Bottleneck effect

The bottleneck effect is a form of genetic drift, the random change in allele frequencies from one generation to the next. It happens when a disaster such as an earthquake, flood, disease outbreak or heavy hunting kills a large part of a population, largely without regard to the genes each individual carries. The survivors pass through the narrow neck of the bottle, and their genes, not those of the original population, become the gene pool from which all later generations descend.

Because the survivors are a small and chance sample, some alleles are lost entirely and others become more or less common than before, purely by luck. Even if the population later grows back to its former size, the lost variation returns only slowly, through new mutations or the arrival of migrants. Low genetic diversity can leave a population less able to adapt to new diseases or a changing climate, and it raises the risk of inbreeding and inherited disorders.

Northern elephant seals were hunted down to perhaps 20 animals by the 1890s. They have since recovered to large numbers, yet they carry far less genetic variation than southern elephant seals, which were never reduced so severely. Cheetahs show a similar genetic signature of past bottlenecks.