Abstract
In promiscuous species, the extent to which males experience stronger sexual selection compared to females may depend on the relative contributions of pre- or post-copulation processes in determining fitness. Moreover, pre-copulatory selection can be reinforced or opposed by post-copulatory selection depending on the extent of male competition, variation in resource availability, and overall degree of promiscuity in the population. However, we do not know which episode of sexual selection is more important in shaping trait variation and if the covariance among corresponding components of fitness is consistent across variable social and ecological contexts. In this dissertation, I disentangle the relative roles of pre- and post-copulatory processes in shaping variance in reproductive fitness and selection on traits in a wild population of brown anole lizards (Anolis sagrei). Brown anole males are larger than females and mate promiscuously, suggesting a strong opportunity for pre- and post-copulatory selection. In Chapter 1, I first assess if the overall opportunity for and strength of phenotypic selection due to reproductive success in males is higher compared to females and the extent to which these patterns are consistent across annual variation in social contexts such as sex ratio, population size, and degree of multiple mating. Using phenotypic measurements and genetic parentage data obtained for a closed island population over five breeding seasons, I find that males have a significantly higher total opportunity for selection, steeper Bateman gradients, and experience stronger selection on body size than females. These sex differences are more pronounced when the population is less female-biased, larger, and less promiscuous. However, a substantial portion of selection on body size is unexplained by mating success (pre-copulatory fitness), suggesting the need to assess the covariance between pre- and post-copulatory selection in males. In Chapter 2, I partition the total opportunity for selection in males into its independent and co-varying pre- and post-copulatory fitness components to find that the opportunity for pre-copulatory selection is consistently greater than the opportunity for post-copulatory selection. Post-copulatory selection is prominent only in years when the population is highly female-biased, smaller in size, and the degree of promiscuous mating is high. Larger males also consistently mate with more females and sire a larger proportion of offspring per mate across all years, indicating that the opportunity for and strength of pre-copulatory selection is reinforced by post-copulatory processes (positive covariance). These results may explain why we observe strong male-biased sexual size dimorphism in this species despite incomplete mate monopolization. Nonetheless, matings that do not result in offspring cannot be detected when deriving metrics of pre-copulatory selection from genetic parentage data alone. Strong reinforcement by post-copulatory selection can also result in mis-estimation of pre-copulatory selection. Thus, it is important to corroborate genetic estimates of mating success with direct observations of copulations, although the latter can be challenging. In Chapter 3, I develop an alternative method to estimate size-specific mating rates. Using the copulatory transfer of fluorescent powder from males to females, I show that larger males attain more copulations and mate with more females rather than more fecund females and that body size is under strong pre-copulatory selection, separate from the effects of post-copulatory selection. Collectively, my dissertation provides key insights into the role of pre-copulatory processes in mediating sexual selection in a population where the natural range of mating interactions can take place in a population and the extent to which social context may modulate it.
