Editing Microevolution (section)

===Genetic drift===
{{Main|Genetic drift}}
[[File:Random genetic drift chart.png|thumb|250px|right|Ten simulations of random genetic drift of a single given allele with an initial frequency distribution 0.5 measured over the course of 50 generations, repeated in three reproductively synchronous populations of different sizes. In general, alleles drift to loss or fixation (frequency of 0.0 or 1.0) significantly faster in smaller populations.]]
Genetic drift is the change in the relative frequency in which a gene variant ([[allele]]) occurs in a population due to [[Sampling (statistics)|random sampling]]. That is, the alleles in the offspring in the population are a random sample of those in the parents. And chance has a role in determining whether a given individual survives and reproduces. A population's [[allele frequency]] is the fraction or percentage of its gene copies compared to the total number of gene alleles that share a particular form.<ref>{{cite book
 | last = Futuyma
 | first = Douglas
 | title = Evolutionary Biology
 | publisher = [[Sinauer Associates]]
 | year = 1998
 | isbn = 978-0-87893-189-7
 | page = Glossary}}</ref>

Genetic drift is an evolutionary process which leads to changes in [[Allele frequency|allele frequencies]] over time. It may cause gene variants to disappear completely, and thereby reduce genetic variability. In contrast to [[natural selection]], which makes gene variants more common or less common depending on their reproductive success,<ref name = avers>{{Cite book | last = Avers | first = Charlotte | year = 1989 | title = Process and Pattern in Evolution | url = https://archive.org/details/processpatternin00aver | url-access = registration | publisher = Oxford University Press | isbn = 978-0-19-505275-6 }}</ref> the changes due to genetic drift are not driven by environmental or adaptive pressures, and may be beneficial, neutral, or detrimental to reproductive success.

The effect of genetic drift is larger in small populations, and smaller in large populations. Vigorous debates wage among scientists over the relative importance of genetic drift compared with natural selection. [[Ronald Fisher]] held the view that genetic drift plays at the most a minor role in evolution, and this remained the dominant view for several decades. In 1968 [[Motoo Kimura]] rekindled the debate with his [[neutral theory of molecular evolution]] which claims that most of the changes in the genetic material are caused by genetic drift.<ref name="Futuyma 1998 320">{{cite book
 | last = Futuyma
 | first = Douglas
 | title = Evolutionary Biology
 | publisher = [[Sinauer Associates]]
 | year = 1998
 | isbn = 978-0-87893-189-7
 | page = 320}}</ref> The predictions of neutral theory, based on genetic drift, do not fit recent data on whole genomes well: these data suggest that the frequencies of neutral alleles change primarily due to [[genetic hitchhiking|selection at linked sites]], rather than due to genetic drift by means of [[sampling error]].<ref>{{cite journal | author = Hahn, M.W. | year = 2008 | title=Toward a selection theory of molecular evolution |journal=Evolution |pages=255–265 |volume=62 |doi=10.1111/j.1558-5646.2007.00308.x | issue = 2 | pmid = 18302709| s2cid = 5986211 | doi-access=free }}</ref>