founder effect

Meaning: When a few individuals colonize a new area or become isolated, they carry only a sample of the source population's alleles. By chance, that sample may over- or under-represent certain alleles. Because the new population is small and descended from those founders, the unusual frequencies persist and are amplified by later drift and inbreeding. The result is lower genetic diversity and, often, a higher frequency of a recessive allele that was rare in the source population.

On the MCAT: A passage may describe a small group, such as a religious community or island colonists, that has a high frequency of a particular recessive trait compared with the original population. You are asked to identify the cause (founder effect, a type of genetic drift), predict reduced heterozygosity or diversity, or explain why a rare allele rose in frequency. It may also appear in a Hardy-Weinberg question: the founder effect is one reason the population is not in equilibrium, because its allele frequencies changed through sampling error rather than selection. Natural selection, mutation, gene flow and non-random mating are the other violations.

Common confusion: Students mix the founder effect up with the bottleneck effect. Both are drift from a small sample. The founder effect follows a few individuals leaving to start a new population. The bottleneck follows a sudden drop in the size of an existing population, through a disaster, for example. Also, the change is random, not adaptive: the allele did not rise because it was beneficial.