Founder effects arise when a new and isolated environment is invaded by only a few members of a species, which then multiply rapidly. In the extreme case, a single fertilised female might arrive in a new environment.
The result of the small number of founders is that there is a sharp loss of genetic variation compared with the parent population. As a result, the new population may be distinctively different, genetically and phenotypically, from the parent population it derived from. In addition, there is a raised probability of inbreeding, resulting in an unusual number of defects due to recessive genes.
Founder effects are common in island ecology, but the isolation need not be geographical. For example, the Amish populations in the United States, which have grown from a very few founders but have not recruited newcomers, and tend to marry within the community, exhibit founder effects: phenomena such as polydactyly (extra fingers and toes, a symptom of Ellis-van Creveld syndrome), though still rare absolutely, are more common in Amish communities than in the US population at large.
In extreme cases, founder effects may lead to the evolution of new species.
Similar effects can be produced by "population bottlenecks" that occur when a species is nearly driven to extinction, either entirely or locally, and then recovers. For example, the Northern Elephant Seal was reduced (by hunting) to between 100 and 1000 individuals by the end of the 19th century; numbers have now recovered to over 100,000 but it is likely that considerable genetic variation was lost.
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