COMPARISONS OF THE MOLECULAR EVOLUTIONARY PROCESS AT RBCL AND NDHF INTHE GRASS FAMILY (POACEAE)

We examine rate heterogeneity among evolutionary lineages of the grass family at two plastid loci, ndhF and rbcL, and we introduce a method to determine whether patterns of rate heterogeneity are correlated bet ween loci. We show both that rates of synonymous evolution are heterog eneous among grass lineages and that rate heterogeneity is correlated between loci at synonymous sites. At nonsynonymous sites, the pattern of rate heterogeneity is not correlated between loci, primarily due to an aberrant pattern of rate heterogeneity at nonsynonymous sites of r bcL. We compare patterns of synonymous rate heterogeneity to predictio ns based on the generation time effect and the speciation rate hypothe ses. Although there is some evidence for generation time effects, neit her generation time effects nor speciation rates appear to be sufficie nt to explain patterns of rate heterogeneity in the grass plastid sequ ences.