Autapomorphy

In phylogenetics, an autapomorphy is a distinctive anatomical feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to the focal taxon (which may be a species, family or in general any clade. It can therefore be considered an apomorphy in relation to a single taxon. The word autapomorphy, first introduced in 1950 by German entomologist Willi Hennig, is derived from the Greek words αὐτός, aut- = "self"; ἀπό, apo = "away from"; and μορφή, morphḗ = "shape".

Discussion

Because autapomorphies are only present in a single taxon, they do not convey information about relationship. Therefore, autapomorphies are not useful to infer phylogenetic relationships. However, autapomorphy, like synapomorph and plesiomorphy is a relative concept depending on the taxon in question. An autapomorphy at a given level may well be a synapomorphy at a less-inclusive level. An example of an autapomorphy can be described in modern snakes. Snakes have lost the two pairs of legs that characterize all of Tetrapoda, and the closest taxa to Ophidia - as well as their common ancestors - all have two pairs of legs. Therefore, the Ophidia taxon presents an autapomorphy with respect to its absence of legs.