Which evolutionary process reduces genetic variation when a population passes through a severe temporary decline?

A population bottleneck is an evolutionary process in which a severe temporary population decline reduces genetic variation.

A bottleneck occurs when events such as famine, disease, habitat loss, hunting, or natural disasters sharply reduce population size. The survivors are a random, often unrepresentative sample of the original population’s genetic diversity. When the population later expands, it may remain genetically less variable than it was before the decline.

The northern elephant seal is a classic example. Commercial hunting reduced the species to a very small number of animals in the nineteenth century, after which its population recovered. Genetic studies have found exceptionally low variation among living northern elephant seals.

Bottlenecks can increase the effects of genetic drift and inbreeding. They may also make populations less able to respond to new diseases or environmental changes. A bottleneck differs from a founder effect: a bottleneck follows a reduction in an existing population, whereas a founder effect begins when a small group establishes a new population elsewhere.