Frequency distribution of SNP density and dN. Graph showing the number of protein-coding genes in the wMel genome within each SNP density (A) and dN (B) category. The frequency axis is drawn on a log scale. The bins containing the transcriptional regulator genes included in the analysis are indicated. Pseudogenes, genes that are potentially inactivated in wAu, IS elements and other genes with multiple ambiguous matches are excluded. The transcriptional regulator gene WD0255 is excluded as its closest wAu sequence contains a frameshift and is more similar to WD0623. dN – number of non-synonymous SNPs per potential site.