A particular group of traits regularly co-occur: species that evolved dioecy via monoecy often show sexual specialization or compensation, they may be able to change sex, they display spatial segregation of the sexes, and they appear to have evolved primarily from ancestors that already produced unisexual flowers and possessed an outbreeding device such as dichogamy (so they would not be candidates for the IA model).
Species that have developed dioecy via monoecy should remain somewhat susceptible to producing flowers of the "opposite sex" under extreme environmental conditions, or, if they are observed over several years (or their reproductive lifetime), they should show more frequent production of flowers of the "wrong" sex.
Some instances of sexual niche partitioning are due to environmental induction of flowering in the male or female phase (see Bierzchudek & Eckhart, 1988, for examples); according to the argument presented above, this should occur only in species that are evolving along the hermaphrodite [right arrow] monoecy [right arrow] dioecy pathway, i.
We would also predict that, because of stabilizing selection exerted by the need for pollinator recognition, floral sexual dimorphism should be less marked in these insect-pollinated plants, compared to wind-pollinated plants that share their derivation via monoecy.
Thus, "dioecy" in this case seems to have evolved via monoecy with heterodichogamy (a known outbreeding device) by increasing the temporal separation of the male and female phase (as postulated by Lloyd, 1979).
Thus, spinach has all the features of the constellation of traits we proposed above for the evolution of dioecy via monoecy, with sexual selection favoring specialization as males and females.