WHY ARE UNISEXUAL FLOWERS ASSOCIATED WITH WIND POLLINATION AND UNSPECIALIZED POLLINATORS

All the hypotheses reviewed that attempt to explain the association of unisexual flowers with pollination by wind or small pollinators do so in terms of advantages for unisexuality that are strongest under those modes of pollination. It is assumed that the selective pressures that actively promote the evolution of dioecy are absent, or weaker, in conditions when dioecy has failed to evolve. In contrast, the idea outlined by the author (that pollination not involving discriminating pollinators favors the evolution of female flowers) explains the associations simply in a terms of the removal of a selective barrier that is usually present. The realization that the presence or absence of necessary preconditions for some evolutionary change can determine the taxonomic and ecological distribution of certain characters is important when one attempts to give an evolutionary account of why some species evolve these characters while others do not. -from Author