheting,<ref>{{cite journal |last=Stoltzfus |first=Arlin |date=1999 |title=On the Possibility of Constructive Neutral Evolution |url=http://link.springer.com/10.1007/PL00006540|journal=Journal of Molecular Evolution |volume=49 |issue=2 |pages=169β181 |doi=10.1007/PL00006540 |pmid=10441669 |bibcode=1999JMolE..49..169S |s2cid=1743092 |access-date=30 July 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730090616/https://link.springer.com/article/10.1007/PL00006540|url-status=live}}</ref><ref>{{Cite journal |last=Stoltzfus |first=Arlin |date=13 October 2012 |title=Constructive neutral evolution: exploring evolutionary theory's curious disconnect |journal=Biology Direct |volume=7 |issue=1 |page=35 |doi=10.1186/1745-6150-7-35 |pmc=3534586 |pmid=23062217 |doi-access=free }}</ref><ref>{{Cite journal |last1=MuΓ±oz-GΓ³mez |first1=Sergio A. |last2=Bilolikar |first2=Gaurav |last3=Wideman |first3=Jeremy G. |last4=Geiler-Samerotte |first4=Kerry |display-authors=3 |date=1 April 2021 |title=Constructive Neutral Evolution 20 Years Later |journal=Journal of Molecular Evolution |volume=89 |issue=3 |pages=172β182 |doi=10.1007/s00239-021-09996-y |pmc=7982386 |pmid=33604782 |bibcode=2021JMolE..89..172M }}</ref> and it has been applied in areas ranging from the origins of the [[spliceosome]] to the complex interdependence of [[Microbial consortium|microbial communities]].<ref>{{Cite journal |last1=LukeΕ‘ |first1=Julius |last2=Archibald |first2=John M.|last3=Keeling|first3=Patrick J.|last4=Doolittle |first4=W. Ford |last5=Gray |first5=Michael W. |display-authors=3 |date=2011|title=How a neutral evolutionary ratchet can build cellular complexity |url=https://onlinelibrary.wiley.com/doi/10.1002/iub.489 |journal=IUBMB Life |volume=63 |issue=7 |pages=528β537 |doi=10.1002/iub.489 |pmid=21698757 |s2cid=7306575}}</ref><ref>{{cite journal |last1=Vosseberg |first1=Julian |last2=Snel |first2=Berend |date=1 December 2017 |title=Domestication of self-splicing introns during eukaryogenesis: the rise of the complex spliceosomal machinery |journal=Biology Direct |volume=12 |issue=1 |page=30 |doi=10.1186/s13062-017-0201-6 |pmc=5709842 |pmid=29191215 |doi-access=free }}</ref><ref>{{Cite journal |last1=Brunet |first1=T. D. P. |last2=Doolittle |first2=W. Ford |date=19 March 2018 |title=The generality of Constructive Neutral Evolution |journal=Biology & Philosophy |volume=33 |issue=1 |page=2|doi=10.1007/s10539-018-9614-6 |s2cid=90290787 }}</ref> The time it takes a neutral allele to become fixed by genetic drift depends on population size; fixation is more rapid in smaller populations.<ref>{{cite journal |last1=Otto |first1=Sarah P. |last2=Whitlock |first2=Michael C. |date=June 1997 |title=The Probability of Fixation in Populations of Changing Size |url=http://www.genetics.org/content/146/2/723.full.pdf |journal=Genetics |volume=146 |issue=2 |pages=723β733 |doi=10.1093/genetics/146.2.723 |pmc=1208011 |pmid=9178020 |access-date=18 December 2014 |url-status=live |archive-url=https://web.archive.org/web/20150319042554/http://www.genetics.org/content/146/2/723.full.pdf |archive-date=19 March 2015}}</ref> The number of individuals in a population is not critical, but instead a measure known as the effective population size.<ref name="Charlesworth">{{cite journal |last=Charlesworth |first=Brian |author-link=Brian Charlesworth |date=March 2009 |title=Fundamental concepts in genetics: effective population size and patterns of molecular evolution and variation |journal=Nature Reviews Genetics |volume=10 |issue=3 |pages=195β205 |doi=10.1038/nrg2526 |pmid=19204717|s2cid=205484393 }}</ref> The effective population is usually smaller than the total population since it takes into account factors such as the level of inbreeding and the stage of the lifecycle in which the population is the smallest.<ref name="Charlesworth" /> The effective population size may not be the same for every gene in the same population.<ref>{{cite journal |last1=Cutter |first1=Asher D. |last2=Choi |first2=Jae Young |date=August 2010 |title=Natural selection shapes nucleotide polymorphism across the genome of the nematode ''Caenorhabditis briggsae'' |journal=Genome Research |volume=20 |issue=8 |pages=1103β1111 |doi=10.1101/gr.104331.109 |pmc=2909573 |pmid=20508143}}</ref> It is usually difficult to measure the relative importance of selection and neutral processes, including drift.<ref>{{cite journal |last1=Mitchell-Olds |first1=Thomas |last2=Willis |first2=John H. |last3=Goldstein |first3=David B. |author-link3=David B. Goldstein (geneticist) |date=November 2007 |title=Which evolutionary processes influence natural genetic variation for phenotypic traits? |journal=Nature Reviews Genetics |volume=8 |issue=11 |pages=845β856 |doi=10.1038/nrg2207 |issn=1471-0056 |pmid=17943192|s2cid=14914998 }}</ref> The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of [[Evolutionary biology|current research]].<ref>{{cite journal |last=Nei |first=Masatoshi |author-link=Masatoshi Nei |date=December 2005 |title=Selectionism and Neutralism in Molecular Evolution |journal=[[Molecular Biology and Evolution]] |volume=22 |issue=12 |pages=2318β2342 |doi=10.1093/molbev/msi242 |issn=0737-4038 |pmc=1513187 |pmid=16120807}} * {{cite journal |last=Nei |first=Masatoshi |date=May 2006 |title=Selectionism and Neutralism in Molecular Evolution |journal=Molecular Biology and Evolution |type=Erratum |volume=23 |issue=5 |pages=2318β42 |doi=10.1093/molbev/msk009 |pmid=16120807 |pmc=1513187 |issn=0737-4038 |ref=none}}</ref> Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. 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