locations are likely to occur, resulting in several possible choices for the most parsimonious ancestry. In such cases, SeqTrack can use an optional rule to select the 'best' ancestor, and select the closest ancestor according to a given proximity measure. For instance, this approach can be used to select, among all equally parsimonious ancestors, the one being geographically closest. Another possibility would consist in favoring ancestors whose haplotype is most frequent in the sampled isolates. By default, SeqTrack resolves ties in the choice of ancestors by maximizing the likelihood of the genetic differentiation observed between the ancestor and the descendent. The number of mutations n occurring between two isolates sampled D t time units apart follows a Poisson distribution: