Evolution of Dioecy in Flowering Plants

Abstract

Dioecy, characterized by the presence of distinct male and female plants, is widespread in angiosperms, being known in 37 out of Engler & Prantl's 51 orders (132). Yet botanists have paid little attention to dioecy, particu­ larly to selective forces underlying its evolution. Several factors have con­ tributed to this neglect. First, though there are many dioecious species, the proportion of such species in the world's flora is reputed to be quite small (37, 53, 132). The presumed rarity of the dioecious condition has led to the belief that it is not a particularly successful mode of reproduction (53, 125). Second, dioecy has been compared with self-incompatibility and, because 50% of the dioecious plants lose the capacity to bear seeds, it has been considered a poor substitute for self-incompatibility (3, 53). Third, selection for outcrossing has been almost universally proposed as the principal selec­ tive force responsible for the evolution of dioecy (3, 16-18, 26, 45, 67, 84-86, 98, 101, 103). The proposal has little empirical support, but one of its consequences was that biologists took for granted the outcrossing advantage as the principal factor and, until recently (10, 21, 128), did not consider other factors influencing the evolution of dioecy. I wish to argue that dioecy is not as rare as is generally assumed. In certain regions, among certain life forms, more than one fourth of all species may be dioecious (see below). In addition, I argue that the evolution of dioecy is not entirely due to selective pressure for increased outcrossing. By