Darwin's abominable mystery: Insights from a supertree of the angiosperms

Abstract

Angiosperms are among the major terrestrial radiations of life and a model group for studying patterns and processes of diversification. As a tool for future comparative studies, we compiled a supertree of angiosperm families from published phylogenetic studies. Sequence data from the plastid rbcL gene were used to estimate relative timing of branching events, calibrated by using robust fossil dates. The frequency of shifts in diversification rate is largely constant among time windows but with an apparent increase in diversification rates within the more recent time frames. Analyses of species numbers among families revealed that diversification rate is a labile attribute of lineages at all levels of the tree. An examination of the top 10 major shifts in diversification rates indicates they cannot easily be attributed to the action of a few key innovations but instead are consistent with a more complex process of diversification, reflecting the interactive effects of biological traits and the environment.

Fig. 1.

One of 10,000 most parsimonious supertrees with dates obtained by used nonparametric rate smoothing transformation of maximum likelihood branch lengths from rbcL sequence data. The time scale was calibrated by using the split between Fagales and Cucurbitales at 84 mya. The strength of shading reflects diversification rates estimated as log (number of species)/age since split from sister clade. See Fig. 2 for a larger figure showing names of all terminal taxa. Diversification rates vary from low (yellow to orange) to high (red to black). Asterisks indicate the top 10 most imbalanced nodes referred to in Table 1.