Editing Genetic drift (section)

==Population bottleneck==
{{Main|Population bottleneck}}
[[File:Population bottleneck.jpg|thumb|350px|Changes in a population's allele frequency following a [[population bottleneck]]: the rapid and radical decline in population size has reduced the population's [[genetic variation]].]]
A population bottleneck is when a population contracts to a significantly smaller size over a short period of time due to some random environmental event. In a true population bottleneck, the odds for survival of any member of the population are purely random, and are not improved by any particular inherent genetic advantage. The bottleneck can result in radical changes in allele frequencies, completely independent of selection.<ref>{{cite encyclopedia | veditors = Robinson R |encyclopedia=Genetics |title=Population Bottleneck |url=https://archive.org/details/genetics0000unse |access-date=2015-12-14 |year=2003 |publisher=[[Gale (publisher)|Macmillan Reference USA]] |volume=3 |location=New York |isbn=0-02-865609-1 |lccn=2002003560 |oclc=614996575 |url-access=registration }}</ref>

The impact of a population bottleneck can be sustained, even when the bottleneck is caused by a one-time event such as a natural catastrophe.
An interesting example of a bottleneck causing unusual genetic distribution is the relatively high proportion of individuals with total [[rod cell]] [[color blindness]] ([[achromatopsia]]) on [[Pingelap|Pingelap atoll]] in [[Federated States of Micronesia|Micronesia]].<ref>{{cite journal | vauthors = Hussels IE, Morton NE | title = Pingelap and Mokil Atolls: achromatopsia | journal = American Journal of Human Genetics | volume = 24 | issue = 3 | pages = 304–309 | date = May 1972 | pmid = 4555088 | pmc = 1762260 }}</ref> After a bottleneck, inbreeding increases. This increases the damage done by recessive deleterious mutations, in a process known as [[inbreeding depression]]. The worst of these mutations are selected against, leading to the loss of other alleles that are [[linkage disequilibrium|genetically linked]] to them, in a process of [[background selection]].<ref name="Masel 2011" /> For recessive harmful mutations, this selection can be enhanced as a consequence of the bottleneck, due to [[genetic purging]]. This leads to a further loss of genetic diversity. In addition, a sustained reduction in population size increases the likelihood of further allele fluctuations from drift in generations to come.

A population's genetic variation can be greatly reduced by a bottleneck, and even beneficial adaptations may be permanently eliminated.<ref>{{harvnb|Futuyma|1998|pp=303–304}}</ref> The loss of variation leaves the surviving population vulnerable to any new selection pressures such as disease, [[climate change (general concept)|climatic change]] or shift in the available food source, because adapting in response to environmental changes requires sufficient genetic variation in the population for natural selection to take place.<ref>{{cite journal | vauthors = O'Corry-Crowe G | title = Climate change and the molecular ecology of Arctic marine mammals | journal = Ecological Applications | volume = 18 | issue = 2 Suppl | pages = S56-76 | date = March 2008 | pmid = 18494363 | doi = 10.1890/06-0795.1 | publisher = [[Ecological Society of America]] | doi-access = free | bibcode = 2008EcoAp..18S..56O }}</ref><ref>{{cite journal | vauthors = Cornuet JM, Luikart G | title = Description and power analysis of two tests for detecting recent population bottlenecks from allele frequency data | journal = Genetics | volume = 144 | issue = 4 | pages = 2001–14 | date = December 1996 | pmid = 8978083 | pmc = 1207747 | publisher = Genetics Society of America | doi = 10.1093/genetics/144.4.2001 }}</ref>

There have been many known cases of population bottleneck in the recent past. Prior to the arrival of [[Europeans]], [[North America]]n prairies were habitat for millions of [[greater prairie chicken]]s. In [[Illinois]] alone, their numbers plummeted from about 100 million birds in 1900 to about 50 birds in the 1990s. The declines in population resulted from hunting and [[habitat destruction]], but a consequence has been a loss of most of the species' genetic diversity. [[DNA]] analysis comparing birds from the mid century to birds in the 1990s documents a steep decline in the genetic variation in just the latter few decades. Currently the greater prairie chicken is experiencing low [[reproductive success]].<ref>{{harvnb|Sadava|Heller|Orians|Purves|2008|loc=chpts. 1, 21–33, 52–57}}</ref>

