Secondary contact is the process in which two allopatricaly distributed populations of a species are geographically reunited. This contact allows for the potential for the exchange of genes, dependent on how reproductively isolated the two populations have become. There are four primary outcomes of secondary contact:  fusion of the two populations back into one, reinforcement, the formation of a hybrid zone, and the formation of a new species through hybrid speciation. Extinction One of the two populations may go extinct due to competitive exclusion after secondary contact. This tends to happen when the two populations have strong reproductive isolation and significant overlap in their niche.   The population that goes extinct may leave behind some of its genes in the surviving population if they hybridize.  However, if hybridization is so common that the resulting population received significant amount of genetic contribution from both populations, the result should be considered a fusion. Fusion The two populations may fuse back into one population. This tends to occur when there is little to no reproductive isolation between the two. During the process of fusion a hybrid zone may occur.  Reinforcement is the evolution towards increased reproductive isolation due to selection against hybridization. This occurs when the populations already have some reproductive isolation, but still hybridize to some extent  .
Secondary contact is the process in which two allopatricaly distributed populations of a species are geographically reunited. This contact allows for the potential for the exchange of genes, dependent on how reproductively isolated the two populations have become. There are several primary outcomes of secondary contact: extinction of one species, fusion of the two populations back into one, reinforcement, the formation of a hybrid zone, and the formation of a new species through hybrid speciation. Extinction One of the two populations may go extinct due to competitive exclusion after secondary contact. This tends to happen when the two populations have strong reproductive isolation and significant overlap in their niche. A possible way to prevent extinction is if there is an advantage to being rare. For example, sexual imprinting and male-male competition may prevent extinction. Yang, Y., Servedio, M. R., Richards-Zawacki, C. L. (2019). Imprinting sets the stage for speciation. Nature, 574(7776), 99-102. The population that goes extinct may leave behind some of its genes in the surviving population if they hybridize. For example, the secondary contact between Homo sapiens and Neanderthals, as well as the Denisovans, left traces of their genes in modern human. However, if hybridization is so common that the resulting population received significant amount of genetic contribution from both populations, the result should be considered a fusion. Fusion The two populations may fuse back into one population. This tends to occur when there is little to no reproductive isolation between the two. During the process of fusion a hybrid zone may occur. This is sometimes called introgressive hybridization or reverse speciation. Concerns have been raised that the homogenizing of the environment may contribute to more and more fusion, leading to the loss of biodiversity. Seehausen, O. (2006). Conservation: losing biodiversity by reverse speciation. Current Biology, 16(9), R334-R337. Reinforcement is the evolution towards increased reproductive isolation due to selection against hybridization. This occurs when the populations already have some reproductive isolation, but still hybridize to some extent . Because hybridization is costly (e.g. giving birth and raising a weak offspring), natural selection favors strong isolation mechanisms that can avoid such outcome, such as assortative mating.Kirkpatrick, M. (2000). Reinforcement and divergence under assortative mating. Proceedings of the Royal Society of London. Series B: Biological Sciences, 267(1453), 1649-1655. Evidence for speciation by reinforcement has been accumulating since the 1990s .
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extinction of one species,
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A possible way to prevent extinction is if there is an advantage to being rare. For example, sexual imprinting and male-male competition may prevent extinction. Yang, Y., Servedio, M. R.,
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Richards-Zawacki, C. L. (2019). Imprinting sets the stage for speciation. Nature, 574(7776), 99-102.
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For example, the secondary contact between Homo sapiens and Neanderthals, as well as the Denisovans, left traces of their genes in modern human.
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This is sometimes called introgressive hybridization or reverse speciation. Concerns have been raised that the homogenizing of the environment may contribute to more and more fusion, leading to the loss of biodiversity. Seehausen, O. (2006). Conservation: losing biodiversity by reverse speciation. Current Biology, 16(9), R334-R337.
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. Because hybridization is costly (e.g. giving birth and raising a weak offspring), natural selection favors strong isolation mechanisms that can avoid such outcome, such as assortative mating.Kirkpatrick, M. (2000). Reinforcement and divergence under assortative mating. Proceedings of the Royal Society of London. Series B: Biological Sciences, 267(1453), 1649-1655. Evidence for speciation by reinforcement has been accumulating since the 1990s