The ornaments and weapons of male animals are among the showiest and seemingly most wasteful of nature's productions. Darwin's theory of sexual selection showed how such traits could be selected through competition for mates even if they were otherwise detrimental1. Flowering displays of plants often show a comparable degree of gaudiness and profligacy, but exploration of the role of sexual selection in plants has only just begun2-5. Bateman6 argued that sexual selection is caused by the greater ability of males than females to increase fitness by mating repeatedly, due to the females' greater energetic commitment to gametes or parental care. Similar reasoning applies to hermaphrodites2,3. In hermaphroditic milkweeds, most young fruits are aborted7-10 and female reproduction (seeds) is limited more by resources than by pollination11. Sexual selection theory therefore predicts that traits increasing mating success will have evolved because they increase male success through pollen. Here I report that a suite of floral traits of a hermaphroditic plant is best interpreted as having evolved through the male competition component of sexual selection, a result with important implications for evolutionary studies of pollination systems.