Bottleneck Effect
This is a severe reduction in population size that causes pronounced drift.
For example, the elephant seal population was hunted down to just 20 individuals. Then the
population rebounded to 30,000. However, electrophoresis has revealed that there is now no allele
variation at 24 genes.
In bottlenecks, some stressful situation greatly reduces the size of a population leaving a few
(typical or atypical) individuals to reestablish the population.
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