Fig. 3.

Illustration of how the age of the common ancestor of a pair of genomes isolated from two distinct hosts is informative about the possibility of transmission between these two hosts. The Upper part shows an example of transmission from individual A to individual B. Within-host diversification is shown by the branching process, and transmission events are represented by the black arrows. Only one genome from each individual is considered at a time, and it is possible to estimate the date of their common ancestor. Because in this example the common ancestor is after the birth of A and before the birth of B, transmission from A to B is possible but not transmission from B to A. The Lower part of the figure gives the full scale of interpretation of possible dates for the common ancestor in term of transmission between individuals A and B.