Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A tree is a testable hypothesis about evolutionary relationships. It is revised when new evidence disagrees (for example, DNA placed whales next to hippos).
- Root = ancestor of all lineages shown. Node = most recent common ancestor of the lineages beyond it, and a speciation event. Clade = an ancestor plus all its descendants.
- Branches can rotate at a node without changing any relationship. Living tips are cousins, never ancestors of each other.
- Trees can be built from morphology of living or fossil species and from DNA or protein sequences. Molecular data are usually more reliable: many more characters, and less often misled by convergent evolution.
Key relationships
| Term | Meaning | Use on a tree |
|---|---|---|
| Ancestral character | present in the outgroup and the common ancestor | cannot separate groups |
| Derived character | new state, gained or lost | placed on the branch where it arose |
| Shared derived character | derived state in two or more lineages | marks a clade |
| Outgroup | lineage less closely related to the rest | shows which state is ancestral |
| Cladogram | branching order only | branch length means nothing |
| Phylogenetic tree | branching order plus time or change | read dates from the axis |
| Molecular clock | Working |
|---|---|
| rate = differences ÷ time since a dated split | calibrate with a fossil-dated split |
| time = differences ÷ rate | estimate other splits |
Assumptions behind the numbers
- A molecular clock assumes mutations build up at a steady rate in that gene on every branch. Real rates vary, so dates are estimates.
- Fossils give a minimum age for a lineage: it existed at least that long ago.
- A derived character shared by several lineages is assumed to have arisen once, in their common ancestor (the simplest explanation).
Mistakes to avoid
- Judging relatedness by which tips are printed side by side.
- Calling one living species the ancestor of another, or calling the outgroup the ancestor.
- Using a character present in the outgroup to define a clade.
- Ignoring losses (eyes, legs, pigment) as derived characters.
- Reading time from the branch lengths of a cladogram.
- Saying a lineage with more derived characters is “more evolved”.
Quick self-check
- What does a node represent? (The most recent common ancestor of the lineages that branch from it; a speciation event.)
- A clock runs at 4 differences per million years. Two species differ by 40. When did they split? (40 ÷ 4 = 10 million years ago.)
- Two unrelated desert plants both store water in thick stems. Why is this weak evidence of close relationship? (Similar conditions can select for similar traits separately: convergent evolution.)
Next: practice questions.