In contrast to the other genotypes, genotype II was not impacted by factors causing population decline in the late 1990s. We expected that vaccination, which started worldwide in the Table 3 . Site-specific selection analysis for each coding gene of vaccine-and nonvaccine-related groups. Strains used for selection analyses in both groups are mentioned in Table S1 . 1950s, should have limited the number of susceptible avian hosts, thus causing a bottleneck for this genotype. However, the impact of NDV vaccination is not seen in the BSP. It is possible that the data available in GenBank is insufficient to capture the population history of this genotype. However, a plausible explanation for the absence of a population bottleneck could be that genotype II NDV is maintained as an asymptomatic infection because it is continually introduced to susceptible populations as a modified live vaccine. Vaccination effectively prevents birds from developing disease when exposed to a virulent strain, but does not prevent shedding [7] . Thus, the number of available susceptible hosts may not dictate genotype II population demographics.