Darwin made the point (widely ignored, despite its persuasiveness) that plants do not need dioecy in order to avoid inbreeding; there are effective alternatives.
In most cases of dioecy studied so far, close relatives that are not dioecious possess one or more of these outbreeding mechanisms (Rennet & Ricklefs, 1995).
Therefore, if any species have evolved dioecy driven by selection for outbreeding, these are likely candidates.
Dioecy is particularly associated with fleshy fruit (Renner & Ricklefs, 1995).
In order for these allocational differences to influence the evolution of dioecy, males and females must differentially respond to resource availability, as has been shown in some species (see Freeman & McArthur, 1982; Vitale et al.
Monoecy and dioecy are disproportionately common among wind-pollinated plants (Darwin, 1877; Freeman et al.
In their recent review of dioecy in the world's flora, Renner and Ricklefs (1995) found (1) that dioecy has evolved disproportionately often from taxa that had previously evolved unisexual flowers (i.
Also the lack of perianth parts makes it easier to evolve dioecy.
Selection for sexual specialization may well play a role in the generation of sexual dimorphisms seen at all of these levels and thus could provide the selective impetus for the evolution of dioecy from monoecy.
Dioecy evolves from gynodioecy via the spread of a gene that suppresses ovary development (B.