Bottlenecks and founder effects. Genetic drift can cause big losses of genetic variation for small populations. Population bottlenecks occur when a population's size is reduced for at least one generation.

Population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events (such as famines, 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 smaller genetic diversity, remains to pass on genes to future generations of offspring through sexual reproduction. Genetic diversity remains lower, increasing only when gene flow with another population occurs or very slowly increasing with time as random mutationsoccur.[1][self-published source] As a consequence of such population size reductions and the loss of genetic variation, the robustness of the population is reduced and its ability to adapt to and survive selecting environmental changes, such as climate change or a shift in available resources, is reduced.[2] Alternatively, if survivors of the bottleneck are the individuals with the greatest genetic fitness, the frequency of the fitter genes within the gene pool is increased, while the pool itself is reduced.

The genetic drift caused by a population bottleneck can change the proportional distribution of alleles by chance and even lead to fixation or loss of alleles. Due to the smaller population size after a bottleneck event, the chances of inbreeding and genetic homogeneityincrease, leading to the potential for inbreeding depression to occur. Smaller population size can also cause deleterious mutations to accumulate[3]

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. Alternatively, invasive speciescan undergo population bottlenecks through founder events when introduced into their invaded range[4]

Population bottlenecks play an important role in conservation biology (see minimum viable population size) and in the context of agriculture (biological and pest control).[5]