A severe reduction in population size that dramatically decreases genetic diversity, as only the alleles present in the surviving individuals are passed to future generations. The population 'squeezes' through a metaphorical bottleneck.
Named for the narrow neck of a bottle that restricts flow, from Middle English 'botel' and 'necke'. The biological usage emerged in the 1960s when population geneticists needed a metaphor for how catastrophic events force populations through periods of extremely small size.
Cheetahs are living proof of an ancient bottleneck - they're so genetically similar that skin grafts between unrelated individuals don't get rejected! The entire species went through such a severe bottleneck around 10,000 years ago that they're essentially genetic clones.
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