The founder effect occurs when a small new population carries only part of the genetic variation of its source population.
It is a form of genetic drift. If a few individuals establish a new population, the alleles they happen to carry may be unrepresentative of the larger source population. Allele frequencies in the new population can therefore differ sharply from the original population, even without natural selection.
The effect is strongest when the founding group is very small and remains isolated. It can reduce genetic diversity and make uncommon inherited conditions more frequent by chance. Researchers have studied founder effects in island populations and in some human communities descended from relatively small founding groups.
The founder effect differs from a population bottleneck. A bottleneck occurs when an existing population is drastically reduced in size; the founder effect begins when a small group starts a new population elsewhere.