Structure and Dynamics of a Natural Hybrid Zone Between the Toads, Anaxyrus americanus and Anaxyrus hemiophrys, in Southeastern Manitoba

Hybrid zones are natural showcases for evolutionary processes. I studied a natural hybrid zone between two species of toads, Anaxyrus americanus and A. hemiophrys in southeastern Manitoba in order to determine the type of selection maintaining the hybrid zone and to test the applicability of the leading descriptive models. I mapped the cline between the two species based on data from morphological measurements, allozyme frequencies and DNA sequences. I also mapped the transition in habitat from one end of the zone to the other based on the extent of forest cover portrayed on topographical maps. The position of the cline centre was determined with maximum likelihood methods, and the coincidence of the clines in particular characters was evaluated with likelihood ratio tests. The history that I documented of back-and-forth movement of the morphological cline independent of the position of the habitat transition indicates the importance of endogenous selection as expected in "tension" zones. However, the general lack of coincidence of single-character clines, based on analysis of molecular data and allozymes, reveals that exogenous selection is the more likely explanation for this hybrid zone's maintenance. Thus, the bounded hybrid superiority model would best describe this particular hybrid zone.