The founder's effect is a specific kind of genetic drift and a force in biological evolution. Evolution concerns changes in allele frequences from one generation to the next. Drift occurs when there are random changes in gene frequencies. In large populations these random changes will often cancel one another out, and have an insignificant effect -- but they will have a significant effect in small populations.

The founder's effect describes what happens when a small population leaves a larger population. If the small population were a random sample of the larger population, it is possible that the allele frequencies in the small population accurately represent the frequencies in the larger population. In fact, in most cases of a small group separating from a larger group, the sample is not random and likely to contain only a small fraction of the total genetic variation of the larger population. In other words, the allele frequencies of the small group will diverge significantly from the frequencies of the larger group. When the small group becomes isolated from the larger group, the founder effect can be an important force in speciation.