Reproductive character displacement and potential underlying drivers in a species‐rich and florally diverse lineage of tropical angiosperms (Ruellia; Acanthaceae)

Reproductive character displacement is a pattern whereby sympatric lineages diverge more in reproductive character morphology than allopatric lineages. This pattern has been observed in many plant species, but comparably few have sought to disentangle underlying mechanisms. Here, in a diverse lineage of Neotropical plants (Ruellia; Acanthaceae), we present evidence of reproductive character displacement in a macroevolutionary framework (i.e., among species) and document mechanistic underpinnings. In a series of interspecific hand pollinations in a controlled glasshouse environment, we found that crosses between species that differed more in overall flower size, particularly in style length, were significantly less likely to produce viable seeds. Further, species pairs that failed to set seed were more likely to have sympatric distributions in nature. Competition for pollinators and reinforcement to avoid costly interspecific mating could both result in these patterns and are not mutually exclusive processes. Our results add to growing evidence that reproductive character displacement contributes to exceptional floral diversity of angiosperms. Our study yields evidence of reproductive character displacement (RCD) in a diverse and widespread lineage of tropical plants. It is set apart from other studies in that we investigate RCD from a macroevolutionary perspective, by comparing data from experimental crosses among 33 different species pairs. We additionally generate and review evidence to help distinguish two primary drivers of RCD in animal‐pollinated plants: pollinator competition and reinforcement. Using this approach, we find evidence that more strongly supports a hypothesis of reinforcement over pollinator competition. Our study builds on prior macroevolutionary studies of RCD and underlying drivers, including landmark research in Drosophila.