The founder effect occurs when a new population is established by a small number of individuals.The founder effect is a special case of genetic drift (random changes in gene frequencies in small groups). When a small group splits off from a large population, it carries only a fragment of the original gene pool. Random fluctuations in this sample can sharply increase the proportion of certain alleles (gene variants). A classic example is the high frequency of Huntington's disease in an isolated community around Lake Maracaibo in Venezuela, where the gene was introduced by one of the early settlers. Another example is deafness on Martha's Vineyard in the 18th–19th centuries, where one in four residents was deaf due to a mutation brought by a few founders.
The concept was formulated by Ernst Mayr in 1942 in his book "Systematics and the Origin of Species," although observations of the distinctiveness of island faunas had been made earlier. Mayr showed that the genetic poverty of founder populations accelerates evolutionary changes and promotes speciation. Later, the effect was confirmed by genetic studies of human populations, such as the Amish in Pennsylvania, who have a high frequency of Ellis–van Creveld syndrome because they all descend from a few dozen 18th-century settlers.
When a small group separates from a large population, a "genetic bottleneck" occurs: the allelic composition of the founders is random and unrepresentative. In subsequent generations, drift and inbreeding (mating between close relatives) amplify the deviations. The effect is maximal when the number of founders is less than 50 individuals. In nature, it explains high endemism (uniqueness of species) on oceanic islands, and in medicine, clusters of rare diseases in ethnic groups.
Explains why isolated communities (e.g., on islands or in religious groups) have higher frequencies of rare hereditary diseases that are almost invisible in large populations.
The founder effect can create new species: if an isolated population accumulates enough differences, it stops interbreeding with the ancestral form.





