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Relating distance, rate and time
Evolutionary rate and time are confounded
Earlier you learned that branch lengths tell us the genetic change that has occurred. The genetic change is calculated by a combination of the rate of substitution and the time that has elapsed, as shown in the equation below:
Genetic change = Evolutionary rate x Divergence time
(substitutions/site) (substitutions/site/year) (years)
This means that without any other prior information about the tree, we do not know whether branch D shown below (Figure 12) represents an extinct or frozen lineage that was sampled a long time ago; a lineage with a relatively slow evolutionary rate; or some combination of both.
This makes it very important for you to consider the biological information and context that you know about when interpreting trees. If you are lucky enough to know the dates when your samples were isolated, then there are some methods available that can help you to disentangle these processes.