Founder Effect
The founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals (founders) from a larger source population. Because the founders carry only a random, limited sample of the original alleles, the new population will differ genetically from the source population and will have lower genetic diversity. The founder effect is a special case of genetic drift and is often responsible for the high prevalence of certain genetic conditions in isolated human populations.
Human Populations Showing the Founder Effect
| Population | Founding Event | Allele/Condition | Frequency in Isolate | Frequency in General Population |
|---|---|---|---|---|
| Amish (Lancaster, PA) | Founded ~1740 by ~200 settlers | Ellis-van Creveld syndrome allele | ~0.07 | ~0.001 |
| Afrikaner (South Africa) | Dutch settlers, 17th century | Familial hypercholesterolaemia | 1 in 72 | 1 in 500 |
| Finnish population | Post-Ice-Age re-colonisation | Lysinuric protein intolerance | High in Finland | Rare globally |
| Ashkenazi Jewish | Repeated bottlenecks in Europe | Tay-Sachs allele | ~1 in 25 carriers | ~1 in 250 globally |
| Pitcairn Islanders | Founded by 9 Bounty mutineers (1790) | Reduced MHC diversity | Very limited | Broad diversity |
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Related Terms
Genetic Bottleneck
A genetic bottleneck is a sharp reduction in the size of a population due to an environmental event (such as a famine, disease, or habitat destruction), resulting in a dramatic loss of genetic diversity in the surviving population. The surviving individuals carry only a small, random subset of the original genetic variation, and subsequent generations inherit this reduced genetic repertoire regardless of population size recovery. Bottlenecks increase homozygosity, reduce adaptive potential, and can cause rare alleles to be lost or increase in frequency by chance.
Speciation
Speciation is the evolutionary process by which populations evolve to become distinct species, typically through the accumulation of reproductive barriers that prevent gene flow between diverging lineages. It is the source of biodiversity and occurs through several modes, most commonly allopatric speciation (geographic isolation) and sympatric speciation (divergence without geographic barrier). The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring, making reproductive isolation the key criterion.
Gene Flow
Gene flow, also called gene migration, is the transfer of alleles or genes from one population to another through the movement and interbreeding of individuals. It tends to homogenise allele frequencies between populations, reducing genetic differentiation, and can introduce new alleles into a population or change the frequencies of existing ones. Gene flow counteracts the genetic divergence produced by natural selection, drift, and mutation, and is a critical factor in whether populations will diverge enough to speciate.
The term "founder effect" was introduced by Ernst Mayr in his 1942 book "Systematics and the Origin of Species". "Founder" derives from Old French "fondeur", from Latin "fundare" (to lay the foundation), from "fundus" (bottom, base). The effect describes how founding individuals "lay the genetic foundation" of new populations.