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It has long been recognized that of the world's five Mediterranean heathland regions, those in Australia (the kwongan) and in South Africa (the fynbos) are the most diverse. The reasons for this however, remain unresolved. We propose that Mediterranean heathland diversity is influenced by how species respond to fire. Fires can occur frequently in Mediterranean heathlands, and obligate reseeders (those species unable to resprout following fire) will consequently have shorter generation times than resprouters and potentially faster speciation rates. High diversity heathlands should therefore be composed of a higher proportion of reseeding species than low diversity heathlands, a prediction confirmed by comparing kwongan and fynbos vegetation to Californian chaparral and Israeli maquis. We speculate that in the kwongan and fynbos, substrate infertility has rendered resprouting too expensive metabolically, promoting obligate reseeders that are capable of elevated rates of speciation.