A cladogram is a phylogenetic tree built from shared traits. To build one you need to tell new traits from old ones.
Key idea
Group organisms by shared derived traits: new features that some of the organisms have and others lack. The trait that all but one organism share marks the first branch (that one is the outgroup). Each trait shared by fewer organisms marks a smaller group nested inside.
Why it works
A derived trait appeared once, in one ancestor, and was passed to all its descendants. So everyone with that trait belongs to one branch. A trait that all the organisms have, like a backbone in a set of vertebrates, came from before the whole group and cannot sort them.
| Organism | Backbone | Jaws | Bony skeleton | Four limbs | Hair |
|---|---|---|---|---|---|
| Lamprey | yes | no | no | no | no |
| Shark | yes | yes | no | no | no |
| Perch | yes | yes | yes | no | no |
| Frog | yes | yes | yes | yes | no |
| Mouse | yes | yes | yes | yes | yes |
Worked example: Building from the table
Build the cladogram from the table above.
- Every organism has a backbone, so it does not split them; it describes the whole group.
- Jaws: all but the lamprey. The lamprey branches first.
- Bony skeleton: all but the lamprey and shark. The shark branches next.
- Four limbs: frog and mouse; the perch branches next.
- Hair: only the mouse; the frog and mouse are the closest pair.
Answer: Lamprey, then shark, then perch, then frog, with the mouse last: each new trait marks the branch to the group that has it.
Worked example: Counting traits
In this cladogram, how many of the marked traits does a frog have?
- Follow the path from the root to the frog.
- It passes the marks for jaws, bony skeleton and four limbs, but not hair.
Answer: 3.
Common mistake
Two mistakes: grouping by overall look ("a shark looks like a perch, so they go together") and grouping by traits everyone shares. Looks can come from similar ways of life. Shared old traits tell you nothing about the branches. Only derived traits, present in some and missing in others, mark branches.