A phylogenetic tree is a diagram of ancestry. It turns the evidence from DNA and proteins and from fossils into a picture you can read. Reading it correctly takes one rule.
Key idea
Each branch point (node) stands for a common ancestor. Two species are more closely related when their most recent common ancestor is more recent, that is, when you reach a shared node sooner by tracing back from the tips. Distance across the tips on the page means nothing.
Why it works
A tree records splits. Tracing back from two tips, the first node where their lines meet is the last population they both came from. Everything after that node happened separately. The more recent that node, the less separate history the two species have.
Worked example: Who is closer?
On the vertebrate tree, is a perch more closely related to a shark or to a mouse?
- Trace the perch and the mouse back: they meet at the node of all bony vertebrates.
- Trace the perch and the shark back: they meet farther back, at the node of all jawed vertebrates.
- The perch-mouse ancestor is more recent.
Answer: To the mouse. (A perch and a mouse share a more recent ancestor than a perch and a shark.)
Worked example: Reading a clade
On the same tree, which organisms descend from the ancestor where four limbs first appeared?
- Find the trait mark for four limbs.
- Every tip to the right of that mark descends from it.
Answer: Frog, lizard and mouse.
A clade is an ancestor and all of its descendants: cut one branch of the tree and everything on it is a clade. Fish, as the word is used every day, is not a clade, because the ancestor of all fish is also the ancestor of frogs, lizards and mice.
Common mistake
Two misreadings are common. One is judging kinship by which tips sit next to each other on the page; rotating a node moves the tips without changing the tree. The other is calling the species at the right end "more evolved". Every living tip has been evolving for exactly the same length of time since the root; only the branch points show kinship.