Because males and females of internally inseminating species interact directly during mating, adaptations in one sex in primary reproductive traits may trigger an evolutionary response in the other sex. Divergent postcopulatory sexual selection is considered the main driving force behind the evolution of many male and female reproductive traits, generating unique morphologies and physiologies that can contribute to reproductive isolation and, ultimately, speciation. 2. The focus of most previous studies of the evolution of primary reproductive characters has been male reproductive traits and ejaculate or sperm characteristics. However, in order to more fully understand the evolution of primary reproductive characters it is crucial that we also include female traits. 3. In insects, both the size and the composition of the ejaculate have been shown to influence female reproduction in numerous ways by affecting female remating behaviour, female fecundity and female life span. Here, we employ a phylogenetic comparative approach to assess correlated evolution between primary reproductive characters in males and those in females in a group of seed beetles (Chrysomelidae: Bruchinae). We further explore correlated evolution between ejaculate size and female fitness in these insects. 4. Our analyses revealed positive correlated evolution between three internal female reproductive traits and ejaculate weight as well as correlated evolution between ejaculate weight and female fitness. We discuss the causal factors behind this correlated evolution and suggest that the evolution of larger ejaculates, primarily by postcopulatory sexual selection, causes selection for larger primary sexual traits in females to allow females to more rapidly process ejaculates. This may then feedback on postcopulatory selection in males, reinforcing selection for larger ejaculates. 5. Our results show that the primary reproductive traits of males and females show correlated evolution and suggest that intersexual co-evolution may affect the evolution of female fitness.
Male-female coevolution is at the heart of biology. It is responsible for much of the diversity we see in behaviour and morphology, and it is thought to be an important engine of speciation. The pattern of intersexual coevolution is well established in many taxa, yet understanding of the processes responsible for male-female coevolution remains incomplete. By studying interspecific variation within a closely related group of species, we can gain important information about how traits and behaviours have evolved. In the work done for this thesis, we studied a group of seed beetle species. Our results show that male-female coevolution has been a strong force in shaping both behaviour and morphological traits that are associated with mating and reproduction such as, morphology of male and female genitalia and remating behaviour. The evolution of harmful male genitalia has often been suggested to be a product of sexually antagonistic coevolution, but understanding of these extraordinary adaptations is limited. By combining comparative and experimental methods we show that as seed beetle males evolve more spiny genitalia, harm to females is elevated. We provide evidence for the correlated evolution between these antagonistic adaptations in males, and a female counter adaptation (the amount of connective tissue in the copulatory duct). We also demonstrate that imbalance of relative armament of the sexes affects evolution of the costs and benefits of reproduction. As males evolve genitalia that are more harmful relative to the level of female counteradaptation, costs associated with mating for females increase and population fitness is depressed. Our results unveil a coevolutionary arms race between the sexes and are consistent with a proposed link between sexual conflict, species’ viability and the risk of extinction.