When morphology stands still: constrained floral evolution in a mega-diverse legume genus

Changes in floral morphology are key for the evolutionary success of flowering plants. However, many species-rich and ecologically diverse groups show low variation in floral traits. Low morphological variation may be associated with selective pressures and constraints to changes. Nonetheless, because our understanding of floral morphological evolution is focused on plant groups with diverse and specialized floral morphology, there has been little progress in identifying factors underlying floral morphological uniformity. Herein we addressed this issue by asking how the morphology of flowers in the mega-diverse legume genus Mimosa is shaped by constraints.

We collected floral morphological data across phylogenetically and geographically representative Mimosa species. By integrating morphology into an evolutionary framework, we quantified morphological diversity and tested whether morphological changes are better explained by neutral or adaptive evolutionary models. Then, we accessed evolutionary model parameters to evaluate the role of evolutionary constraints in limiting morphological changes across Mimosa floral whorls.

Our results showed that floral morphology is remarkably homogeneous, and evolved under evolutionary constraint. Levels of morphological variation and evolutionary parameter estimates vary across floral whorls, indicating that constraint strength differs throughout the flower.

The homogeneous and evolutionarily constrained flowers of Mimosa are unlikely to have been primarily driven by pollinator-mediated selection. We hypothesized that the development and function of flowers in the mimosoid inflorescence constrained trait evolution in this mega-diverse yet morphologically uniform group.