Topology

The topology is the branching structure of the tree. It is of particular biological significance because it indicates patterns of relatedness among taxa, meaning that trees with the same topology and root have the same biological interpretation. 

The three trees in the top box of Figure 5 have topologies in common. This is evident from the statement “A and B are more closely related to one another than they are to C”, which is true for all three trees. In the lower box, all three trees have different topologies from one other. For example, in the left-most tree, A and B are more closely related to one another than they are to C. However, in the middle tree, it is C and A who are each other’s closest relatives, and in the right hand tree it is B and C which are most closely related.

Making a model of a tree

If you find it difficult to imagine which trees have the same topology, it can be useful to make a model of a tree using either string or pipe cleaners, and a pin. An example of one we made earlier is shown below (Figure 6).

The pipe cleaners represent branches and the pin indicates the root position. Rotating any combination of branches around the root always results in the same topology, and thus patterns of relatedness.