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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 reported in 21 of 32 species studied, but this may overestimate its true natural frequency. Few studies have sought evidence that an observed pattern of SSS evolved in response to selection, and few have ruled out nonadaptive differential mortality as the cause of the pattern observed. No study to date has demonstrated that competition between males and females is ultimately responsible for SSS. We outline the design of the critical experiments that need to be done to implicate niche partitioning as an ultimate cause of SSS. The evidence currently available indicates that most reported cases of SSS (whether due to adaptation or differential mortality) are caused by differences in the reproductive biology of male and female plants.
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