Diversity in substitution rates. Synonymous substitution rates per site per billion years (SSB) for different organisms and genomes plotted on a log scale. Black bars represent seed plant mitochondrial genomes. Average rates for animal taxa from Lynch et al. ; angiosperm chloroplast and nuclear estimates from Wolfe et al. ; mitochondrial rates for individual plant species taken from Cho et al.  and Mower et al. .