Theoretical models suggest that geographic overlap with different heterospecific assemblages can promote divergence of mate recognition systems among conspecific populations. Divergence occurs when different traits undergo reproductive character displacement across populations within a contact zone. Here, I tested this hypothesis by assessing patterns of acoustic signal divergence in two- and three-species assemblages of chorus frogs (Pseudacris), focusing in particular on P. feriarum and P. nigrita. In addition, I tested one criterion for reinforcement, by examining the evolution of female P. feriarum preferences in the contact zone. Patterns of signal evolution indicated that in each of the four sympatric populations studied, only the rarer species displaced substantially (P. feriarum in three cases and P. nigrita in one instance). Moreover, the three displaced P. feriarum populations diverged in different signal traits across the contact zone, evolving in directions that increased the energetic cost of calling relative to the allopatric call, and in ways that maximized differences from the particular heterospecific assemblage present. Consistent with reinforcement, divergence of female preferences in sympatry was estimated to reduce their propensity to hybridize by 60%. Together, signal and preference data suggest that interactions between species can promote diversification within species, potentially contributing to reproductive isolation among conspecific populations.
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Vol. 63 • No. 5