The diversity of sexual systems in plants has been generally attributed to selection for an optimal amount of genetic recombination. However, sexual systems such as hermaphroditism (including heterostyly), monoecism, andromonoecism, gynomonoecism, dioecism, androdioecism, and gynodioecism may also be viewed as different patterns of relative resource allocation to paternal and maternal functions to optimize paternal and maternal reproductive success in different ways. These different patterns may arise in large part in response to reproductive competition resulting from sexual selection. But the efficacy of sexual selection in zoophilous species is mainly determined by pollinator behavior. It follows then that the evolution of a particular sexual system must be influenced by the dynamics of the pollination system. The role of pollinators in the evolution of sexual systems is examined by considering several types of interactions between flowers and pollinators. The role of cost-sharing between paternal and maternal functions in pollinator attraction is stressed in the evolution of hermaphroditism. Andromonoecism is considered in terms of loss of pistils in that part of the flower crop which is produced merely to attract pollinators and/or to fulfill male function. In the evolution of andromonoecism to monoecism, the role of stamens of hermaphroditic flowers in the functional integrity of the pollination system is evaluated. The importance of long mouth parts of pollinators to promote compatible pollinations in the evolution of heterostyly is pointed out. The evolution of protogyny is considered in relation to long inhabitation of pollinators in flowers and inflorescences. The evolution of dioecism is examined in relation to the ability of pollinators to respond to minor changes in floral resources thereby altering the patterns of pollen donation and pollen receipt. Finally, the importance of stamens in hermaphroditic plants in attracting pollen collecting bees is emphasized in the maintenance of androdioecism. The development of a general hypothesis to explain the diversity of sexual systems will require not only a comprehensive knowledge of pollination ecology but also a revision of the sexual system classification that will take into account functional gender rather than intrinsic gender estimates based solely on morphology.The flowering plants display a wide variety of sexual systems ranging from obligate selfing in association with self-compatibility to obligate outcrossing in conjunction with self-incompatibility (

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Sexual systems of tropical lowland rain forest trees were investigated to estimate the relative proportions of hermaphroditic, monoecious, and dioecious species. Controlled hand pollinations were performed on hermaphroditic species to determine the proportions of self‐compatible and self‐incompatible species. A total of 333 species was examined; 65.5% species were found to be hermaphroditic, 11.4% monoecious, and 23.1% dioecious. There were no differences between canopy and subcanopy habitats in the distribution of various sexual systems. Out of the 28 species subjected to controlled pollinations, 24 showed the presence of self‐incompatibility. The site of the incompatibility barrier in distylous species was either in the stigma or in the style. In most of the monomorphic species, the incompatibility barrier was in the ovary. Although a majority of tree species in the rain forest appear to be obligate outcrossers (xenogamons), the potential for inbreeding due to small effective population size still remains to be explored.