Population of hybrids beyond first hybrid generation
A red-bellied paradise flycatcher (Terpsiphone rufiventer) found in the gallery forests of Abuko, The Gambia (where many of the T. rufiventer are part of a hybrid swarm with T. viridis).
A hybrid swarm is a population of hybrids that has survived beyond the initial hybrid generation, with interbreeding between hybrid individuals and backcrossing with its parent types. Such population are highly variable, with the genetic and phenotypic characteristics of individuals ranging widely between the two parent types.[1] Hybrid swarms thus blur the boundary between the parent taxa. Precise definitions of which populations can be classified as hybrid swarms vary, with some specifying simply that all members of a population should be hybrids, while others differ in whether all members should have the same or different levels of hybridization.[2]
Hybrid swarms occur when the hybrid is viable and at least as vigorous as its parent types; and there are no barriers to crossbreeding between the hybrid and parent types. Swarms cannot occur if one of these conditions is not met: if the hybrid type has low viability, the hybrid population cannot maintain itself except by further hybridisation of the parent types, resulting in a hybrid population of low variability. On the other hand, if hybrids are vigorous but cannot backcross with parent populations, the result is hybrid speciation, which, aside from the contribution of new hybrids, evolves independently of its parent types. In either situation, it is possible for the hybrid population to overtake the parent populations.[3] In some cases, this can even occur within the span of just a few generations.[3] If the hybrid species has the greater viability, they can simply outcompete the parent species for resources, resulting in competitive exclusion, whereas if the parent species has the greater viability, gene flow between the two populations can result in decreased genetic variation.[3]
Hybrid swarms form within hybrid zones, an area where two similar species come into contact and hybridize.[4][5] These develop as a result of secondary contact between the parent species.[4] After a long period of geographic isolation, the cause of which may be either natural or man-made, the reoccurrence of the parent species in the same environment can lead to interbreeding, hybridization, and potentially, a hybrid swarm.[4][6] Hybrid swarms can lead to introgressive hybridization, or introgression, in which there is gene flow from the hybrid gene pool to the parental gene pool, or vice versa, occurring due to backcrossing between the populations.[6] When introgression has occurred, there will be a high level of diversity in alleles near the vicinity of the hybrid swarm.[7]
^Cockayne, L.; Allan, H.H. (1926). "The Naming of Wild Hybrid Swarms". Nature. 118 (2974): 623–624. doi:10.1038/118623a0. S2CID 4125412.
^Kalinowski, S T; Powell, J H (March 2015). "A parameter to quantify the degree of genetic mixing among individuals in hybrid populations". Heredity. 114 (3): 249–254. doi:10.1038/hdy.2014.93. ISSN 1365-2540. PMC 4815573. PMID 25388141.
^ abcWard, Jessica L; Blum, Mike J; Walters, David M; Porter, Brady A; Burkhead, Noel; Freeman, Byron (June 2012). "Discordant introgression in a rapidly expanding hybrid swarm". Evolutionary Applications. 5 (4): 380–392. doi:10.1111/j.1752-4571.2012.00249.x. ISSN 1752-4571. PMC 3353357. PMID 25568058.
^ abcLi, Yue; Tada, Fumito; Yamashiro, Tadashi; Maki, Masayuki (2016-01-22). "Long-term persisting hybrid swarm and geographic difference in hybridization pattern: genetic consequences of secondary contact between two Vincetoxicum species (Apocynaceae–Asclepiadoideae)". BMC Evolutionary Biology. 16: 20. doi:10.1186/s12862-016-0587-2. ISSN 1471-2148. PMC 4724111. PMID 26801608.
^ abHarrison, Richard G.; Larson, Erica L. (2014-01-01). "Hybridization, Introgression, and the Nature of Species Boundaries". Journal of Heredity. 105 (S1): 795–809. doi:10.1093/jhered/esu033. ISSN 0022-1503. PMID 25149255.
^Twyford, A. D.; Ennos, R. A. (2011-09-07). "Next-generation hybridization and introgression". Heredity. 108 (3): 179–189. doi:10.1038/hdy.2011.68. PMC 3282392. PMID 21897439.
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