Testing mechanisms of speciation and hybridization in terrestrial vertebrates

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There is abundant evidence for the process of speciation, whereby a single species evolves into two, but the forces that usually drive this process are controversial. Some propose that the evolution of reproductive isolation between species is driven by adaptation to differing habitats. Others propose that reproductive isolation evolves as a result of sexual selection driving continual change within species, resulting in incompatibilities when they come into contact. We will test these ideas by comparing patterns of trait divergence and reproductive isolation in a variety of species pairs in western Canada and central Siberia. Both regions were not forested during the Ice Ages. Now, each contains many contact zones between distinct western and eastern forms of birds that diverged in separate refugia. These contact zones provide a 'natural laboratory' for the study of how new species arise. Our goal is to study whether and how reproductive isolation is developing in multiple species pairs in these regions, and then to draw general conclusions regarding the factors that promote or hinder speciation. We will study geographic variation in genetics, songs, ecology, migratory behavior, and body shape and size with the goal of measuring reproductive isolation and its causes. In addition to helping resolve the relative importance of sexual and natural selection in speciation, these studies will examine the importance of (1) hybridization and (2) seasonal migratory behavior in speciation. To examine hybridization, we will compare three warbler hybrid zones and determine whether there are common parts of the genome under selection. We will also test whether the Audubon's warbler is the first known case of hybrid speciation in birds; preliminary data suggest it is. To examine seasonal migratory behavior, we will study variation in migratory behavior across a hybrid zone between two forms of Swainson's thrush; we hypothesize that the pure forms will have divergent migratory routes, while the hybrids will have intermediate routes and will suffer higher mortality. There are many migratory divides known in birds, and our study will be the first to directly test the idea that hybrids are selected against because of inferior migratory behaviors.