Faster speciating cacti have faster evolving flowers

The rise of biodiversity is shaped by variation in speciation rates. Across many taxonomic groups, both trait values and the rates at which traits evolve have been proposed to influence diversification, but these factors can act independently. Here, we test two competing hypotheses in the cactus family, that speciation depends on flower length variation or on the rate of evolutionary change in flower length. Across >750 species in 107 genera, we find that flower length is only weakly related to speciation, whereas the rate of flower-length evolution is a strongly positive predictor. Moreover, flower length and rate of evolutionary change in flower length are only weakly correlated, indicating that rapid change, rather than any particular floral morphology, underlies cactus diversification. These results challenge expectations that specialized morphologies accelerate diversification, suggesting instead that in cacti, it is the tempo of floral change, rather than any particular floral form, that explains their extraordinary diversity.