Evolutionary rates (ER) in the pol gene within- and between-hosts in 10 datasets. Within-host evolutionary rates are in red. The letters above the boxes indicate significant differences between datasets (t.test with a p-value <0.001). The dashed lines indicate the median ER of the WH data (in red) and of the BH data (in black) and these are significantly different (Welch two sample t-test, t = 17.3, df = 1418, p-val. < 0.001).