If in a particular generation, the size of a population suddenly and drastically reduces this is known as a population bottleneck, an extreme form of genetic drift. This results in a reduction of genetic diversity. When the population recovers there will be fewer alleles in the gene pool. Examples are northern elephant seals which were hunted almost to extinction in the 19th century and cheetahs which experienced a bottleneck about 10,000 years ago.New colonies founded by a few individuals give rise to the founder effect. The populations that arise have a much smaller gene pool e.g. the Amish in Pennsylvania and the Afrikaner population in South Africa, closed communities that rarely intermarry.