XVI International Botanical Congess

New parametric and nonparametric inference procedures were used to examine rates of molecular evolution in chloroplast rbcL, psaA and psbC genes, from all five lineages of seed plants. All genes showed striking departures from rate-constancy in both synonymous and nonsynonymous substitutions, with the former showing the largest differences between lineages. Sources of these departures include deviations from a Poisson process (overdispersion), random variation in lineage rates, and nonrandom variation in lineage rates (due to generation-time effects or other factors). Nonrandom variation is pervasive, with angiosperms and Gnetales showing higher rates than other seed plant groups. Significant challenges remain in distinguishing random lineage effects in a Poisson model from those due to episodic evolution.