| Divergence in sexually selected traits can lead to shifts in mating strategies, long-term phenotypic change, and speciation, yet the causes of variation in sexually selected traits in different environments is poorly studied. In this thesis I use measurements of song and plumage in two species of Asian warblers to examine how sexual selection interacts with environmental variation within populations over time and among populations in different habitats. I show that climate at different time points has large and opposing effects on the size of a sexually selected plumage trait in Phylloscopus humei, suggesting that climate change can disrupt mating systems and parental investment strategies. Song is a pre-mating barrier to reproduction in Phylloscopus trochiloides. I find that the relative roles of male competition and female choice in shaping song vary in different habitats, resulting in geographical variation in song traits. Divergence in song among populations is thus driven by the interaction between sexual selection and ecology. My results highlight the dynamic nature of the relationship between sexual and natural selection, and indicate that considering ecological context is necessary to understand variation in the strength, direction, and targets of sexual selection. |
