Accelerated floral evolution in habitats with low pollinator availability

Background/Question/Methods

Many plant genera that today grow in Atacama desert and high Andes of northern and central Chile have a high a number of species with a great diversity of floral morphologies and colors.  Both environments have stressful conditions for biotic pollinators and their high floral diversity contrast with the generalized idea that pollinator mediated selection is the main driving force for floral diversification. In this study, we compared the rates of floral evolution among habitats with contrasting conditions for biotic pollination. We investigated the patterns of floral differentiation of Schizanthus and Leucocoryne,  two southern southamerican genera with species in mediterranean-type climates, winter rainfall deserts, and above treeline habitats. We characterized floral morphology of all species using both linear and geometric morphometric analyses. We mapped habitats and morphological traits on time-calibrated phylogenies of each genus, and then, we estimated the rates of morphological change associated with each habitat. 

Results/Conclusions

We found that the rates of floral evolution were significantly greater for desert and andean species than for mediterranean species. Rates were up to 7 times higher in the case of Schizanthus, and up to 3 times higher in the case of Leucocoryne.  We discuss several factors that would promote floral diversification in environments with stressful conditions for biotic pollinators, including historical factors and the evolution of autonomous selfing.  We also revisited the first ideas of Charles Darwin about floral evolution, showing that variation in pollinator service can promote coevolution between flowers and insects in generalist systems.