symplesiomorphy

Symplesiomorphy is a shared ancestral character state found in two or more taxa because it was inherited from a distant common ancestor. It is a plesiomorphic (ancestral) trait that may also be present in other lineages outside the group being studied, so it is not diagnostic for defining that group.

In cladistics, symplesiomorphies are distinguished from synapomorphies, which are shared derived character states that define a clade. Autapomorphies are traits unique to a single lineage. A symplesiomorphy can complicate phylogenetic inference because its presence does not indicate a more recent common ancestry within the group of interest.

An illustrative idea is helpful but not tied to a specific real-world taxon. Suppose two species, A and B, share a trait X because their distant common ancestor possessed X, and X is also found in other lineages outside the A+B lineage. In this case, X is a symplesiomorphy for the A+B grouping rather than a synapomorphy that would support a closer A+B relationship. Therefore, such a trait is not used to diagnose a clade.

Practically, recognizing symplesiomorphies helps avoid misplacing taxa based on non-diagnostic characters. Phylogenetic analyses often polarize characters with outgroups to determine ancestral states, and emphasis is placed on synapomorphies to infer relationships. Understanding the distinction between ancestral and derived traits is essential for interpreting homology versus homoplasy and for constructing robust evolutionary trees.