Interspecific hybridization and polyploidy are key processes in plant evolution and are responsible for ongoing genetic diversification in the genus Sorbus (Rosaceae). Ludwig et al. study mating systems of diploid, triploid and tetraploid taxa in a diversity ‘hotspot’ for Sorbus in south-west England and find mating inter-relationships to be complex, and to provide the driving force for hybridization and ongoing genetic diversification. In particular, the presence of self-incompatibility (SI) in triploid pseudogamous apomicts imposes a requirement for interspecific cross-pollination, thereby facilitating continuing diversification and evolution through rare sexual hybridization events. This is the first report of naturally occurring pseudogamous apomictic SI plant populations, and suggests that interspecific pollination, in combination with a relaxed endosperm balance requirement, is the most likely route to the persistence of these populations.