What are the features of a cladogram?

1 Answer
Nov 11, 2017

Cladograms are made up of a root, nodes, and sister groups.


Cladograms are made up of a root, nodes, and sister groups.

The root shows the ancestor that all organisms in the cladogram share. The root in the cladogram below is the last common ancestor of Species A, B, C, D, and E.

Internal nodes or intersections on the cladogram show a point of divergence. The first internal node on the cladogram below occurs when the ancestor of Species E diverges from the ancestor of Species D, C, B, and A. The next internal node is when the ancestor of Species C and D splits from the ancestor of Species B and A.

Terminal nodes are the end points of the cladogram. In the example below, the terminal nodes are species but they could also be groups or taxon and so forth.

An outgroup may be included on a cladogram to compare the other groups to. In the example cladogram, the outgroup is Species E. It is related to the root organism but it is not as closely related to the other terminal nodes or species as those terminal nodes are to each other.

Cladograms also have sister groups or sister species, which are the closest relatives. Species D and C are sister species in the cladogram below and so are Species B and A. If the terminal nodes represented groups rather than species, we would say "sister groups."

Created by Kate M

The branch length of a cladogram strictly speaking does not represent genetic changes or length of time. If this were a phylogenetic tree, the branch length would represent change. However, cladograms and phylogenetic trees are terms increasingly used interchangeably, so the branch length may represent the amount of change an organism has undergone since the last node. Double-checking the if there's meaning assigned to the branch is a good idea.