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. ROOTED UNROOTED