| The process of how and why populations diverge and become different species is a topic of widespread interest. A complete understanding of the process of speciation requires that we understand the causes of divergence between populations, as well as the consequences of that divergence. Regardless of the causes of divergence, the consequences must include some degree of reproductive isolation in order for speciation to proceed. For two species of songbird, the American Redstart (Setophaga ruticilla) and the Black-throated Blue Warbler (Dendroica caerulescens), we find evidence that recent glacial cycles have contributed to allopatric and parapatric divergence, respectively. For the Black-throated Blue Warbler, we find that the causes of the divergence between populations may include sexual selection, as songs in the northern population have faster trills, and sexual selection favors fast trills in this population. The consequences of this divergence may include reproductive isolation, as males in the northern population respond more to local songs compared to non-local songs. However, in the southern population males do not discriminate between northern and southern songs, thus reproductive isolation may be asymmetric and incomplete. These results provide support for the hypothesis that recent glacial cycles may be contributing to diversification and speciation. In addition, we find a rare example of sexual selection acting to contribute to differences across populations, as well as a rare example of asymmetric discrimination, which may be one of the very first steps in the process of speciation. |
