Substitution rates are compound products of at least four factors: generation time, effective population size, underlying mutation rate and mutation fitness [44] . The last factor can be assessed indirectly by studying the level of selection pressure on the variable sites. The low positive selection is a well documented aspect of the mode of evolution of vector-borne RNA viruses [45] [46] [47] , which demonstrate a lack of immune-driven positive selection [46] and a very effective purifying selection [48] . Our analyses did not identify any site under positive selection. Moreover, the substitution rate analysis yielded a median of 3.3610 25 (HPD: 1.5610 25 -6.1610 25 ) subs./site/year for E, significantly lower than the previously reported rates of 1.6610 24 , within S-TBEV [49] and FE-TBEV [50] and the 8.0610 24 found W-TBEV [21] . The main difference with the previous studies can be pinpointed to our use of a relaxed clock, which was chosen because Bayes factor comparisons indicated that the strict clock performed significantly worse than relaxed models. It is known that incorrect clock assumption may lead to spurious rate estimates [51] and dating analyses effectuated under a strict clock and the same set of priors as in inference 1, yielded a mean rate estimate twice as high as under a relaxed clock and, consequently underestimated all divergence times by about a factor two (data not shown).