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Wallace effect
 
 
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The Wallace Effect is the hypothesis that natural selection can contribute to the reproductive isolation
Reproductive isolation

An important concept in evolutionary biology, reproductive isolation is a category of mechanisms that prevent two or more populations from exchanging genes....
 of incipient species by encouraging varieties to develop barriers to hybridization.

889, A. R. Wallace wrote the book Darwinism, which explained and defended natural selection
Natural selection

Natural selection is the process by which favorable heritable trait become more common in successive generations of a population of Reproduction organisms, and unfavorable heritable traits become less common, due to differential reproduction of genotypes....
. In it he proposed that natural selection could cause the reproductive isolation
Reproductive isolation

An important concept in evolutionary biology, reproductive isolation is a category of mechanisms that prevent two or more populations from exchanging genes....
 of two varieties by encouraging the development of barriers against hybridization, and thus contribute to the development of new species.






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The Wallace Effect is the hypothesis that natural selection can contribute to the reproductive isolation
Reproductive isolation

An important concept in evolutionary biology, reproductive isolation is a category of mechanisms that prevent two or more populations from exchanging genes....
 of incipient species by encouraging varieties to develop barriers to hybridization.

Historical development

In 1889, A. R. Wallace wrote the book Darwinism, which explained and defended natural selection
Natural selection

Natural selection is the process by which favorable heritable trait become more common in successive generations of a population of Reproduction organisms, and unfavorable heritable traits become less common, due to differential reproduction of genotypes....
. In it he proposed that natural selection could cause the reproductive isolation
Reproductive isolation

An important concept in evolutionary biology, reproductive isolation is a category of mechanisms that prevent two or more populations from exchanging genes....
 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.

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 an important factor in speciation
Speciation

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages....
, especially sympatric speciation
Sympatric speciation

Sympatric and sympatry are terms from biogeography, referring to organisms whose ranges overlap or are even identical, so that they occur together at least in some places....
. 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
Sex linkage

Sex linkage is the phenotype expression of an allele that is related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosome chromosomes, where both sexes have the same probability of expressing the trait....
 species preference in flycatchers
Collared Flycatcher

The Collared Flycatcher, Ficedula albicollis, is a small passerine bird in the Old World flycatcher family, one of the four species of Western Palearctic black-and-white flycatchers....
.