A Comparative Approach to Understanding Floral Adaptation to Climate and Pollinators During Diversification in European and Mediterranean Silene

Abstract

Pollinator selection on floral traits is a well-studied phenomenon, but less is known about the influence of climate on this species interaction. Floral trait evolution could be a result of both adaptation to climate and pollinator-mediated selection. In addition, climate may also determine pollinator communities, leading to an indirect influence of climate on floral traits. In this study, we present evidence of both direct and indirect effects of climate on plant morphology through a phylogenetic comparative analysis of the relationships between climate, pollinators, and morphology in 89 European and Mediterranean Silene species. Climate directly influences vegetative morphology, where both leaf size and internode length were found to be smaller in habitats that are warmer in the driest quarter of the year and that have more precipitation in the coldest quarter of the year. Similarly, flower size was directly influenced by climate, where smaller calyxes were also associated with habitats that are warmer in the driest quarter of the year. These results suggest that reduced leaf and flower size promote water conservation in species that occupy arid climates. Floral traits also evolved in response to pollinators, with elongated calyxes associated with nocturnal pollination, though we also found evidence that climate influences pollinator distribution. Nocturnal pollinators of Silene are found in habitats that have more temperature evenness across seasons than diurnal pollinators. Correspondingly, nocturnally-pollinated Silene are more likely to occur in habitats that have lower daily temperature fluctuation and more temperature evenness across seasons. Altogether these results show that climate can directly influence vegetative and floral morphology, but it can also affect pollinator distribution, which in turn drives floral adaptation. Our study therefore suggests that climate mediates the influence of species interactions on trait evolution by imposing direct selective demands on floral phenotypes and by determining the pollinator community that imposes its own selective demands on flowers.