Sex-Specific Genetic Architecture and Fitness Consequences of Subtly Dimorphic Traits in the Long-Lived Alpine Swift

Previous research has been disproportionately focused on the evolution and maintenance of male secondary sexual traits in species with polygynous mating systems and pronounced sexual dimorphism (SD), where sexual selection plays an obvious role. Further, the evolution of SD is seen as a potential resolution of intra-locus sexual conflict, which has the potential to be especially strong in species with weak SD. Consequently, our understanding of sexual selection and sexual conflict has remained incomplete. In this thesis, I contributed to addressing these gaps using a quantitative genetic approach to investigate how shared genetic architecture may drive sex-specific responses to selection in a bird previously characterized as monomorphic, the Alpine swift Tachymarptis melba. I first quantified SD in morphometric traits and suggest that the Alpine swift is rather subtly dimorphic. I found that fork length and body mass have the potential to be shaped by sexual selection and that their SD has changed over the past two decades. Thus, I further investigated these two traits. In regard to fork length, I found a cross-sex genetic correlation nearing one suggesting a strong genetic constraint, directional selection suggesting a possible role of mutual selection, and an absence of microevolution change in either sex despite phenotypic changes in females. An explanation for this change in morphology counter to evolutionary predictions—directional selection coupled with a strong cross-sex genetic correlation would be expected to result in shared evolutionary trajectories between the sexes—goes beyond the scope of this thesis. In regard to body mass, I found shared genetic architecture and that the shape of selection differed between the sexes suggesting the possible presence of an intra-locus sexual conflict. As SD in body mass has increased over the past two decades, I suggest that this may reflect a weakening of the genetic constraints imposed on this trait and that body mass therefore has the potential to evolve to sex-specific fitness optima. These results further suggest a need to consider the impact of pairing decisions, which could result in the observed pattern of reproductive success. Hence, I considered the fitness consequences of mate switching (‘divorce’), as this is the only way for these otherwise monogamous birds to modify pairing decisions and potentially increase individual reproductive success. I found that divorce was most likely to occur after poor reproductive success, suggesting that these birds follow a ‘win-stay lose-switch’ decision making paradigm, and that the benefits of divorce outweigh the costs of mate replacement itself. However, heritability was low in both sexes, suggested little scope for evolutionary change in this behavior. Overall, my thesis highlights the need to consider both sexes in studies of sexual selection and to broaden our current scope of model organisms to explicitly include species with cryptic SD, as only then can we reach a more complete understanding of these complex evolutionary processes.