Cladograms

Cladograms are visual representations of genetic similarity. Evolutionary theory posits that any two organisms share a common ancestor in history, which would have some traits shared by both modern organisms today. The ancestor is represented at the intersection of two lines.

Terminating lines, those that connect to an organism, represent a characteristic unique to that organism. Branching lines are usually representative of a characteristic all following organisms share.

Example
The cladogram on the right is a typical example. When the line begins at the bottom, it is labelled Vertebrae because all organisms branching off after the label are vertebrates.

Lampreys are the first organisms to branch off, because the next characteristic is the presence of jaws, which lampreys lack. The category of jawed organisms is given the label Gnathostomata. Following that, the ray-finned fish is filtered out by the next criterion: bony ribs and shoulder girdle. This continues until a single organism remains at the end of the original line.

In some cases, it may be necessary to have smaller lines branching off the branches.