AP Biology · Unit 7 · Topic 7.9
A phylogenetic tree is a hypothesis with a shape. Build one from a character matrix, then find out what it does and does not assert.
Learning objective 7.9.A Describe the types of evidence that can be used to infer an evolutionary relationship, and 7.9.B explain how phylogenetic trees and cladograms can be used to infer evolutionary relatedness.
The lancelet is the outgroup: the lineage known to have branched off before all the others. It has none of these characters, which is what tells you that having them is the derived condition. Step through the characters and watch each one mark a branch — until the last one, which marks nothing.
The dolphin swims, has a streamlined body and paddle-shaped appendages, and lives its whole life in water. One tree puts it with the fish. The other is the one the six nested characters gave. Counting changes decides between them — not opinion, and not the character that seems most obvious.
| Character | Changes on the inferred tree | Changes on the rival tree |
|---|
| Morphological characters | Molecular characters |
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Notice the last row. Morphology is not simply worse — it is the only kind of evidence a fossil can give you, and every calibration point on a molecular clock comes from a fossil that was dated and identified by its form.
Pick a pair. The tool marks their most recent common ancestor — the node they both descend from — and reports how many taxa share it. A smaller number means a more recent ancestor and a closer relationship. Where the two names happen to be printed has nothing to do with it.
| Does it show… | Cladogram | Phylogenetic tree |
|---|
The reason the tips mislead is that they can be swapped. Any node on a tree can be rotated — its two branches drawn in either order — without changing a single thing the tree claims. A tree drawn with the dolphin printed next to the shark and one with it printed next to the mouse are the same tree. If rotating a node can change what you would conclude, you are reading the wrong feature.
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