Founder Effect

Founder effect is the term for what happens when a few individuals are removed from a much larger population and used to found a new population group descended only from the individuals thus removed. In such instances, the allele frequencies of various genes in the population change from the allele frequencies found in the source population, and reflect the proportions in the new founder group. Population bottlenecks also cause founder effect even though no new population has been founded, strictly speaking.

The following is a quotation from Purebred Dog Breeds Into the Twenty-First Century: Achieving Genetic Health for Our Dogs, explaining "Founder Events":

"Perhaps the most crucial concept in population genetics for dog-breeders is the founder event, for its theory describes perfectly what takes place when a breed is "recognised" by CKC or a similar registry. Whatever may be the state of genetic balance or the frequency with which particular alleles are found in the general canine population, it all changes when a founder event occurs. In nature such events happen when individuals of a species occupy and reproduce in territory new to the species, losing contact with the source population of the migrants (as when small birds are deposited by hurricane winds on mid-ocean islands). The founder event describes the establishing of a small population, although later on it may grow to be a large one. When a finite number of individuals found a new population group, the genome of the new group will necessarily reflect the genes brought to it by the founder animals; gene frequencies within that population will reflect the gene frequencies within the founder group rather than that of the source population. In this way, when a founder event occurs, a gene quite rare in the source population may have a much higher frequency in the new population; conversely, genes common in the source population may be infrequent or even absent from the new population. It all depends on the genes of the founders! Thus a genetic defect extremely rare in the overall canine population can come to be common in a particular breed simply because one or more individuals of a small breed foundation carried that gene."