Figure 6. Neutrality plot.

Evolution of large DNA viruses is primarily governed by mutational pressure. Scatter plot demonstrating a strong, near-perfect correlation between GC at the synonymous third codon position (GC₃) (X-axis) and non-synonymous first/second codon positions (GC_1,2) (Y-axis) among (a) large DNA viruses infecting invertebrate hosts and (b) large DNA viruses infecting vertebrate hosts. This finding suggesting that all codon positions are similarly affected and hence mutational pressure and not translational selection is primarily responsible for the observed differences in nucleotide composition among large DNA viruses.