Cladistics is a relatively new tool for classifying organisms. It was
controversial when first developed, but in recent years the
controversy has largely been removed as its use became better
For many, Cladistics provides a more useful look at evolutionary
relationships than do older methods, as it results in a diagram which resembles a bush rather
than using the straight lines that have characterized lineage diagrams
in the past. The results of evolution over time are much more closely
related to a multi-branching bush than they are to a straight line.
Here are some pertinent definitions, from The Evolution and Extinction of the Dinosaurs David E. Fastovsky and
David B. Weishampel, Cambridge University Press, 1996.
Clade. Group of organisms in which all members are
more closely related to each other than they are to anything else. All
members of a clade share a most recent common ancestor that is itself
the most basal member of that clade. Synonymous with "monophyletic
group" and "natural group."
Cladistic analysis. Analysis of the
ancestor-descendant (evolutionary) relationships among organisms using
hierarchies of shared, derived characters.
Cladogram. A hierarchical, branching diagram that
shows the distribution of shared, derived characters among selected
And here is a simplified cladogram showing the placement of Aves
(birds) within the larger Theropod clade: