Title: On the Evolution of Sex and its Consequences
Abstract: This dissertation is an empirical and theoretical examination of the evolution of sex, sex-determining mechanisms, and sexual selection. Chapter 1 surveys adult sex ratios across species and populations of Anolis lizards to determine whether increased sexual selection — as proxied by sexual dimorphism — is associated with skewed adult sex ratios. Chapter 2 develops an evolutionary model to explain the simultaneous evolution of asexuality and increased ploidy in angiosperms, based on the biology of the well-studied asexual genus Taraxacum (dandelions). Chapter 3 analyzes the stochastic evolutionary dynamics of transitions between male and female heterogamety (XX/XY and ZW/ZZ), and discovers that evolution along the paths of (deterministic) equilibria that link these two systems is non-neutral, even when all genotypes are equally fit. Instead, an emergent ‘drift-induced’ selective force biases substitution rates in favor of dominant sex-determining mutations. Chapter 4 proposes a theoretical explanation for the prevalence of genetic versus environmental sex determination. Under environmental sex determination, a gene that increases the probability that its bearers develop as one of the sexes couples with a sexually antagonistic gene that is beneficial in that sex but detrimental in the other. The coupled haplotype invades, and recruits more sex biasing and sexually antagonistic alleles, eventuating in a sex chromosome, i.e., genetic sex determination. Chapter 5 proposes a novel explanation for the evolution of female preferences for costly male traits, based on the selfish genetic interests of sex chromosomes within the genome. Female-biased genetic elements, such as the W and X sex chromosomes, evolve mating preferences for males displaying traits that reduce those males’ fitness (or that of their male offspring) but increase the fitness of female offspring. This process may explain differences in costly ornamentation and behavior across species with divergent sex-determining mechanisms. Chapter 6 reanalyzes the logic underlying another theory for the evolution of female mating preferences for costly male traits, Fisherian runaway selection, and identifies two distinct channels through which it operates in diploids.
Committee: Jonathan Losos and David Haig (Advisors), Naomi Pierce, Scott Edwards, Martin Nowak