Scientists have strived to understand the nature of the mating systems that allow plant species to flourish in their respective environments. The annual wildflower genus Clarkia contains several pairs of outcrossing and selfing sister taxa with different mating systems. The goal of this project is to: (1) determine the relationship between pollen performance and seed size in two self-compatible Clarkia sister taxa (C. exilis, a predominant selfer) and C. unguiculata (a predominant outcrosser) and (2) whether the nature of this relationship depends on the mating system of a given taxon. Numerous pollen samples and seed samples from 190 individual plants were extracted from eight sites across the Sierra Nevada foothill region in 2008. We weighed 30 seeds from each of the samples to obtain the average seed size of the taxa. In addition, fluorescence microscopy was used to measure pollen tube growth rates of each Clarkia taxon. We predicted that there would be a positive linear correlation between seed size and pollen performance for each taxon. Furthermore, we expected this relationship to be stronger in C. unguiculata (the outcrosser) taxa because of the potential for intra-male pollen competition to promote the evolution of rapid pollen tube growth.  Contrary to our expectation, we found that there was no relationship between pollen performance and seed size in either taxon.  We did detect, however, differences between the sister taxa in pollen performance and in seed size.  C. unguiculata had faster pollen tube growth rates and heavier average seed weight than its selfing sister species, C. exilis. Our project will provide information regarding the effects of environmental change and the evolutionary outcomes of mating system transitions in plants.