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=== Population bottleneck ===
[[File:Population bottleneck.jpg|thumb|left|250px|Model of population bottleneck illustrates how [[allele]]s can be lost.]]
A [[population bottleneck]] occurs when the population of a species is reduced drastically over a short period of time due to external forces.<ref>{{cite encyclopedia |editor-last=Robinson |editor-first=Richard |encyclopedia=Genetics |title=Population Bottleneck |url=https://archive.org/details/genetics0000unse |access-date=2011-04-07 |year=2003 |publisher=[[Gale (publisher)|Macmillan Reference US]] |___location=New York |isbn=978-0-02-865606-9 |lccn=2002003560 |oclc=49278650 |url-access=registration }}</ref> In a true population bottleneck, the reduction does not favour any combination of alleles; it is totally random chance which individuals survive. A bottleneck can reduce or eliminate genetic variation from a population. Further drift events after the bottleneck event can also reduce the population's [[genetic diversity]]. The lack of diversity created can make the population at risk to other selective pressures.<ref>{{harvnb|Futuyma|1998|pp=303–304}}</ref>
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