Plesiomorphies

Plesiomorphy is a term used in phylogenetics to denote an ancestral or primitive character state within a lineage and its ancestors. The concept helps researchers distinguish inherited traits from those that arose more recently in particular lineages. Plesiomorphies are contrasted with apomorphies, which are derived states that evolved within a lineage after it split from its relatives.

In cladistic terms, plesiomorphies are often contrasted with synapomorphies and autapomorphies. A synapomorphy is a shared derived state that defines a clade, whereas an autapomorphy is a derived trait unique to a single lineage. A symplesiomorphy refers to a shared ancestral character state within a group, which can be informative about relationships only in a broad sense and is usually not considered diagnostic for resolving the finer structure of a clade. Because plesiomorphies can be shared with outgroups, they are typically less useful for determining branching patterns within a group than synapomorphies.

Determining whether a state is plesiomorphic or apomorphic requires character polarity, commonly inferred by comparing the ingroup to an outgroup. If a trait state is present in both the outgroup and the ingroup, it is often treated as a plesiomorphy for the ingroup; if it is present only in the ingroup, it is considered apomorphic. Plesiomorphies can illuminate ancestral conditions and help polarize character evolution, but they may provide limited information about the internal relationships of the studied clade.