Genetic drift can cause big losses of genetic variation for small populations.
Population bottlenecks occur when a population’s size is reduced for at least one generation.
Because genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck can reduce a population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many generations.
This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually small draw creates a bottleneck.
