Theory suggests that coevolution drives diversification in obligate pollination mutualism, but it has been difficult to disentangle the effects of coevolution from other factors. We test the hypothesis that differential selection by two sister species of pollinating yucca moths (Tegeticula spp.) drove divergence between two varieties of the Joshua tree (Yucca brevifolia) by comparing measures of differentiation in floral and vegetative features. We show that floral features associated with pollination evolved more rapidly than vegetative features extrinsic to the interaction and that a key floral feature involved in the mutualism is more differentiated than any other and matches equivalent differences in the morphology of the pollinating moths. A phylogenetically based, ancestral states reconstruction shows that differences in moth morphology arose in the time since they first became associated with Joshua trees. These results suggest that coevolution, rather than extrinsic environmental factors, has driven divergence in this obligate pollination mutualism.