What Is a Cladogram?

Cladistics is a powerful modern tool for studying and understanding evolution. Cladistics attempts to do for the entire history of life what genealogy does for the history of human families: to disentangle the relationships between all living beings. But while genealogy focuses on detailed lineages of ancestry and descent, cladograms focus on identifying the common ancestry of related groups.

 

This cladogram represents the archosaurs, the common ancestors of crocodilians, birds, and all their descendants.

Cladogram Questions and Segments

Cladograms depend on two main scientific ideas. The first is that time flows in one direction only. The cladogram represents this by moving strictly from left to right. Thus, common ancestors of related groups must arise prior to these descendants in time, just as in genealogy parents arise before their children. Just as parents cannot inherit characteristics from their children, an hypothesis of ancestry requires that the “ancestor”; occurred earlier in time than its first “descendants.”

The second idea is that life forms are closely related if they share new features, feathers for instance; that is, characteristics that first appeared in an ancestor common to them both. Cladograms group life forms in terms of shared new characteristics that indicate commonality of ancestry. All of the descendants to the right of a branch point share that new feature; none of the creatures to the left does.

In your family tree, if your grandparents had a son late in life he would be your uncle, even though you might be older than he. So it is with cladograms: in many lineages, a “related” species can live before, during or after its relatives did. In the case of the archosaur family tree, so-called primitive forms are shown having appeared later than advanced ones, the uncle born after his nephew or niece.

For example, most scientists believe that Archaeopteryx was the earliest dinosaur capable of flight, although how well it flew is debatable. Most of the well-known extinct relatives of the flying dinosaurs—Deinonychus, Velociraptor, Oviraptor—first appeared much later than Archaeopteryx did, proving that none of these theropods can be the non-flying ancestor of flying dinosaurs. But that does not mean that they are not related, perhaps distantly, to birds. Indeed, the evidence suggests that both Archaeopteryx and its cousins—Deinonychus, Velociraptor, Oviraptor—had a common ancestor not yet found in the fossil record.