Synonymous nucleotide divergence: what is “saturation”?

The nucleotide divergence at synonymous third sites between two lineages will increase with time since the latest common ancestor, up to some saturation level. The “null-hypothesis divergence” is defined as the percentage of difference predicted at synonymous third sites, allowing for amino acid composition and codon bias, but assuming that codon bias is the same at all sites occupied by a given amino acid, when equilibrium has been reached between forward and backward substitutions. For two highly expressed genes, gapA and ompA, in the enterobacteria, the estimated values of the null-hypothesis divergence are 39.3 and 38.15%, respectively, compared to estimated values of saturation divergence of 19.0 and 25.4%. A possible explanation for this discrepancy is that different codons for a given amino acid are favored at different sites in the same gene.