Reinforcement generates reproductive isolation between neighbouring conspecific populations of spadefoot toads.

Reproductive character displacement is the adaptive evolution of traits that minimize deleterious reproductive interactions between species. When arising from selection to avoid hybridization, this process is referred to as reinforcement. Reproductive character displacement generates divergence not only between interacting species, but also between conspecific populations that are sympatric with heterospecifics versus those that are allopatric. Consequently, such conspecific populations can become reproductively isolated. We compared female mate preferences in, and evaluated gene flow between, neighbouring populations of spadefoot toads that did and did not occur with heterospecifics (mixed- and pure-species populations, respectively). We found that in mixed-species populations females significantly preferred conspecifics. Such females also tended to prefer a conspecific call character that was dissimilar from heterospecifics. By contrast, females from pure-species populations did not discriminate conspecific from heterospecific calls. They also preferred a more exaggerated conspecific call character that resembles heterospecific males. Moreover, gene flow was significantly reduced between mixed- and pure-species population types. Thus, character displacement (and, more specifically, reinforcement) may initiate reproductive isolation between conspecific populations that differ in interactions with heterospecifics.