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Autapomorphy

A derived trait carried by only a single species - a novelty unique to one tip, shared with no one else. Because no second species carries it, an autapomorphy says nothing about kinship *between* species, so it is useless for grouping. But it is dangerous: when two lineages change fast and long, they pile up autapomorphies, and some coincidentally match - fooling maximum parsimony and causing long-branch attraction.