Founder effect

Definition

In population genetics, the reduction in genetic diversity that occurs when a new population is established by a small number of individuals from a larger source population. Because the founders carry only a subset of the original gene pool, the descendant population exhibits reduced heterozygosity and altered allele frequencies relative to the parent population, potentially leading to phenotypic divergence, inbreeding depression, or rapid adaptive evolution in the new environment.

Etymology

From 'founder' meaning one who establishes or originates, and 'effect' in the statistical sense; coined in population genetics to describe the evolutionary consequences of colonization events.

Example

A small number of European honey bees (Apis mellifera) introduced to Australia in the 1820s established feral populations with markedly lower mitochondrial DNA diversity than their European ancestors, demonstrating founder effect in an invasive insect. Similarly, island populations of spiders often show reduced genetic variation compared to mainland relatives, reflecting serial founder events during colonization.

Related Terms

• genetic bottleneck
• genetic drift
• Gene flow
• population bottleneck
• inbreeding depression
• effective population size
• Colonization
• Island biogeography

Usage Notes

Distinguished from genetic bottleneck: a bottleneck can occur in any population regardless of origin, whereas founder effect specifically describes reduced diversity in newly established populations derived from small colonizing groups. The effect is stronger when fewer founders are involved and when multiple founder events occur sequentially (founder chain). In conservation entomology, founder effects complicate reintroduction efforts because released individuals may carry insufficient genetic diversity for long-term population viability.