Sliding window representing the rate of synonymous and nonsynonymus nucleotide substitutions per site from serotype A sequences. The extent of positive and negative selection on individual codons. The value above and below the abscissa, indicates the number of per-site nonsynonymus substitutions (dN) over per-site synonymous ones (dS) in an individual codon. The zero value means dN = dS. Site 151 shows evidence for significant positive selection (P < 0.05) based on the extended binomial distribution.