Editing Genetic drift (section)

==Versus natural selection==
In natural populations, genetic drift and natural selection do not act in isolation; both phenomena are always at play, together with mutation and migration. Neutral evolution is the product of both mutation and drift, not of drift alone. Similarly, even when selection overwhelms genetic drift, it can act only on variation that mutation provides.

While natural selection has a direction, guiding evolution towards heritable [[adaptation]]s to the current environment, genetic drift has no direction and is guided only by the [[probability|mathematics of chance]].<ref>{{cite web |url=http://www.actionbioscience.org/evolution/futuyma.html |title=Natural Selection: How Evolution Works |work=[[Actionbioscience]] |publisher=[[American Institute of Biological Sciences]] |location=Washington, D.C. |access-date=2009-11-24 |url-status=live |archive-url=https://web.archive.org/web/20100106013805/http://www.actionbioscience.org/evolution/futuyma.html |archive-date=6 January 2010}} An interview with [[Douglas J. Futuyma]]. See answer to question: Is natural selection the only mechanism of evolution?</ref> As a result, drift acts upon the [[genotype frequency|genotypic frequencies]] within a population without regard to their phenotypic effects. In contrast, selection favors the spread of alleles whose phenotypic effects increase survival and/or reproduction of their carriers, lowers the frequencies of alleles that cause unfavorable traits, and ignores those that are neutral.<ref name="humangenes">{{harvnb|Cavalli-Sforza|Menozzi|Piazza|1996}}</ref>

The [[law of large numbers]] predicts that when the absolute number of copies of the allele is small (e.g., [[small population size|in small populations]]), the magnitude of drift on allele frequencies per generation is larger. The magnitude of drift is large enough to overwhelm selection at any allele frequency when the [[selection coefficient]] is less than 1 divided by the effective population size. Non-adaptive evolution resulting from the product of mutation and genetic drift is therefore considered to be a consequential mechanism of evolutionary change primarily within small, isolated populations.<ref>{{harvnb|Zimmer|2001}}</ref> The mathematics of genetic drift depend on the effective population size, but it is not clear how this is related to the actual number of individuals in a population.<ref name="gillespie 2001" /> [[Genetic linkage]] to other genes that are under selection can reduce the effective population size experienced by a neutral allele. With a higher [[homologous recombination|recombination]] rate, linkage decreases and with it this local effect on effective population size.<ref>{{harvnb|Golding|1994|p=46}}</ref><ref>{{cite journal | vauthors = Charlesworth B, Morgan MT, Charlesworth D | title = The effect of deleterious mutations on neutral molecular variation | journal = Genetics | volume = 134 | issue = 4 | pages = 1289–303 | date = August 1993 | pmid = 8375663 | pmc = 1205596 | url = http://www.genetics.org/content/134/4/1289.full.pdf | publisher = Genetics Society of America | doi = 10.1093/genetics/134.4.1289 | author-link3 = Deborah Charlesworth | access-date = 9 December 2015 | archive-date = 12 March 2020 | archive-url = https://web.archive.org/web/20200312232806/https://www.genetics.org/content/genetics/134/4/1289.full.pdf | url-status = live }}</ref> This effect is visible in molecular data as a correlation between local recombination rate and [[genetic diversity]],<ref>{{cite journal | vauthors = Presgraves DC | title = Recombination enhances protein adaptation in Drosophila melanogaster | journal = Current Biology | volume = 15 | issue = 18 | pages = 1651–6 | date = September 2005 | pmid = 16169487 | doi = 10.1016/j.cub.2005.07.065 | publisher = Cell Press | s2cid = 15120927 | author-link = Daven Presgraves | doi-access = free | bibcode = 2005CBio...15.1651P }}</ref> and negative correlation between gene density and diversity at [[noncoding DNA]] regions.<ref>{{cite journal | vauthors = Nordborg M, Hu TT, Ishino Y, Jhaveri J, Toomajian C, Zheng H, Bakker E, Calabrese P, Gladstone J, Goyal R, Jakobsson M, Kim S, Morozov Y, Padhukasahasram B, Plagnol V, Rosenberg NA, Shah C, Wall JD, Wang J, Zhao K, Kalbfleisch T, Schulz V, Kreitman M, Bergelson J | title = The pattern of polymorphism in Arabidopsis thaliana | journal = PLOS Biology | volume = 3 | issue = 7 | pages = e196 | date = July 2005 | pmid = 15907155 | pmc = 1135296 | doi = 10.1371/journal.pbio.0030196 | publisher = [[PLOS|Public Library of Science]] | doi-access = free }} {{open access}}</ref> Stochasticity associated with linkage to other genes that are under selection is not the same as sampling error, and is sometimes known as [[genetic hitchhiking|genetic draft]] in order to distinguish it from genetic drift.<ref name="gillespie 2001" />

Low allele frequency makes alleles more vulnerable to being eliminated by random chance, even overriding the influence of natural selection. For example, while disadvantageous mutations are usually eliminated quickly within the population, new advantageous mutations are almost as vulnerable to loss through genetic drift as are neutral mutations. Not until the allele frequency for the advantageous mutation reaches a certain threshold will genetic drift have no effect.<ref name="humangenes" />