founder effect

Scoop a small handful of marbles from a huge mixed bag and your handful probably will not match the bag's exact color proportions—and might miss some colors entirely. The founder effect is that sampling accident applied to populations: when a new population is started by just a few individuals, it carries only a small, possibly skewed slice of the original gene pool's variation.

Formally, the founder effect is the loss of genetic variation and the shift in allele frequencies that occur when a new population is founded by a small number of colonizers. It is a special case of genetic drift, acting in a single sharp event at the moment of founding.

The consequences can persist for many generations. Alleles that were rare in the source population can become common in the founded one simply by the luck of which founders carried them, while other alleles may be absent altogether. This can make certain inherited conditions unusually frequent in geographically or culturally isolated communities.

The founder effect is a major reason that small, newly established, or island populations often differ genetically from their parent populations far more than selection alone could explain.