plesiomorphic

Plesiomorphic refers to an ancestral character state in a lineage, meaning the trait was present in the last common ancestor and retained by its descendants. The word comes from Greek pleision, meaning near, and morph, meaning form. In phylogenetics, plesiomorphies are contrasted with apomorphies, which are derived states that evolved after a lineage diverged.

Apomorphies can be further divided into synapomorphies, which are shared derived traits that define a clade, and autapomorphies, which are unique to a single lineage. Plesiomorphies are often less informative for resolving relationships within a group because they do not indicate where new lineages split; they are features inherited from an ancestor rather than features that define reproductive boundaries among descendant lineages.

An example commonly cited is pentadactyly, or five digits on limbs, which is often treated as the plesiomorphic condition for tetrapods. While many tetrapod descendants retain five digits, some lineages modify or reduce digits, illustrating how plesiomorphies can persist alongside apomorphic changes in different branches.

Determining whether a trait is plesiomorphic or apomorphic requires polarizing characters against an appropriate outgroup—the taxon or group outside the study clade whose relationship helps establish the ancestral state. In practice, plesiomorphies help reconstruct ancestral conditions and the polarity of character changes, but they are usually not relied upon to resolve the branching order within a clade.