Fig. 2

The anatomy of a phylogeny. An evolutionary tree includes several components. At the right (in this case; see Figs. 4 and 5 for alternatives) are the terminal nodes or “tips” of the tree. These typically represent individual species or larger taxonomic groups, and all are contemporaries of one another (for example, all may be living at present, in which case A through F would all represent modern species). The terminal nodes are connected to one another through branches that join at “internal nodes.” Internal nodes represent inferred lineage splitting (speciation) events that give rise to descendant sister groups—in other words, they represent the common ancestors from which two or more related lineages are descended. In this figure, the node marked with an asterisk represents the most recent common ancestor of species A and B, and the one marked with two asterisks is the most recent common ancestor of species A, B, and C. The pattern of branching—known as the “topology” of the tree—indicates evolutionary relatedness. For example, species A and B share a recent common ancestor that was not shared by the other species and are therefore called “sister taxa.” Similarly, species D and E are sister taxa. Species F is the most distantly related of the sample of species and is known as the “outgroup.” Outgroup species are necessary to “root” an evolutionary tree—that is, to indicate the last common ancestor (i.e., the deepest internal node) shared by the entire group of species. The term “basal lineage” is sometimes used to describe the branch leading to the outgroup, but this is not recommended as it is often incorrectly taken to imply that it has undergone less change and is therefore more “primitive” or “ancestral” than the other lineages (Fig. 15; see also Crisp and Cook 2005)