Median ratio of nonsynonymous nucleotide diversity (πN) to synonymous nucleotide diversity (πS) in virus sequence datasets classified by virus and host characteristics. Vert. = vertebrate host. NE = non-exposed protein; E = exposed protein. Kruskal-Wallis test for overall difference among medians: P < 0.001. Individual comparisons to E for RNA (Vert., vector): * P < 0.05; ** P < 0.01; *** P < 0.001. Numbers of datasets were as follows: DNA (Vert., no vector): 39 NE, 29 E; RNA (Vert., no vector): 57 NE, 34 E; RNA (Vert., vector) 9 NE, 15 E; RNA (Plant) 13 NE, 18 E.