How to read a cladogram
Most mistakes with cladograms come from reading the tips instead of the nodes. Once you know what a node means, the rest follows quickly.
The parts of a cladogram
- Tips (or leaves), the taxa being compared, at the ends of the branches.
- Nodes, the branch points. Each represents the most recent common ancestor of everything beyond it.
- Branches, lines of descent connecting ancestors to descendants.
- Root, the base of the diagram, the common ancestor of everything shown.
- Clade, any node plus every descendant of that node. Snip a branch anywhere and what falls off is a clade.
Which organisms are most closely related?
Two taxa are more closely related when they share a more recent common ancestor, that's, when you can reach both by travelling back to a nearer node. Relatedness is measured through the nodes, never by how close the labels sit on the page.
To compare two taxa, trace back from each until the paths meet. The pair whose paths meet sooner are the closer relatives. It's a mechanical procedure and it always works.
Why the same tree can look different
Every node is a free hinge. Swapping the two branches at a node produces a diagram that looks different but states exactly the same thing. This is why two textbooks can show the same relationships in what appear to be two different pictures.
The test is simple: if you can turn one diagram into the other purely by rotating branches at nodes, they are the same cladogram. If you have to detach a branch and reattach it somewhere else, they aren't.
Reading clades
A clade, also called a monophyletic group, is an ancestor together with all of its descendants. Two other terms show up in the same discussion (see what is a clade for a diagram of each):
| Group | Contains | Example |
|---|---|---|
| Monophyletic (a clade) | An ancestor and all its descendants | Mammals; birds |
| Paraphyletic | An ancestor and some of its descendants | "Reptiles" excluding birds |
| Polyphyletic | Taxa grouped on a trait their common ancestor lacked | "Warm-blooded animals" (birds + mammals) |
This is why birds are, cladistically, reptiles: birds descend from within the reptile lineage, so a group of reptiles that leaves birds out is paraphyletic rather than a true clade.
What branch lengths mean
In a cladogram, nothing. Branches are drawn to make the diagram legible, and the tips are usually lined up neatly. A long branch doesn't mean more time or more change.
In a phylogram, branch length represents the amount of evolutionary change. In a chronogram, it represents elapsed time. If a diagram has a scale bar, the lengths mean something; if the tips are flush, they usually don't.
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Open the cladogram makerCharacters marked on branches
Tick marks along a branch show where a derived character is inferred to have appeared. Everything beyond that mark inherits the trait, which is what defines the clade below it.
Reading these in order gives you the sequence of evolutionary innovations: a vertebral column, then four limbs, then an amniotic egg, then hair, each one nesting a smaller group inside the last.