Figure 21.5 shows a proposed phylogeny for the great apes, Hominidae, taken in part from Purvis [Purvis, 1995]. The tree consists of a number of nodes (also termed vertices) and branches (also termed edges). These nodes can represent either an individual, a species, or a higher grouping and are thus broadly termed taxonomical units. In this case, the terminal nodes (also called leaves or tips of the tree) represent extant species of Hominidae and are the operational taxonomical units (OTUs). The internal nodes, which here represent extinct common ancestors of the great apes, are termed hypothetical taxonomical units since they are not directly observable.
The ordering of the nodes determine the tree topology and describes how lineages have diverged over the course of evolution. The branches of the tree represent the amount of evolutionary divergence between two nodes in the tree and can be based on different measurements. A tree is completely specified by its topology and the set of all edge lengths.
The phylogenetic tree in figure 21.5 is rooted at the most recent common ancestor of all Hominidae species, and therefore represents a hypothesis of the direction of evolution e.g. that the common ancestor of gorilla, chimpanzee and man existed before the common ancestor of chimpanzee and man. In contrast, an unrooted tree would represent relationships without assumptions about ancestry.