Amino acid divergence observed within subtypes over time.

Estimates of evolutionary divergence for representative amino acid sequences between influenza A virus HA subtype lineages from the past (≤1989) and present (≥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.