Speciation by reinforcement.

Speciation has a central place in evolution, linking genetic processes within populations to the generation of biological diversity. The formation of new species must involve the evolution of barriers to gene flow within existing species, but how these barriers arise remains a problem. In the case of prezygotic isolation it is possible that natural selection directly favours characters that decrease gene flow. Where two populations have diverged to such an extent that they produce unfit hybrids, individuals will leave more offspring if they mate within their own population. Characters increasing assortive mating will be favoured until eventually two species may result. This is the widely accepted model of speciation by reinforcement. However, recent studies suggest serious limitations on the efficacy of reinforcing selection and a lack of well-substantiated examples.