There has been remarkable progress recently in clarifying phylogenetic relationships among basal angiosperm lineages using DNA sequences from multiple sources. Two major questions, however, have not been addressed using this massive amount of data: substitution rates of these genes in all lineages of basal angiosperms and use of these data to date major divergences within angiosperms. Here, we use a data set of seven genes, from the chloroplast (atpB, rbcL), mitochondrial (atp1,matR, and SSU and LSU rDNA), and nuclear genomes (18S rDNA), to examine patterns of substitution rates of these genes and to estimate the age of angiosperms. A global molecular clock can be rejected for all genes, and all combinations of genes, under all models of evolution explored. This is due in part to an apparent increase in substitution rate in 3-4 lineages, including monocots, Piperales, Ceratophyllum, and perhaps Ranunculales in the eudicots. This increase in rate is found in all three plant genomes. Nevertheless, by excluding these clades, it is possible to find local rate constancy among the remaining lineages. Using this pruned phylogeny, we attempt to estimate the age of angiosperms and divergence times of some basal angiosperm clades. We compare these estimates to those derived from an alternative approach of using models that allow for heterogeneous rates of evolution.

Key words: angiosperms, evolutionary rates, molecular clocks