genetic bottleneck

Picture pouring a jar of mixed beads through a narrow funnel: only a few squeeze through at a time, and whatever comes out the other side is a random, reduced sample of what went in. A genetic bottleneck is that funnel for a population—a sharp, often sudden drop in numbers that lets only a fraction of the original genetic variation pass to the survivors.

More precisely, a genetic bottleneck is an event that drastically reduces a population's size, and with it the diversity of its gene pool. Even if numbers later recover, the rebuilt population draws all its alleles from the few survivors, so much of the lost variation is gone for good.

Bottlenecks are a powerful instance of genetic drift acting over a short interval. Rare alleles are especially likely to vanish, overall heterozygosity falls, and previously rare alleles can rise to high frequency by chance. The population can also lose adaptive potential, making it more vulnerable to disease and environmental change.

Famous cases include species pushed to the brink and then recovered—such as the cheetah and the northern elephant seal—which still show strikingly low genetic diversity, a lingering signature of bottlenecks in their past.