In other viruses such as SARS-CoV, it was observed that they can develop through positive selection through the Fig. (1) . Pairwise identity frequencies for (a) Hemagglutinin and (b) Neuraminidase genes, respectively. cross-species transmission in early epidemics, and negative selection during late epidemics [16] . It is possible that the same mechanism was the driven force of evolution of 2009