Molecular evolution in small steps under prevailing negative selection - A nearly-universal rule of codon substitution.

The widely accepted view that evolution proceeds in small steps is based on two premises: 1) negative selection acts strongly against large differences and 2) positive selection favors small-step changes. The two premises are not biologically connected and should be evaluated separately. We now extend a previous approach to studying codon evolution in the entire genome. Codon substitution rate is a function of the physicochemical distance between amino acids (AA5), equated with the step size of evolution. Between nine pairs of closely related species of plants, invertebrates, and vertebrates, the evolutionary rate is strongly and negatively correlated with a set of AA distances (Au, scaled to [0, 1]). Au, a composite measure of evolutionary rates across diverse taxa, is influenced by almost all of the 48 physicochemical properties used here. The new analyses reveal a crucial trend hidden from previous studies: Au is strongly correlated with the evolutionary rate (R-2 > 0.8) only when the genes are predominantly under negative selection. Because most genes in most taxa are strongly constrained by negative selection, Au has indeed appeared to be a nearly universal measure of codon evolution. In conclusion, molecular evolution at the codon level generally takes small steps due to the prevailing negative selection. Whether positive selection may, or may not, follow the small-step rule is addressed in a companion study.