The shift from outcrossing to self-fertilization is a commonly observed evolutionary transition in angiosperms. While extensive research has examined the evolutionary associations between selfing and other plant traits, very few studies have investigated how mating systems influence sexual selection on pollen performance. In this dissertation, I evaluated previously unstudied aspects of mating system evolution and sexual selection in two pairs of Clarkia sister taxa with contrasting mating systems: the outcrosser, C. unguiculata and its selfing sister species, C. exilis, as well as C. xantiana ssp. xantiana and its selfing counterpart, C. x. parviflora. To test the prediction that, due to comparatively strong intrasexual competition for access to ovules, outcrossing taxa should evolve to produce faster growing pollen than their selfing relatives, I assessed pollen competition in natural populations of C. unguiculata and C. x.

xantiana and evaluated pollen performance in natural populations of all four focal taxa. I evaluated pollen competition by measuring pollen limitation in multiple populations of C. unguiculata and C. x. xantiana in 2008-2010. Pollen limitation was consistently absent in C. unguiculata populations, but reproduction in C. x. xantiana populations was pollen-limited in two of the three years of the study. This suggests that pollen competition may be strong in C. unguiculata populations, but weak in C. x. xantiana. In 2008, I compared pollen germination and pollen tube growth rates between sister taxa following one-donor crosses. Clarkia unguiculata exhibited faster pollen germination than C. exilis. By contrast, C. x. parviflora, a selfing taxon that exhibits rapid floral development rates, demonstrated faster pollen tube growth than C. x. xantiana. I next examined the relationships between pollen performance and physiological performance within each taxon. In C. x.

parviflora, an individual's mean pollen tube growth rate was positively correlated with its photosynthetic rate. In general, however, physiological performance was not associated with pollen performance. Finally, I used allozyme markers to generate outcrossing estimates for one population of each focal taxon and provide evidence that sister taxa differ in mating system. Collectively, the studies presented here further our understanding of mating system evolution and sexual selection in flowering plants.