Codon based evolutionary distances for maize and sorghum genes. A. Synonymous substitutions per synonymous site estimations for maize and sorghum were used to calculate B., estimates of synonymous substitutions per synonymous site per year (KS), by assuming that maize and sorghum separated from rice and barley approximately 50 mya . This allowed the synonymous substitution data to be “normalised”. C. Illustrates the estimated time in years since separation of the maize and sorghum orthologous gene pairs and maize homoeologues. D. Mutation rates for the maize sorghum gene orthologues. E. Comparisons of synonymous substitution rates between maize and maize Cel gene orthologues, maize and sorghum gene orthologues and barley and rice gene orthologues. Orthologous gene sets are presented in the same colour and the graph shows that orthologues across all four species have very similar rates of synonymous substitutions per synonymous site per year.