Sexual selection is a branch of natural selection which acts upon variation in reproductive success. Sexual selection is a complex field of study in biology as each species has their own mating system and strategies. Models of sexual selection theory are not mutually exclusive, and often times there are multiple layers of selection within a given mating system. For instance, both direct and indirect benefits of sexual selection can occur simultaneously, and selection can act both before and after mating occurs. Postcopulatory sexual selection, which is not as well understood, can be comprised of both the male-male interaction of sperm competition and the male-female interaction of cryptic female choice. Although there are many studies which show the existence of postcopulatory sexual selection, there is limited knowledge of its underlying mechanisms or genetic basis. Although we know of the physical male traits that females prefer, the relationships among male trait, female preference, and postcopulatory sexual selection are often unknown. Here I use Drosophila melanogaster to show accurate alternative measurements for female lifetime reproductive success (Chapter 2) and the genetic architecture underlying lifetime reproductive success (Chapter 3). I found that the short term reproductive success measure of 5 days can accurately predict the lifetime reproductive success of females, and that lifetime reproductive success is a result of additive genetic variation. In Chapter 4, I compared lifetime reproductive success to mating success in a multi-generational study and found that males who were more successful at mating produced sons with lower fitness. I then examined mechanisms of sperm competition, specifically the role of Acps (accessory gland proteins) in sperm competition. I discovered that Acps from the first male to mate are beneficial to the second mated male, contributing to second male advantage (Chapter 5). Lastly, in Chapter 6, I assessed male quality based on five fitness measures and determined male performance in both pre- and postcopulatory sexual selection. I concluded that a combined fitness measure most accurately predicted male offspring production. This thesis characterizes the various factors that contribute to variation in lifetime reproductive success, specifically from a sexual selection perspective
