Summary• Measures of selection on floral traits in flowering plants are often motivated by the assumption that pollinators cause selection. Flowering plants experience selection from other sources, including herbivores, which may enhance or oppose selection by pollinators. Surprisingly, few studies have examined selection from multiple sources on the same traits.• We quantified pollinator-mediated selection on six floral traits of Lobelia cardinalis by comparing selection in naturally and supplementally (hand-) pollinated plants. Directional, quadratic and correlational selection gradients as well as total directional and quadratic selection differentials were examined. We used path analysis to examine how three herbivores -slugs, weevils and caterpillars -affected the relationship between floral traits and fitness.• We detected stronger total selection on four traits and correlational selection (c ij ) on three trait combinations in the natural pollination treatment, indicating that pollinators caused selection on these traits. Weak but statistically significant selection was caused by weevil larvae on stem diameter and anther-nectary distance, and by slugs on median-flower date.• In this study, pollinators imposed stronger selection than herbivores on floral traits in L. cardinalis. In general, the degree of pollen limitation and rate of herbivory are expected to influence the relative strength of selection caused by pollinators or herbivores.

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 tested the hypothesis using two approaches. First, we analysed the published studies containing information on both pollen limitation and selection. Second, we explored how natural selection measured in one Ontario population of Lobelia cardinalis over 3 years and two Michigan populations in 1 year relates to pollen limitation. For the Ontario population, we also explored whether pollinator-mediated selection is related to pollen limitation. Consistent with the hypothesis, we found an overall positive relationship between selection strength and pollen limitation both among species and within L. cardinalis. Unexpectedly, this relationship was found even for vegetative traits among species, and was not found in L. cardinalis for pollinator-mediated selection on nearly all trait types.

Clonal propagation allows some plant species to achieve massive population sizes quickly but also reduces the evolutionary independence of different sites in the genome.We examine genome-wide genetic diversity in Spirodela polyrhiza, a duckweed that reproduces primarily asexually.We find that this geographically widespread and numerically abundant species has very low levels of genetic diversity. Diversity at nonsynonymous sites relative to synonymous sites is high, suggesting that purifying selection is weak. A potential explanation for this observation is that a very low frequency of sex renders selection ineffective. However, there is a pronounced decay in linkage disequilibrium over 40 kb, suggesting that though sex may be rare at the individual level it is not too infrequent at the population level. In addition, neutral diversity is affected by the physical proximity of selected sites, which would be unexpected if sex was exceedingly rare at the population level.The amount of genetic mixing as assessed by the decay in linkage disequilibrium is not dissimilar from selfing species such as Arabidopsis thaliana, yet selection appears to be much less effective in duckweed. We discuss alternative explanations for the signature of weak purifying selection.

21• Clonal propagation allows some plant species to achieve massive population 22 sizes quickly but also reduces the evolutionary independence of different sites in 23 the genome. 24• We examine genome-wide genetic diversity in Spirodela polyrhiza, a duckweed 25 that reproduces primarily asexually. 26• We find that this geographically widespread and numerically abundant species 27 has very low levels of genetic diversity. Diversity at nonsynonymous sites relative 28 to synonymous sites is high, suggesting that purifying selection is weak. A 29 potential explanation for this observation is that a very low frequency of sex 30 renders selection in effective. However, there is a pronounced decay in linkage 31 disequilibrium over 40 kb, suggesting that though sex may be rare at the 32 individual level it is not too infrequent at the population level. In addition, neutral 33 diversity is affected by the physical proximity of selected sites, which would be 34 unexpected if sex was exceedingly rare at the population level. 35• The amount of genetic mixing as assessed by the decay in linkage disequilibrium 36 is not dissimilar from selfing species such as Arabidopsis thaliana, yet selection 37 appears to be much less effective in duckweed. We discuss alternative 38 explanations for the signature of weak purifying selection. 39 40

Self-fertilization is expected to reduce genetic diversity within populations and consequently to limit adaptability to changing environments. Little is known, however, about the way the evolution of self-fertilization changes the amount or pattern of the