Fig. 8

Information other than topology requires different trees. The tree shown in a is known as a “cladogram” and is the same as the others in the paper in that the information contained within it is limited to branching order; the lengths of the branches in such trees do not convey any information. However, other types of trees can be drawn to indicate additional information, such as the degree of genetic or morphological divergence between species. b shows a special kind of phylogeny known as a “phylogram,” in which branch length is proportional to some measure of divergence. Phylograms typically include a scale bar to indicate how much change is reflected in the lengths of the branches. Total divergence between two species is determined on a phylogram by adding up the entire length of branches separating them: from one species to the common ancestor and then from the ancestor to the second species. In this tree, the lineage leading to species U has undergone less change than the lineage leading to species V since these lineages split from a common ancestor. Conversely, the lineage leading to X has undergone more change than the lineage leading to W since these lineages diverged from their most recent shared ancestor (which includes another split that led to Z—recall that neither Z nor W is an ancestor of X). It is important to note that, as with the cladogram in a, all species U–Z in b are contemporary species. To make this clearer, trees such as those in b are sometimes “ultrametricized” as in c, meaning that the terminal nodes are aligned with each other and the internal branch lengths are scaled to show the degree of divergence among sister groups rather than among individual species. Alternatively, a tree like that in c can be scaled to time (e.g., in millions of years before the present), for example if fossil or “molecular clock” data are available for calibrating the specific timing of branching events (Benton and Ayala 2003; see also Fig. 9)