Autapomorphy

An autapomorphy is a derived character state that is unique to a single taxon or lineage within a broader group. It represents an evolutionary novelty that arose after the lineage diverged from its relatives and is not shared with other taxa in the study.

In cladistics, autapomorphies are contrasted with synapomorphies and plesiomorphies. A synapomorphy is a shared derived trait that unites two or more taxa, providing information about relationships within a clade. A plesiomorphy is an ancestral trait retained from earlier ancestors. An autapomorphy, by contrast, helps diagnose and distinguish one terminal taxon but usually provides little information about how that taxon is related to others elsewhere in the tree.

Identification of autapomorphies relies on comprehensive sampling and the use of appropriate outgroups to determine whether a trait is truly derived and unique to a given lineage. Autapomorphies can arise from novel adaptations, developmental changes, or other evolutionary processes after a lineage splits from its relatives. They may be sensitive to sampling bias: if related taxa are not included, a trait might appear autapomorphic when it is actually shared with unsampled relatives.

Examples are typically described in the context of a study group. For instance, within a genus of lizards, a species that exhibits a distinctive scale pattern not found in any other species of the genus would be diagnosed with an autapomorphy for that species.

See also: synapomorphy, autology, homology, homoplasy, pleisiomorphy.