6. Phylogenetic trees

Terms: Tree, node (ancestor), leaf (taxon), branch, divergence, outgroup.

The length of each branch to the next node represents the number of sequence changes* that occurred prior to the next level of separation.

* Molecular clock hypothesis: If there is a constant pace of the rates of evolution, the length of the branches in some methods could be presented along a time-axis.

Rooted tree: The direction of the path from the root to any other node indicates the passage of evolutionary time (common ancestor).

Unrooted tree: There is no hierarchy among the nodes showing how close or distant the species are. By introducing an outgroup (distant from all the species) the unrooted trees can be rooted.