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The concept of sexual selection (intrasexual competition for mates and mate preference) is used in revising the classical explanation of dioecy in plants. Male and female functions of hermaphroditic flowers can be subject to different sexual selection pressures, which may conceivably lead to the separation of male and female structures. This suggestion does not exclude genetic advantages that may accrue from outcrossing; the two aspects of selection probably operated together in the evolution of dioecy. The hypothesis regarding the importance of sexual selection could be tested by monitoring variation in individual fitness via pollen donation and via ovule maturation. The evolution of pollinia and other large pollen dispersal units is also viewed in terms of sexual selection. Availability of a reliable pollen vector is a prerequisite to the evolution of pollen packages. From the point of view of a flower functioning as a male, large PDU's ensure fertilization of more ovules and possible preemption of stigmatic surfaces. From the point of view of a flower functioning as a female (perhaps the same flower), less energy may be devoted to fruits with a partial complement of seeds. Furthermore, large PDU's may provide a means by which females can assess the relative masculinity of potential mates.
Mary F. Willson (Fri,) studied this question.