Diversification rates in a temperate legume clade: Are there “so many species” of Astragalus (Fabaceae)?

Abstract

Astragalus, the largest genus of flowering plants, contains upwards of 2500 species. Explanations for this exceptional species diversity have pointed to unusual population structure or modes of speciation. Surprisingly, however, three different statistical analyses indicate that diversification rates in Astragalus are not exceptionally high compared to its closest relatives. Instead, rates are high throughout the “Astragalean clade,” a much broader radiation distributed throughout the temperate zone. The increase in diversification rate is associated with the origin and divergence of this clade from common ancestors of it and several much less diverse and more narrowly distributed Asian genera. This suggests that causal factors in the shift toward higher rates of diversification must be due not to factors unique to Astragalus, but to characteristics common to the entire Astragalean clade. However, this larger clade has never been circumscribed in classifications based on morphological data. This raises the possibility that the causes of increased diversification may not be due to morphological innovation, but may instead be related to ecological factors or cryptic physiological or biochemical features.