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In cladistics, an autapomorphy is a derived trait that is unique to a given terminal group. That is, it is found only in one member of a clade, but not found in any others or outgroup taxa, not even those most closely related to the group (which may be a species, family or in general any clade).

An autapomorphy is not present in the closest relative of the terminal group and also was not present in their common ancestor.

[edit] See also

• Homoplasy – a trait that is found in several terminal groups but evolved independently (i.e., was not present in their common ancestor)
• Synapomorphy – a trait that is found in some or all terminal groups of a clade, and inherited from a common ancestor, for which it was an autapomorphy (i.e., not present in its immediate ancestor).
  • Underlying synapomorphy – a synapomorphy that has been lost again in many members of the clade. If lost in all but one, it can be hard to distinguish from an autapomorphy.