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Island animal populations are well-known for the extent of their divergence from each other and from source populations on the mainland. Mayr argued that these differences are often triggered by random sampling when island populations are founded by only a few colonizing individuals. The resultant founder effects-changes in the genetic and phenotypic composition of a population due to founding by a limited number of individuals-have been proposed as an important cause of evolutionary divergence for the past half-century. However, an alternative explanation is that island environments differ from one another and from the mainland environment, and these ecological differences result in divergent natural selection. The long-term evolutionary significance of founder effects also has been questioned because their imprint may be short-lived.