| Summary: | Sexual selection is responsible for the evolution of formidable adaptations across the tree of life. Traditionally, sexual selection research has focused on male sexual displays and female choice. However, this approach ignores important social and environmental factors that can influence the operation of sexual selection. In this thesis, I aim to contribute to our knowledge of the effects of social and environmental factors, particularly those related to nutrition, on sexual selection. I investigate how the sexual behaviour of females and the nutritional and social environments of both sexes during larval and adult stages modulate sexual selection in the fruit fly Drosophila melanogaster. First, I assess how female promiscuity ("polyandry") affects the strength of sexual selection and patterns of assortative mating in freely interacting populations. There is no current consensus for the conflicting results of previous studies showing that polyandry can have positive, negative or have negligible effect on the opportunity for sexual selection. Using a genetic knockout that increases female sexual receptivity, I show that increasing polyandry reduces the opportunity for sexual selection and shifts the relative role of sexual selection from pre- to post-copulatory in males without affecting assortative mating patterns in freely interacting populations. Next, I consider how plastic responses to nutritionally poor larval environments and adult social environments modulate sexual selection, an area that has been considerably ignored. I find that although nutritionally poor larval environments reduce individuals' mating and reproductive success, plastic responses to social environments might mitigate against these disadvantages. Moreover, I show that plastic responses to larval and social environments influence the relative role of pre- and post-copulatory sexual selection in males, regulate offspring traits through trans-generational effects and determine population fate. Finally, I consider whether male reproductive traits have distinct macronutrient (protein and carbohydrate) requirements, and whether males can regulate their feeding to attain a diet that satisfies the requirements for these traits. I find that both a high short-term rate of offspring production when males mate with virgin females, and a high total number of offspring sired when males mate with previously mated females, require carbohydrate-rich diets, whereas male attractiveness requires a balanced (1:1) macronutrient diet. Furthermore, I show that male protein intake can negatively affect female long-term reproduction when males mate with virgins, but not with previously mated females, revealing a novel intersexual effect of male nutrition. Thus, this thesis deepens our understanding of key evolutionary processes by revealing the negative effects of high polyandry and nutritionally poor larval environments on the operation of sexual selection as well as uncovering male nutritional compromise in the expression of reproductive traits.
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