Speciation

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Speciation is the formation of new species. In allopatric speciation, populations are separated by a geographical barrier, such as a river or mountain, and diverge by mutation, selection and genetic drift until they can no longer interbreed. In sympatric speciation it occurs without physical separation, as in polyploidy in plants.

Reproductive isolation, either before or after mating, defines species. Darwin's finches and the cichlids of African lakes are classic examples. Adaptive radiation produces many species rapidly from a common ancestor.

Darwin's finches are a case of allopatric speciation: birds from the mainland reached islands, and populations on different islands faced different foods, evolving different beak shapes until they no longer interbred. Reproductive isolation can be geographic, behavioural, such as different mating songs, or genetic, such as hybrids being sterile. A mule, from a horse and a donkey, is sterile.

Sympatric speciation, with no physical barrier, is common in plants through polyploidy, where chromosome numbers double in one generation, as happened in wheat. Speciation usually takes thousands of years, but sometimes happens faster, such as in cichlid fish in African lakes. The biological species concept defines a species as a group that can interbreed to produce fertile offspring, though it has limits for asexual organisms.