Population bottleneck

A population bottleneck is a sharp reduction in the size of a population caused by an environmental event, such as famine, earthquake, flood or disease, or by human actions such as violent conflict or deliberate culling. Because a bottleneck sharply shrinks the number of breeding individuals, it reduces the genetic variation carried in the gene pool of the population, leaving the survivors more vulnerable to environmental change and disease, and it can also increase inbreeding, which raises the risk of inbreeding depression and the accumulation of harmful mutations. Northern elephant seals demonstrate both the damage and the limits of recovery: overhunting reduced the population to roughly 30 individuals in the 1890s, and although numbers have since rebounded into the hundreds of thousands, genetic variation remains extremely low, in part because a small number of dominant males mate with as many as 100 females each, concentrating offspring into a few ancestral lines and leaving the species more susceptible to disease.