Population bottleneck

A sharp reduction in population size that reduces genetic variation and makes chance changes in allele frequencies more influential.

Genetic drift: Random changes in allele frequencies caused by chance differences in survival and reproduction. Drift intensifies when a bottleneck leaves few individuals to pass on alleles.
Northern elephant seal: A pinniped species whose population was reduced to a few dozen individuals by nineteenth-century hunting. Its recovery from near-extinction produced a well-known example of sharply reduced genetic variation.
Population genetics: The study of genetic variation within and between populations and how evolutionary forces change it. It provides the framework for describing bottleneck effects on variation and allele frequencies.
Genetic rescue: An increase in population fitness after gene flow introduces genetic variation into a small or inbred population. It is a management response when bottleneck-associated inbreeding threatens persistence.
Founder effect: A change in allele frequencies when a new population is established by a small subset of a larger one. It is the closest demographic counterpart, caused by founding rather than a population crash.
Effective population size: The size of an idealized population that would experience the same genetic drift or inbreeding as a real population. It captures how strongly a bottleneck affects genetic change, beyond the census count.
Cheetah: A large African and Asian cat species with unusually low genetic variation across much of its genome. A proposed ancient bottleneck has been used to explain its low diversity, though its timing and severity remain debated.
Allele: One of two or more alternative forms of a genetic sequence at a particular locus. A bottleneck changes which alleles survive and how common they become.
Inbreeding: Reproduction between related individuals, increasing the chance that offspring inherit identical copies of alleles. Small populations left by bottlenecks are more likely to contain close relatives that mate.
Population expansion: An increase in population size over time, often following a period of stability or decline. Growth can follow a bottleneck, but it does not immediately recreate the lost genetic variation.

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