Co-occurring species of flowering plants may compete for pollination, which can cause character displacement by altering natural selection on floral traits. In a gynodioecious species, competition for pollination may also affect the evolution of sexual dimorphism of floral traits by influencing sex-specific selection. I demonstrated that Mimulus ringens did not affect seed set of gynodioecious Lobelia siphilitica, indicating that it is not a competitor for pollination. The presence of M. ringens did not alter selection upon most floral traits of L. siphilitica. I detected sex-specific selection upon five floral traits, supporting the hypothesis that sexual dimorphism evolves in response to sex-specific selection, rather than pleiotropic effects. My results also suggest that the presence of a co-flowering species may provide a context for sex-specific selection, and therefore, influence sexual dimorphism. Thus, my results suggest a link between the fields of study of competition for pollination and the evolution of sexual dimorphism. / NSERC, OGS, Ontario Innovation Trust, Canada Fund for Innovation