Protein evolution in essential and dispensable genes. Boxplots of nonsynonymous substitution rates dN of coding sequences averaged for all 36 strains. Data were grouped by gene essentiality as annotated in the Database of Essential Genes . 87 genes showing a very low read depth (≤ 25% of the average read depth) were neglected in this analysis. Boxplots indicate 0.25 and 0.75 quantiles and the median (red line) with notches giving an estimate for the variance of the median. The green dashed line indicates the median dN for all genes. Values with dN above 20 × 10-3 are not shown.