Investigation of interspecific reproductive barriers is crucial to understand origins and evolution of species. Pinheiro et al. examine the contribution of transitions between self-compatibility and self-incompatibility and genetic divergence in the evolution of reproductive isolation (RI) in Dendrobium, one of the largest orchid genera. They find that genetic distance among species is significantly correlated with interspecific seed production, suggesting a gradual evolution of RI. Significant reproductive barriers between self-compatible and self-incompatible species are detected, preventing gene flow among Dendrobium species, and they conclude that changes in the compatibility system may constitute an important evolutionary process contributing to the high levels of species diversity in this tropical orchid group.