Lower species richness in dioecious clades

Despite the extensive research on the potential benefits of dioecy to individuals, little is known about the long-term success of dioecious lineages in relation to their hermaphroditic or monoecious relatives. This study reports on the evolutionary success of worldwide dioecious flora in light of recent phylogenetic work by performing sister-group comparisons of species richness between clades of angiosperms with different breeding systems. Whether this analysis is performed at the family or genus level, species richness is generally far lower in dioecious taxa when compared to their hermaphroditic or monoecious sister taxa. Despite the advantages of avoiding inbreeding depression and of allocating resources separately to male and female function, dioecy in angiosperms does not appear to be a key innovation promoting evolutionary radiation. A potential explanation for the low representation of dioecious lineages is that dioecious plants may have lower colonization rates. Baker's Law states that self-compatible lineages will have higher rates of successful long-range dispersal since they do not require a mate; consequently, self-compatible lineages may have higher rates of allopatric speciation. However, identical analyses performed with hermaphroditic self-incompatible angiosperms did not produce similar results, suggesting that Baiter's law is not the reason for the poor representation of dioecy among angiosperm species.