Plesiomorphy

Plesiomorphy is a term used in comparative anatomy and phylogenetics to describe an ancestral character state that is retained by two or more lineages because it was present in their common ancestor. Plesiomorphies are considered primitive or ancestral traits within the group being studied. They contrast with apomorphies, which are derived character states that evolved after a lineage diverged.

In cladistics, a shared derived state is called a synapomorphy, which helps to define monophyletic groups. A shared ancestral state, when used to describe relationships within a group, is often referred to as a symplesiomorphy or simply a plesiomorphy, depending on the terminology preferred by authors. Because plesiomorphies are inherited from distant ancestors, they are often uninformative for resolving relationships within a particular clade, since many lineages may retain the same primitive trait.

Examples commonly cited include the pentadactyl limb in tetrapods, which is the ancestral condition for the group and is retained in many descendants, even though some lineages later evolved fewer digits. Another example is the general presence of a vertebrate backbone, a trait inherited from early vertebrate ancestors and retained by its descendants. While plesiomorphies can illuminate deep evolutionary history, they can also obscure relationships if used alone to define clades.

Plesiomorphy remains a foundational concept for distinguishing primitive versus derived traits and for interpreting the evolutionary significance of shared character states in phylogenetic analyses.