Factors promoting the evolution of sexually dimorphic traits and their consequences on lineage diversification in Phyllophaga beetles (Coleoptera: Scarabaeidae)

Date of Completion

January 2008


Biology, Ecology|Biology, Entomology




The goal of my dissertation is to analyze the evolution of a sexually dimorphic trait in both males and females at multiple hierarchical levels. This work is divided into four chapters beginning with the systematics of the genus Phyllophaga (Coleoptera: Scarabaeidae) and working through analysis of a species group (fraterna), followed by an intraspecific comparative investigation, and concluding with a behavioral study at the individual level. ^ Chapter one generates a phylogenetic hypothesis for the genus Phyllophaga to trace the evolution of male and female genitalia and evaluate patterns of diversity. The results show that the ancestral form of male genitalia is symmetrical, evolving toward asymmetry in several major clades. In contrast, the evolution of female genitalia is more chaotic, with each distinct form appearing in several or all species groups. ^ Chapter two focuses on the fraterna species group and tests for hybridization and lineage sorting to explain rampant polyphyly of the species P. fraterna and P. hirticula. The results provide evidence for introgression between P. fraterna and P. hirticula, and these results are used to hypothesize the role of introgression and lineage sorting in the evolutionary history of the fraterna group. ^ The third chapter investigates genitalic divergence at the population level in the species P. hirticula by testing the hypothesis that genitalia are more geographically structured than a rapidly evolving mitochondrial marker. The results show that while female genitalia are more geographically structured than mtDNA, male genitalia are not. This pattern suggests that selection on female genitalic variation may have caused divergence of these structures among populations. ^ The last chapter focuses on polyandrous behavior in P. hirticula in an effort to determine the potential for post-copulatory sexual selection in this mating system. Paternity analysis of individual families reveals that broods are sired by an average of two males and that paternity share among fathers is not heavily biased. This result is surprising because beetles in the genus Phyllophaga are characterized by highly diverse species-specific male and female genitalia, an oft-cited trademark of post-copulatory sexual selection. Alternative hypotheses to explain genitalic diversity are discussed.^