Distributions of normalized 4-fold substitution rates. These graphs indicate that most mammals have heterogeneous mutation rates and most fungi have uniform mutation rates. The normal Gaussian distribution (thick green line) is what would be expected if all 4-fold sites in each gene have an equal and independent probability of being substituted. (a) All mammalian distributions have a bias toward high and low substitution rates, consistent with regional mutation biases in mammalian genomes (b) Most fungal distributions fit more closely to the Gaussian, except for a tail of genes with low substitution rates stemming from codon usage selection. (c) The distributions of C. albicans – C. dubliniensis (dotted purple line), C. tropicalis – C. dubliniensis (solid red line), and N. crassa – C. globosum (dashed blue line). These species pairs each have a wider distribution, similar to the mammalian distributions.