Founder Effect

The founder effect is the loss of genetic variation that occurs when a new population is started by a small number of individuals from a larger one. The founders carry only a sample of the original allele pool, so the new population can differ from its source simply by chance.

A small group is unlikely to represent the original population's allele frequencies exactly. Rare alleles may be absent, or by chance very common among the founders. As the population grows, these frequencies are inherited by all the descendants. The effect is a special case of genetic drift, and it was emphasized by the biologist Ernst Mayr in 1942 in connection with the formation of new species on islands.

Among the Old Order Amish of Lancaster County, Pennsylvania, who descend from about 200 founders, a form of dwarfism with extra fingers, Ellis-van Creveld syndrome, is much more common than elsewhere. Several rare diseases are enriched in Finland, and a hereditary form of color blindness is common on the island of Pingelap in Micronesia after a typhoon in the 1770s left about 20 survivors. Afrikaners in South Africa have a high rate of porphyria variegata, traced to a Dutch couple.

Founder events on islands and in new habitats can start populations along different paths, so they may contribute to speciation, though this is debated. A founder effect differs from a bottleneck, where an existing population is reduced, but both lower the effective population size. They help to explain patterns in population genetics and the deviations from the Hardy-Weinberg principle.

• Genetic Drift — the broader process
• Effective Population Size — a measure reduced by founder events
• Speciation — a possible outcome