Population bottleneck

A population bottleneck (or genetic bottleneck) is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide). Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population, with a correspondingly smaller genetic diversity, remains to pass on genes to future generations of offspring through sexual reproduction. Genetic diversity remains lower, only slowly increasing with time as random mutations occur. In consequence of such population size reductions and the loss of genetic variation, the robustness of the population is reduced and its ability to survive selecting environmental changes, like climate change or a shift in available resources, is reduced.

Conversely, depending upon the causes of the bottleneck, the survivors may have been the genetically fittest individuals, hence increasing the frequency of the fitter genes within the gene pool, while shrinking it. This genetic drift can change the proportional distribution of an allele by chance and even lead to fixation or loss of alleles. Due to the smaller population size after a bottleneck event, the chance of inbreeding and genetic homogeneity increases and unfavoured alleles can accumulate.

A slightly different form of a bottleneck can occur if a small group becomes reproductively (e.g. geographically) separated from the main population, such as through a founder event where for example a few members of a species successfully colonize a new isolated island, or from small captive breeding programs such as animals at a zoo.

Population bottlenecks play an important role in conservation biology (see minimum viable population size) and in the context of agriculture (biological and pest control).

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