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. Hybrid swarms thus blur the boundary between the parent taxa.
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.
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