The founder effect

The founder effect is a particular example of the influence of random sampling. It was defined by Ernst Mayr as

"The establishment of a new population by a few original founders (in an extreme case, by a single fertilized female) which carry only a small fraction of the total genetic variation of the parental population."

Two reasons why a population may be descended from a small number of ancestral individuals:

1. A small number of individuals may colonize a place previously uninhabited by their species. The 250 people making up the human population on the island of Tristan da Cunha, are mostly descended from one Scottish family who arrived in 1817.

2. An established population may fluctuate in size: the founder effect occurs when the population passes through a “bottleneck” in which only a few individuals survive, and later expands again under more favorable conditions. This process is illustrated in the following animation.