Distribution of Ka, Ks and Ka/Ks (a) The cumulative density of Ka, Ks and Ka/Ks derived from human-mouse orthologous pairs. Ka, the number of non-synonymous substitutions per non-synonymous sites. Ks, the number of synonymous substitutions per synonymous site, and the Ka/Ks ratio. Three groups of human genes are represented in different colors and the number of genes in each group is listed right to the line symbols. (b) The box plots are drawn based on the same data. For each category, the central box depicts the middle 50% of the data between the 25th and 75th percentile, and the enclosed red horizontal line represents the median value of the distribution. Extreme values are indicated by solid blue dots that occur outside the main bodies of data.