Multistage scenarios for the evolution of polymorphisms in birds

One of the most striking characteristics of the animal kingdom is the extent of phenotypic differentiation between the sexes. Selection acts on the phenotype and sex dependent polymorphisms, such as sexual-size dimorphism, result from selection favouring different optima for males and females. Macroevolutionary patterns of morphological variation are inherently fascinating and warrant explanation because they contribute importantly to the phenotypic component of biodiversity. Understanding the evolution of these patterns is complicated because the majority of the genome is shared between males and females, and this could constrain the evolution of sex-specific phenotypes. In this thesis, I develop tools and use existing evolutionary theory to derive testable predictions regarding the evolution of a small but diverse set of morphological polymorphisms in birds. First, I develop tools, in the form of explicit phylogenetic hypotheses, to analyze the evolution of sexual-size dimorphism (SSD) within the order Galliformes (landfowl). I examine the indirect effect that sexual selection acting on males can have on female life history traits (egg size and clutch size), and the roles that sexual selection and divergence in the reproductive roles of males and females contribute to SSD. Second, I evaluate whether intersexual competition on the wintering grounds is implicated in the evolution of disproportionate bill-length dimorphism, which is common in migratory shorebirds (Charadriiformes). To do this, I test predictions derived from the niche differentiation hypothesis, using the western sandpiper (Calidris mauri) as a model shorebird differential migrants. Finally, I consider an enigma in avian reproductive biology, the evolution of an extreme form of intraclutch egg-size dimorphism that is unique to the six species of Eudyptes penguins (Sphenisciformes). Through these examples I provide evidence that apparently simple polymorphisms, such as SSD, often require multistage explanations.