Pollen Removal, Paternity, and the Male Function of Flowers

Abstract

Much recent research suggests that floral characteristics of hermaphroditic plants are often selected primarily because of their effects on male reproduction. Two recent challenges to this view have been based on the fate of pollen after it is removed by pollinators, something that is rarely measured in field studies. One objection is theoretical, holding that the large variance in the success of pollen after it is removed weakens selection through the male side (Wilson et al. 1994). The other objection is empirical. In a species in which pollen removal studies had suggested that flowers are primarily male, new paternity data have been used to argue that they are not (Broyles and Wyatt 1990a, 1990b, 1995). Here I argue that both objections are invalid, the theoretical argument because it uses the wrong measure of selection, and the empirical study because a confounding variable was not controlled. Bateman (1948) provided a foundation for understanding why males often act as if they take a greater interest in having many mates than females do. This difference arises because of an asymmetry in reproductive costs. Males commonly invest little or nothing in offspring, whereas female investment may be large. Therefore, reproductive success of females is limited by resources, and mating frequently has comparatively little effect on their fitness, whereas male success is limited by access to females and may increase linearly with the number of mates. Bateman's principle helped fuel an explosion of interest in sexual selection in animals, recently summarized by Andersson (1994). Bateman believed his principle to be nearly universal and explicitly included plants among the relevant organisms. However, sexual selection research on plants lagged behind that on animals, partly for historical reasons and perhaps partly because it was unclear how to apply the ideas to hermaphroditic organisms that were both male and female (reviewed in Willson 1994). It was eventually recognized how Bateman's principle might apply to these organisms (e.g., Charnov 1979; Wilson 1979; Willson and Burley 1983). For example, consider an animal-pollinated hermaphroditic plant. It needs to attract pollinators for two reasons: to receive pollen for its own seed (female function) and to have its pollen