A population bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts, or human activities such as specicide, widespread hunting, or habitat destruction. The bottleneck can also occur if a small group becomes reproductively separated from the main population, such as through a founder event, e.g., if a few members of a species successfully colonize a new isolated island, or from small captive breeding programs such as animals at a zoo.

The genetic drift caused by a population bottleneck can change the proportional random distribution of alleles and even lead to the loss of alleles. Due to the loss of genetic variation, the remaining population can become genetically distinct from the original population, which has led to the hypothesis that population bottlenecks can lead to the evolution of new species. Following a population bottleneck, the remaining population faces a higher level of genetic drift, which describes random fluctuations in the presence of alleles in a population. In small populations, infrequently occurring alleles face a greater chance of being lost, which can further decrease the gene pool.

Population bottlenecks play an important role in conservation biology, where the minimum viable population size is a key concept. They are also relevant in the context of agriculture, biological and pest control. A classic example of a population bottleneck is that of the northern elephant seal, whose population fell to about 30 in the 1890s. Although it now numbers in the hundreds of thousands, the potential for bottlenecks within colonies remains.