Understanding the mechanisms underlining pathogenicity and persistence of pathogens in human populations is an important aspect of disease epidemiology and control. Fixation of mutations into nucleotide substitutions, a key principle behind phylogenetic signatures, is shaped by major evolutionary forces such as selection (molecular adaptation deriving from an increasing fitness of a corresponding phenotypic trait) and genetic drift (stochastic gene sampling process at reproduction) [23, 24] . A useful tool to detect and measure selection in viral gene sequences is the ratio between synonymous (dS) and nonsynonymous (dN) substitutions. Whereas a ratio above 1.0 is an indicator of positive selection operating at the amino acid sequence level [25] , significantly lower values are generally referred to as purifying selection and refer to preservation of the phenotypic trait.