Edaphic specialization onto bare, rocky outcrops as a factor in the evolution of desert angiosperms

The environmentally stressful conditions found in desert regions have often been implicated as the main factor in the evolution of drought tolerance in desert plants. Yet many iconic desert plant lineages evolved prior to the recent emergence of widespread arid climates, suggesting an important role for pre-adaptation (exaptation). In the desert rock daisies (Perityleae), we provide empirical support for this view by showing that life history evolution associated with their ecological specialization onto rock outcrops was a precursor to their establishment and extensive diversification in North American deserts. We caution against assuming the presence of ancient dry biomes based on time-calibrated phylogenies and we emphasize the fundamental roles that exaptation and dispersal play during community assembly in novel environments.