Fast evolving flowers drive cactus diversification

The rise of biodiversity is shaped by variation in diversification rates. Across the Tree of Life, numerous forces are thought to influence these rates, including the evolution of adaptive traits, climate change, and interactions with other organisms. In the flowering plants, a longstanding hypothesis favoured by Darwin suggests that floral evolution is a driving force for plant diversity. However, this remains to be tested in detail across many diverse plant groups. Here, we test this in the cactus family, an iconic and rapidly diversifying plant family, by using the widely recorded but underexplored metric of floral length. Floral length itself shows no association with speciation rate. In contrast, the underlying rate of floral-length evolution is a strong positive predictor of speciation. The rate at which floral traits change is more important for generating new species than any particular floral form. Specific morphologies, which are shaped by adaptation to pollinators, are less important than previously thought. This work emphasises the role of “evolvability” in plant macroevolution and provides new evidence for the floral diversification hypothesis. Trait rate is more important than trait state.