The founder effect was defined by Ernst Mayr in 1963 to be the effect of establishing a new population by a small number of individuals, carrying only a small fraction of the original population's genetic variation. As a result, the new population may be distinctively different, both genetically and phenotypically, than the parent population from which it is derived.
In the figure shown, the original population has nearly equal numbers of blue and red individuals. The three smaller founder populations show that one or the other color may predominate (founder effect), due to random sampling of the original population. A population bottleneck can also cause a founder effect even though it isn't strictly a new population.
In addition to founder effects, the new population is often very small and shows random genetic drift, and an increase in inbreeding due to small population size.
Founder effects are common in island ecology, but the isolation need not be geographical. For example, the Amish populations in the United States, which have grown from a very few founders but have not recruited newcomers, and tend to marry within the community, exhibit founder effects. Though still rare absolutely, phenomena such as polydactyly (extra fingers and toes, a symptom of Ellis-van Creveld syndrome) are more common in Amish communities than in the US population at large.
Another example is the large amount of cleidocranial dysostosis in the Muslim population section in South Africa, caused by descent from one affected Chinese man who settled in South Africa and converted to Islam.
In extreme cases the founder effect is thought to lead to the speciation and subsequent evolution of new species.
The founder effect is a feature that can also occur in memetic evolution.
Simple illustration of founder effect. The original population is on the left with three possible founder populations on the right.