The structure and dynamic history of a mosaic hybrid zone are investigated using an analysis of morphometric variation to determine the extent of interbreeding between the species involved in different regions. The hybrid zone between the toads Bufo americanus and Bufo fowleri is geographically widespread. Natural hybrids occur in Ontario at Long Point but not along the nearby Niagara peninsula although the species are sympatric in both places. Discriminant analysis of variation in 27 morphological characters from 286 preserved museum specimens of male toads and putative hybrids was compared with previous allozyme genetic characterization of the same specimens. Reference samples from allopatric populations were used for comparison. Bufo americanus from Long Point were significantly different from other samples of the same species, demonstrating introgression in the direction of sympatric B. fowleri. Long-term hybridization in this region has been confined to Long Point despite sympatry elsewhere. Thus, the hybrid zone between these toads does not conform to gradient or “tension zone” models. Rather, it is a mosaic in that hybrids appear at some times and in some places but not in others and it conforms to the concept of a “localized sympatric” hybrid zone.

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