The d_Letter/d_S ratio (?), otherwise known as the “acceptance rate,” provides a sensitive measure of selection pressure at the amino acid level. ?=1 indicates neutral expectation, ?< 1 suggests negative (purifying) selection, while ?> 1 suggests positive (diversifying) selection. The hemagglutinin gene from influenza A virus is probably one of the fastest evolving genes in terms of the rate of nucleotide substitution, which was estimated at 5.7?10 ? 3 per site per year. Continue reading →