Intensity of Reproductive Isolation between Sympatric and Allopatric Populations of Bufo americanus and Bufo fowleri

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Abstract
Hybrid crosses were made between different geographical representatives of the two partly sympatric species of toads, Bufo americanus and Bufo fowleri, to test the applicability of two evolutionary tenets; namely, Muller''s contention that the initial signs of reproductive isolation originate as by-products of adaptation, and the postulate, first advanced by Dobzhansky and Roller, that reproductive isolation should be stronger between two species in the areas of cohabitation than in the noncontiguous localities. The experimental results of the various intercrosses have indicated that genetical differentiation in certain geographically separated populations of toads has been associated with the early development of isolating mechanisms, and that no positive correlation exists between the territorial adjacency of the two species and the degree of reproductive isolation between them. In particular, crosses between sympatric populations of the two Bufo species from North Carolina, where natural hybridization occurs with some frequency, were far more successful than crosses between certain allopatric populations of the two species. A partial hybrid inviability has been demonstrated in crosses between B. fowleri from Louisiana (where only fowleri occurs) and B. americanus from North Carolina. However, there were no traces of hybrid incompatibility in crosses between B. americanus from Wisconsin (where only americanus occurs) and B. fowleri from North Carolina.