The Wallace effect is a hypothesis developed by British naturalist Alfred Russel Wallace which posits that natural selection can contribute to the reproductive isolation of incipient species by evolving barriers against hybridization.
In Wallace's 1889 book Darwinism, he explained and defended the theory of natural selection, first widely presented in 1859 by Charles Darwin in On the Origin of Species. In it he proposed that natural selection could cause the reproductive isolation of two varieties by encouraging the development of barriers against hybridization, and thus contribute to the development of new species. He suggested the following scenario: When two varieties of a species had diverged beyond a certain point, each adapted to different conditions, hybrid offspring would be less well adapted than either parent form. At that point natural selection will tend to eliminate the hybrids. Under such conditions natural selection would also favor the development of barriers to hybridization, since individuals that avoided hybrid matings would tend to have more fit offspring. This would contribute to the reproductive isolation of the two incipient species.
Sometimes also called "reinforcement", the Wallace effect continues to be a topic of research in evolutionary biology, as it is potentially an important factor in speciation, especially sympatric speciation. Its validity has been supported by mathematical models, and recently by empirical field data on the evolution of differing flowering times as a reproductive isolation mechanism, as well as sex-chromosome linked species preference in flycatchers.
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