Abstract
A central goal of evolutionary biology is to understand the sources of phenotypic variation within and among species. Sexually selected traits, those that confer an advantage in both securing a mate and in fertilization, are often highly diverse, especially in males. Though research has historically focused on extravagant traits, such as plumage or horn size, relatively little research has focused on traits that experience selection after mating, or during postcopulatory selection. Sperm is the most diverse of all cell types, ranging four orders of magnitude in size across all animals, and is central to male reproductive success. Because males of all animal species produce sperm, studying this cell provides a unique model for exploring how sexual selection shapes the evolution of traits associated with postcopulatory selection. In this dissertation, I use an integrative approach to compare the processes that generate variation within a species, to the patterns of evolution in sperm morphology among species of Anolis lizards. As sexual selection occurs in two episodes (pre- and postcopulatory sexual selection), I first tested for correlated evolution between targets of each episode of selection in snakes and lizards. I found a negative relationship between sexual size dimorphism and residual testis size, suggesting that precopulatory selection constrains the opportunity for postcopulatory selection to occur, and/or that targets of each episode of selection experience an energetic trade-off. Among species of anoles, I then demonstrated that the sperm midpiece length evolves faster than the rest of the sperm cell, but evolves much more slowly than residual testis size. The differences in these rates of evolution indicate that sperm production may be more evolutionarily labile, or more important for reproductive success than sperm morphology in Anolis lizards. In both experimental diet treatments and in wild populations of Anolis sagrei I demonstrate that the sperm midpiece length is condition-dependent. In this same study, I showed that fertilization success is condition-dependent, which may be partially mediated by condition-dependent variation in sperm morphology, sperm count, or mating frequency. I also found consistent differences in sperm morphology between native and introduced populations of three species of Anolis lizards, suggesting that the environment may influence sperm morphology either through phenotypic plasticity or by genetic adaptation. Finally, I tested for correlations between sperm morphology and sperm velocity in a wild population of brown anoles and found that sperm midpiece length was positively correlated with sperm velocity in this population. These studies reveal high variation in sperm morphology within individuals, between populations, and across species, and demonstrate that this variation is phenotypically plastic, is related to sperm function, and may be associated with male reproductive success. These results also suggest that sperm number and/or copulation rate are more important for fertilization success than sperm morphology in anoles.