However, the genetic loss caused by bottleneck and genetic drift can increase fitness, as in ''[[Ehrlichia]]''.<ref name="DaleMoran2006">{{cite journal | vauthors = Dale C, Moran NA | title = Molecular interactions between bacterial symbionts and their hosts | journal = Cell | volume = 126 | issue = 3 | pages = 453–65 | date = August 2006 | pmid = 16901780 | doi = 10.1016/j.cell.2006.07.014 | s2cid = 15985536 | doi-access = free }}</ref>

Over-hunting also caused a severe population bottleneck in the [[northern elephant seal]] in the 19th century. Their resulting decline in genetic variation can be deduced by comparing it to that of the [[southern elephant seal]], which were not so aggressively hunted.<ref>{{cite web |url=http://evolution.berkeley.edu/evolibrary/article/bottlenecks_01 |title=Bottlenecks and founder effects |work=Understanding Evolution |publisher=University of California, Berkeley |access-date=2015-12-14 |url-status=live |archive-url=https://web.archive.org/web/20151204051014/http://evolution.berkeley.edu/evolibrary/article/bottlenecks_01 |archive-date=4 December 2015}}</ref>

===Founder effect===
{{Main|Founder effect}}
[[File:Founder effect with drift.jpg|thumb|right|350px|When very few members of a population migrate to form a separate new population, the founder effect occurs. For a period after the foundation, the small population experiences intensive drift. In the figure this results in fixation of the red allele.]]
The founder effect is a special case of a population bottleneck, occurring when a small group in a population splinters off from the original population and forms a new one. The random sample of alleles in the just formed new colony is expected to grossly misrepresent the original population in at least some respects.<ref>{{harvnb|Campbell|1996|p=423}}</ref> It is even possible that the number of alleles for some genes in the original population is larger than the number of gene copies in the founders, making complete representation impossible. When a newly formed colony is small, its founders can strongly affect the population's genetic make-up far into the future.

A well-documented example is found in the [[Amish]] migration to [[Pennsylvania]] in 1744. Two members of the new colony shared the recessive allele for [[Ellis–Van Creveld syndrome]]. Members of the colony and their descendants tend to be religious isolates and remain relatively insular. As a result of many generations of inbreeding, Ellis–Van Creveld syndrome is now much more prevalent among the Amish than in the general population.<ref name="humangenes" /><ref name="PBS_Evolution_founder">{{cite web |url=https://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html |title=Genetic Drift and the Founder Effect |year=2001 |website=Evolution Library |series=Evolution |publisher=[[WGBH Educational Foundation]]; Clear Blue Sky Productions, Inc. |location=Boston, MA |type=Web resource |oclc=48165595 |access-date=2009-04-07 |url-status=live |archive-url=https://web.archive.org/web/20090314040513/http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html |archive-date=14 March 2009}}</ref>

The difference in gene frequencies between the original population and colony may also trigger the two groups to [[genetic divergence|diverge]] significantly over the course of many generations. As the difference, or [[genetic distance]], increases, the two separated populations may become distinct, both genetically and [[phenotype|phenetically]], although not only genetic drift but also natural selection, gene flow, and mutation contribute to this divergence. This potential for relatively rapid changes in the colony's gene frequency led most scientists to consider the founder effect (and by extension, genetic drift) a significant driving force in the evolution of [[speciation|new species]]. Sewall Wright was the first to attach this significance to random drift and small, newly isolated populations with his [[shifting balance theory]] of speciation.<ref>{{harvnb|Wolf|Brodie|Wade|2000}}</ref> Following after Wright, [[Ernst Mayr]] created many persuasive models to show that the decline in genetic variation and small population size following the founder effect were critically important for new species to develop.<ref>{{harvnb|Hey|Fitch|Ayala|2005}}</ref> However, there is much less support for this view today since the hypothesis has been tested repeatedly through experimental research and the results have been equivocal at best.<ref>{{harvnb|Howard|Berlocher|1998}}</ref>