Sympatric sister species are predicted to have greater divergence in reproductive traits than allopatric sister species, especially if mating system shifts, such as the evolution of self-fertilization, are more likely to originate within the geographic range of the outcrossing ancestor. We present evidence that supports this expectation-sympatric sister species in the monkeyflower genus, Mimulus, exhibit greater divergence in flower size than allopatric sister species. Additionally, we find that sympatric sister species are more likely to have one species with anthers that overtop their stigmas than allopatric sister species, suggesting that the evolution of automatic self-pollination may contribute to this pattern. Potential mechanisms underlying this pattern include reinforcement and a stepping stone model of parapatric speciation.