Genetic Bottlenecks

A bottleneck can reduce a population's genetic variation by a lot, even if the bottleneck doesn't last for very many generations.
A founder effect occurs when a new colony is started by a few members of the original population, sort of an artificial bottleneck.This small population size means that the colony may have:

• reduced genetic variation from the original population.
• a non-random sample of the genes in the original population.

For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington's disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

Bags of marbles where, in generation 2, an unusually small draw creates a bottleneck.