Plesiomorfia

Plesiomorfia, or plesiomorphy, is a term in evolutionary biology used to describe a character state that is ancestral for a given group. Such a trait is inherited from the last common ancestor of the group and tends to be widespread among its members, often remaining unchanged across many lineages. The concept relies on comparing a group to outgroups to determine which characters were present in the common ancestor and which arose later in some descendants.

Plesiomorphies are contrasted with apomorphies, which are derived or novel states that evolved after the lineage split. Within that framework, synapomorphies are shared derived traits that define clades, while autapomorphies are derived traits unique to a single lineage. Plesiomorphies are not typically informative for resolving close relationships within a group because they do not distinguish one member from another; however, they help in understanding character polarity and the direction of evolutionary change when rooted with appropriate outgroups.

In practice, identifying a plesiomorphy requires phylogenetic context. The same trait can be a plesiomorphy for one clade and an autapomorphy or synapomorphy for another, depending on which lineage and which outgroups are being considered. For example, the tail is often cited as a plesiomorphic feature for many tetrapods, present in their early ancestors and retained in many descendants, even as some lineages show tail reduction or loss.

Plesiomorphy is a relative concept used to interpret character evolution and to root phylogenetic trees, emphasizing ancestral states rather than deriving relationships based on those states alone.