Environmental energy and evolutionary rates in flowering plants

Abstract

The latitudinal gradient in species richness is a pervasive feature of the living world, but its underlying causes remain unclear. We evaluated the hypothesis that environmental energy drives evolutionary rates and thereby diversification in flowering plants. We estimated energy levels across angiosperm family distributions in terms of evapotranspiration, temperature and UV radiation taken from satellite and climate databases. Using the most comprehensive DNA-based phylogenetic tree for angiosperms to date, analysis of 86 sister-family comparisons shows that molecular evolutionary rates have indeed been faster in high-energy regions, but that this is not an intermediate step between energy and diversity. Energy has strong, but independent effects on both species richness and molecular evolutionary rates.

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“…1). These results are in accordance with previous studies of a range of other taxa, which have found significant correlations between rates of molecular evolution and net diversification (6,(9)(10)(11)16).…”

“…Previous studies have noted that clades of flowering plants containing more species tend to have longer molecular branch lengths (6,11), and that path lengths on molecular phylogenies from a range of taxa tend to be positively correlated to the number of nodes through which they pass (16). These results suggest that net diversification-the balance between speciation and extinction rates that gave rise to the extant diversity-is somehow linked to rates of DNA change over time.…”

“…1-4). However, the link between diversification and the rate of molecular evolution is still debated, with much of the attention focused on the role speciation plays in driving genetic change (5)(6)(7)(8)(9)(10)(11)(12)(13). Changes to specific genes have been linked to the development of reproductive isolation during species formation, and in some cases such genes have been shown to be under strong positive selection (reviewed in refs.…”

“…For example, a previous study of plants suggested that environmental energy drives both rates of molecular evolution and net diversification, but that there is no direct link between the two (6).…”

“…By definition, each of the two lineages of the sister pair originated at the same time, so they have had the same amount of time to accumulate genetic differences or to accumulate species through the net effects of speciation and extinction. Therefore, any difference in the amount of molecular change accumulated along each lineage of the sister pair represents a difference in the rate of molecular evolution, and any difference in the species richness of these two sister lineages represents a difference in net diversification (6,21). Phylogenetically independent sister pairs represent separate evolutionary instances of the association between molecular rates and net diversification, so we can use them as datapoints in a statistical analysis to test for an association between molecular rates, net diversification, and life-history traits (22).…”