Evolutionary shift in petal function from pollinator attraction to water drainage in small buzz-pollinated flowers

Floral morphological variation has long shaped our understanding of evolution, with most studies focusing on how structural changes facilitate pollinator interactions. Less appreciated, however, is how floral morphology changes through time to sustain pollination under specific environmental conditions. We explored this topic by testing whether buzz-pollinated species occurring in humid areas and/or blooming during wet seasons have evolved mechanisms for water drainage, using acuminate petals in tribe Miconieae (Melastomataceae) as a study case. We analyzed petal shape and flower size in the context of phenologies and climatic niches for 502 species using a set of trait evolution models. We found that species with drip-tip (i.e., acuminate) petals are more likely to occur in more humid and less seasonal habitats, and that flower size and phenology significantly impacted the evolution of petal shape in the clade, with small flowered species that bloom during wet seasons being more likely to evolve drip-tip petals. We discuss how petals in which drip-tips evolved may have adapted from their primary function of pollinator attraction to additionally drain water. Our study offers a new perspective of how flower morphological evolution responds not only to pollinator interactions but also to climatic conditions where species live.