Phylogeny
A phylogenetic tree is a hypothesis about ancestry, and Topic 7.9 is where you learn to read one for what it actually says. The single move that trips almost everyone is deciding who is related to whom by how close two tips sit along the row. That is not what the tree encodes. Relatedness is set by shared ancestry — you trace back from each tip and find the node where their lineages last split. Two tips that sit side by side can share a much older common ancestor than a pair drawn far apart; proximity along the tips is just a drawing choice.
Nodes are the whole story. A node marks a most recent common ancestor, and the pattern of nesting — which lineages join at which node — is the only relationship information the tree carries. Because of that, you can rotate the branches at any node and the tree means exactly the same thing: the order of the tips changes, the ancestry does not. The other habit to break is reading the tree left-to-right as a ladder of improvement. A lineage that is more-derived simply accumulated more changes since a shared node; it is not "more evolved," not more advanced, and no closer to a goal than its cousins. Every living tip has been evolving for exactly the same amount of time.
Interactive · Phylogeny Tree Builder
Build a tree from shared traits, then trace any pair of tips back to the node where their lineages last split. Rotate the branches at a node and watch the tip order flip while every relationship stays put — proof that ancestry, not tip proximity, is what the tree encodes.
Phylogeny Tree Builder · Open the full sandbox →The mistakes here cluster around two failure modes. Students judge relatedness by how close two tips are drawn instead of tracing back to the shared node, so they call adjacent tips "most related" when a rotation could put a distant cousin next door (U7-BIO17); and they read the tree as a ladder of progress, treating a more-derived lineage as "more evolved" or more advanced when it has simply accumulated different changes since a common ancestor (U7-BIO18). Every scenario in this topic asks you to reason from nodes and shared ancestry — never from the left-to-right order of the tips.
The work
3 ways in · any order
Lesson
Phylogeny
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A phylogenetic tree encodes relatedness through shared ancestry — the nodes where lineages split — not through how close two tips sit along the row. The lesson walks the ways students misread that: calling adjacent tips "most related," forgetting that a node can be rotated without changing anything, and treating a more-derived lineage as "more evolved." It closes with a ten-scenario applet that asks you to reason from nodes, not from the order of the tips.
Diagnostic
10-item topic check
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Ten items on reading a phylogenetic tree — that relatedness comes from shared ancestry and the nodes where lineages split, not from how close two tips are drawn, and that rotating branches at a node changes nothing (U7-BIO17); and that a more-derived lineage is not "more evolved" or more advanced, just differently changed since a common ancestor (U7-BIO18). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.