Figure 7

Relative synonymous codon usage in the direct 1918 human H1N1 lineage for the four alanine codons, after adjusting the “ frozen” portion of the lineage ( 1976–2009) by 21 years. Note that the two decreasing in frequency over time are exactly matched by the two increasing over time (i.e., non-synonymous changes are having a negligible effect). Also, three of these lines (GCA, GCC, and GCG) indicate these codons are drifting even farther away from the human average.