Darwin’s early work on heterostyly and related style polymorphisms (the presence of two or three style morphs within a population) generated much interest to understand how precise interactions between ecological and genetic mechanisms influence the evolution of floral diversity. Here we tested three key hypotheses proposed to explain the evolution of heterostyly: (i) the presence of self-incompatibility; (ii) the role of pollinators in promoting dissasortative mating; and (iii) floral architecture, which restricts pollinators’ movements and ensures more exact pollen deposition on their bodies. We combined data from experiments, field observations, and published studies to test whether evolution of style polymorphism in Narcissus is driven by the incompatibility system, pollinator guilds, or floral architecture, within a phylogenetic framework. Neither differences in pollinator environment nor the presence of genetic self-incompatibility were correlated with presence of style polymorphism. However, our results indicate that the evolution of style polymorphism was driven by the presence of a narrow and long floral tube.