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Scaffold S13 — Reading trees, with rotated nodes

Five rounds. In each, click the two tips whose most recent common ancestor is most recent — i.e. the two that are each other's closest relatives on the tree. The visual layout is designed to fool you. After each answer, the same tree is redrawn with a different set of node rotations to show that the layout was arbitrary; the ancestry is not.

Locked — answer the pretest above first.

Running tally

What you just did has a name

Rotating a node in a phylogenetic tree — swapping which clade sits on top at any internal node — produces an equivalent tree. The relationships are identical. Only the drawing changes. If you answered by visual proximity on the page, the rotations would flip your answer; if you answered by tracing back to the MRCA, they never would.

Sister taxa are defined by a shared most recent common ancestor — depth(MRCA) — not by how far apart their names are printed. Crocodiles and birds are each other's closest relatives on the vertebrate tree. They share an archosaur ancestor that is nested deeply inside the "reptiles." Hippos and whales share an artiodactyl ancestor that post-dates the fish split; whales are nested tetrapods that happen to look fish-shaped. Convergent morphology is not ancestry.

The only things that cannot change without altering a tree's meaning are (1) which taxa share an MRCA, and (2) the relative depth of each internal node. Rotate any branch at any node — the tree means the same thing. That is the whole of "reading trees."