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Pollinators may promote floral evolution by selectively visiting preferred floral phenotypes. Herbivores may also influence the evolution of floral traits, by reducing the fitness of poorly-defended plants, such as those without protective sepals. In this dissertation, I investigate the reproductive and evolutionary consequences of floral visitors for two floral traits of the Neotropical herb Chrysothemis friedrichsthaliana (Gesneriaceae). I focus on the trait male-biased nectar production, which results from coincident temporal shifts in nectar production rates and sexual function within flowers. First, I review theory relevant to male-and female-biased nectar production, and I generate hypotheses to explain its evolution in 41+ representative species. Next, I test natural and sexual selection hypotheses using C. friedrichsthaliana and its hummingbird pollinator Phaethornis striigularis (Trochilidae). Sexual selection theory predicts …