Lower species richness in dioecious clades

Despite the extensive research on the potential benefits of dioecy to indiv iduals, little is known about the long-term success of dioecious lineages i n relation to their hermaphroditic or monoecious relatives. This study repo rts on the evolutionary success of worldwide dioecious flora in light of re cent phylogenetic work by performing sister-group comparisons of species ri chness between clades of angiosperms with different breeding systems. Wheth er this analysis is performed at the family or genus level, species richnes s is generally far lower in dioecious taxa when compared to their hermaphro ditic or monoecious sister taxa. Despite the advantages of avoiding inbreed ing depression and of allocating resources separately to male and female fu nction, dioecy in angiosperms does not appear to be a key innovation promot ing evolutionary radiation. A potential explanation for the low representat ion of dioecious lineages is that dioecious plants may have lower colonizat ion rates. Baker's Law states that self-compatible lineages will have highe r rates of successful long-range dispersal since they do not require a mate ; consequently, self-compatible lineages may have higher rates of allopatri c speciation. However, identical analyses performed with hermaphroditic sel f-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.