Fig. 4

Hypothetical phylogenies showing how molecular clocks can be used to estimate the date of divergence of the children’s HIV virus strains. In Scenario 1, the children’s sequences show little divergence and so probably began diverging from one another after the medics’ arrival. In Scenario 2, the children’s sequences show substantial divergence and so probably began diverging from one another before the medics’ arrival. Illustration adapted with permission from the Understanding Evolution website