Abstract
A review of the literature on vertebrate hybridization reveals the existence of a number of narrow hybrid zones. Three hypotheses have been suggested to explain the occurrence of these zones. The ephemeral-zone hypothesis states that hybridization will end either in speciation or fusion of the hybridizing taxa by means of introgression. The dynamic-equilibrium hypothesis allows the possibility that narrow hybrid zones might be stable: where hybrids are confined to a small area by steep selection gradients, crystalization of an antihybridization mechanism might be prevented by naive immigrants from the parental populations eve though hybrids are selected against. The hybrid-superiority hypothesis states that hybrids are more fit than parental phenotypes in some environments. The ephemeral-zone hypothesis fails to explain the antiquity and apparent stability of several hybrid zones. The dynamic-equilibrium hypothesis does not adequately explain the persistence of hybrid populations that do not receive a s...
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