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Several recent studies have shown that males and females in some populations of dioecious plants are spatially segregated with respect to an environmental gradient. The inference is often made that such spatial segregation of the sexes (SSS) is favored by selection because it reduces competition between individuals of opposite sex (sexual "niche partitioning"). This paper was written to clarify the evolutionary interpretation of cases of SSS. We describe the possible proximate mechanisms capable of producing SSS and evaluate their plausibility. Then, we identify the selective factors that could favor the evolution of SSS. We argue that SSS can be favored if male fitness and female fitness respond differently across environments (because of differences in reproductive biology), that a reduction in competition between males and females is unlikely to be an evolutionary cause of SSS, and that differential mortality is unlikely to evolve as a proximate mechanism for achieving adaptive SSS. Such a pattern is r...

Arisaema triphyllum plants change sex: small individuals are males while large plants are females. Contrary to the expectations generated by the size-advantage model, though, reproductive success is independent of size for existing males and females, because it is pollinator-limited rather than resource-limited. Estimates of the likely reproductive effort that would be borne by nonexistent small females suggest, however, that there is a size threshold below which female reproduction is prohibitively costly. This threshold creates selection for sex-changing, but because of temporal fluctuations in seed production and sex ratio, the magnitude and even the direction of this selection are not constant. Such fluctuations make it unlikely that jack-in-the-pulpits ever attain the simple sort of optimum sex-changing strategy described by theory predicated on assumptions of population stability.