An evaluation of narrow hybrid zones in vertebrates

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, "crystallization" of an antihybridization mechanism might be prevented by naive immigrants from the parental populations even 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 adeguately explain the persistence of hybrid populations that do not receive a substantial influx of genes from both parental populations. The hybrid-superiority hypothesis is consistent with the various sizes, shapes, and positions reported for stable hybrid zones because, under this hypothesis, the range of a hybrid population is determined by the range of environmental conditions within which the hybrids are superior. Although there are exceptions, most vertebrate hybrid zones are, in fact, narrow. The hybridsuperiority hypothesis must accommodate this fact. The additional hypothesis is offered that hybrids, in some cases, can succeed in environments where competition from parental phenotypes is weak. Thus, hybrid populations are often found in areas devoid of stable ecological communities. Ecotones are one such area, and I suggest that stable hybrid zones are often narrow because they tend to occur in ecotones which are themselves nllrrow. INTRODUCTION T HE EVENTS which ensue secondary contact between two morphologically and presumably genetically distinct vertebrate populations are obscure. Certainly, hybridization is to be expected if the taxa in question have not diverged too far, but the final outcome is unpredictable. It is widely believed that natural hybridization is ephemeral, leading ultimately to either speciation i.e., the perfection of reproductive isolationor to fusion of the two races through introgressive hybridization (e.g., Dobzhansky, 1940; Sibley, 1957; Wilson, 1965; Remington, 1968). This hypothesis no longer seems tenable, however, because there are several examples of hybrid zones which appear to be stable (see Short, 1970, and below p. 266 ff.). Short ( 1969) made a distinction between a hybrid zone and a zone of overlap and hybridization; both are subcategories of zones of secondary intergradation. Although this could be an important distinction, it can rarely be made with regard to published reports of secondary intergradation zones. In this article I have used the term hybrid zone interchangeably with zone of secondary intergradation in order to reduce verbiage. The hypothesis that hybrid zones are always ephemeral is based on two concepts: the concept of coadapted gene complexes; and the concept that gene flow is a strong cohesive force