Fig. 5

Temporal directionality on a phylogeny. Regardless of the type of tree depicted (Fig. 4), a rooted tree can be read as indicating the earliest ancestor at the root, from which are descended the internal nodes and, more recently, the terminal nodes. That is to say, evolutionary trees indicate the passage of time beginning from the root (oldest) to the terminal nodes (youngest). Time cannot be read in any other direction on the tree (for example, across the tips), because all terminal nodes represent contemporary species (see Figs. 11 and 16). On all four trees shown here, the arrow indicates the direction of time from earliest ancestor (at the root) to modern species (at the tips). Trees are most commonly oriented to face up or right, but this is convention only, and downward or leftward trees would be equally accurate. Note that trees such as these do not imply specific amounts of time per branch, nor do they indicate when particular branching events occurred; they merely indicate the historical order of branchings within lineages. For example, this tree indicates that the split between the lineage leading to species D and E occurred sometime after the split of lineages from the common ancestor of D + E + C + F + B. By contrast, it does not indicate that the D + E and C + F splits occurred at the same time