Bottlenecks occur as a result of a populations size being reduced for atleast one generation. Geographic bottlenecks can cause populations to become isolated. When organisms from the original population are separated the population begins to populated and the population reproduces independently. The Founders Effect is the result of new traits dominating the new isolated population. See the following video on Founder Effect: You tube founder effect explanation
Example of the: FOUNDERS EFFECTEdit
Polydactyly -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome. The syndrome is commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect." Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population. Children are therefore more likely to inherit two copies of the particular recessive genes that lead to genetic disease.