GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
Systematics II: The Tree of Life

Life is organized as a pattern of a tree, with lineages diverging from common ancestors, and many branches ending (becoming extinct).

Phylogeny: a "family tree" of taxa.

Traditional taxa name different parts of the tree of life. Taxa represent different kinds of groupings:

Monophyletic taxa prefered by most biologists today, because they represent whole historical parts of the tree.

Problem: the true shape of the Tree of Life is not known, because of missing data.

How to reconstruct the Tree?

Over time, the lineages acquire new adaptations that are passed on (sometimes with modification) to their descendants.


This can be shown as a set of nested diagrams:

Or by a braching diagram:

In 1950s, German entomologist Willi Hennig realized that one could use this method to organize taxonomy (systematics) by reconstructing the phylogeny of life.

The branching diagram above is called a cladogram.

Here is another, more comprehensive, cladogram of living terrestrial vertebrates:

The above arrangement can also be written in outline form:


For example, some evidence suggests that, unlike the cladogram above, turtles might be closer to lepidosaurs than lepidosaurs are to archosaurs, or that turtles might be closer to archosaurs than archosaurs are to lepidosaurs. This uncertainty can be shown by the following polytomy:

To Next Lecture.
To Previous Lecture.
To Syllabus.

Last modified: 14 July 2006