Amino acid divergence observed between sister subtypes over time.

Estimates of evolutionary divergence for representative amino acid sequences between influenza A virus HA sister subtype lineages from the past (2009). Distances represent the average pairwise amino acid substitutions per site over all sequence pairs between groups. Standard error estimates were obtained by using a bootstrap procedure (1000 replicates). A total of 592 positions (non-ambiguous) over 604 sequences were used. A Poisson correction model was implemented. All analyses were conducted in MEGA5.