Although overall all genes of class II aPMV-1 are evolving under purifying selection consistent with other paramyxoviruses [33, 56, 63] , distinctive profiles of positively selected codons were shown in both vaccine-and nonvaccine-related groups. Notably, P gene had the highest dN/dS ratio of any gene and had three sites predicted to be under selection in the nonvaccine-related group; there were no sites under selection in P gene in the vaccine related group. In contrast, HN had three sites predicted to be under selection in the vaccine related group. Of interest, codon 115 in the F cleavage domain is positively selected only in vaccine-related groups. Previous studies have reported that a single amino acid substitution at codon-115, which falls within the F0 cleavage site, resulted in a dramatic change from an avirulent infection to highly virulent NDV [64, 65, 66] . In contrast to the results reported in the present study, Miller et al [33] did not identify codon115 in F gene under positive selection. This is likely due to differences in the data sets because factors such as sequence length, sequence divergence, and the number of sequences can determine the ability to detect positively selected sites [67] . Thus, the vaccine-related genotypes I and II maintain a phenotypic mixture of strains with different infection and pathogenic potential and selection profiles.