The role of sex ratio as a context for selection in Silene vulgaris

When the sex ratio varies among populations, it is likely that the fitness consequences of sexual phenotypes will also differ among those populations. Sex ratio can also vary at a fine scale within populations, and so there may be multiple, potentially overlapping scales over which sex ratio can affect the evolutionary process. I explored the fitness consequences of sex ratio variation in Silene vulgaris, a perennial herb that maintains a sexual polymorphism between plants that produce both pollen and seed, and plants that are only capable of producing seed. My results demonstrate that there is variance in the local sex ratio individuals experience within wild populations, which creates an opportunity for selection to drive differences in fitness among individuals at a fine scale. Further, I show that nocturnal insects, which are the effective pollinators of this species, do not exhibit preferences for the sex of individuals or the sex ratio of groups. This lack of preference in pollinator behavior means that pollinator service should be uniform among individuals regardless of sex or sex ratio, and so fine-scale variation in pollen availability is likely to generate heterogeneous fitness consequences for sexual phenotypes. Finally, I use experimental populations to show that both sexes have the highest fitness when they are rare, and that the direction and magnitude of selection on floral traits is substantially altered by sex ratio. Thus, the sex ratio an individual experiences is a critical context in which selection on sexual phenotypes occurs, and should have dramatic effects on the evolution of those phenotypes at multiple scales within and among populations.