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 whether the morphology of flowers in the mega-diverse legume genus Mimosa is shaped by constraints. By investigating patterns and processes underlying trait variation along floral whorls, we evaluated the role of development and/or function in limiting morphological change. · Our results showed that floral morphology is remarkably homogeneous and likely shaped by convergent evolution. Morphological variation is unevenly distributed across floral whorls, reflecting differences in the levels of constraints. · Floral morphology in Mimosa is unlikely to have been primarily driven by pollinator-mediated selection. Instead, we suggest that the development and function of flowers in the mimosoid inflorescence constrained morphological evolution in this mega-diverse yet morphologically uniform group.