Pollen limitation and its influence on natural selection through seed set

Stronger pollen limitation should increase competition among plants, leading to stronger selection on traits important for pollen receipt. The few explicit tests of this hypothesis, however, have provided conflicting support. Using the arithmetic relationship between these two quantities, we show that increased pollen limitation will automatically result in stronger selection (all else equal) although other factors can alter selection independently of pollen limitation. We then test the hypothesis using two approaches. First, we analyze published studies containing information on both pollen limitation and selection. Second, we explore how natural selection measured in one Ontario population of Lobelia cardinalis over three years and two Michigan populations in one year relates to pollen limitation. For the Ontario population we also explore whether pollinator-mediated selection is related to pollen limitation. Consistent with the hypothesis, we find an overall positive relationship between selection strength and pollen limitation both among species and within L. cardinalis. Unexpectedly, this relationship holds even for vegetative traits among species, and was not found in L. cardinalis for pollinator-mediated selection on nearly all trait types.