2 ]. Uncertainty in the association between an internal node and the fossil record can be accommodated by providing a prior probability distribution for the age of the node. Previous studies have used a uniform distribution with upper and lower bounds on the age [
54 (link)], although other distributions may be suitable [
35 (link)]. In the above Results section, we presented examples in which calibration times are treated with parametric prior distributions (normal and lognormal). Assigning an age to a particular node is only possible when the tree itself is assumed to be known and fixed, a limitation of previous relaxed-clock implementations [
15 (link),
17 (link),
54 (link)]. In the framework presented here, the tree itself is being sampled and thus we cannot define the age of a particular internal node. Instead we specify the age, or the prior distribution of age, for the most recent common ancestor of a set of taxa. Every time a new tree is proposed in the MCMC chain, the most recent common ancestor of the specified taxa is located in the tree, and the prior probability of the age of this node is used to assess the acceptance probability of the proposed tree. (2) Known ages of the sequences: Recently it has also been demonstrated that calibrations can be associated with the sequences at the tips of the tree if they are sampled at significantly different times [
29 (link),
30 (link),
66 ] with respect to their rate of evolution. Again, there may be uncertainty in calibration dates [
67 (link)]. The RNA virus data in this study provide examples of this form of calibration information. (3) A strong prior on the substitution rate: If the mean substitution rate is known from a previous study on independent data, then this can be incorporated as prior knowledge. In the simplest case this can be achieved by fixing the rate of evolution to a known value. It is also straightforward to sample the rate from a parametric distribution obtained from a previous (independent) analysis [
68 (link),
69 (link)]. If there is no prior information about the mean substitution rate, then it can be fixed to 1, resulting in time being in units of substitutions per site.
All of these forms of calibration information can be incorporated into our MCMC implementation either on their own or in any combination, as appropriate.