Wallace effect

The Wallace Effect is the hypothesis that natural selection can contribute to the reproductive isolation of incipient species by encouraging varieties to develop barriers to hybridization.

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Historical development

In 1889, A. R. Wallace wrote the book Darwinism, which explained and defended natural selection. 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 particular 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. ^[1]

Current status

It is sometimes also called "reinforcement" and it continues to be a topic of research in evolutionary biology today as it is potentially important in sympatric speciation. Its validity has been supported by mathematical models, and recently by empirical field data on the evoluton of differing flowering times as a reproductive isolation mechanism^[2], as well as sex-chromosome linked species preference in flycatchers^[3]

Notes

References

• Slotten, Ross A. (2004) The heretic in Darwin's court: the life of Alfred Russel Wallace Columbia University Press: New York. ISBN 0-231-13010-4