A population bottleneck, also known as a genetic 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, or from small captive breeding programs such as animals at a zoo. Genetic bottlenecks occur when populations show a drastic reduction in population size, for example, due to temporally deteriorating environmental conditions.

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. Due to the loss of genetic variation, the new 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. Reduced genetic variation means that the population may not be able to adapt to new selection pressures, such as climatic change or a shift in available resources, because the genetic variation that selection would act on may have already drifted out of the population.

Population bottlenecks play an important role in conservation biology, where the minimum viable population size is a key concept. In the context of agriculture, population bottlenecks are important for 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.