in several inner segments of influenza A we typically only see limited (~10)
amino-acid changes in wild avian flu so that these segments are close to the calculated
"index"-protein-sequence here:
http://www.flutrackers.com/forum/sho...d.php?t=135205
Synonymous changes are not affected and do occur at normal rates.
No such restriction is observed in mammalean flu,
which accumulates ~2.5*10^-3 nucleotide changes
and ~4*10^-4 amino-acid changes per position per year.
(6 nucleotide changes and 0.8 amino-acid changes in PB2 per year in humans)