Apomorphies

Apomorphy is a derived character state that evolves in a lineage after its divergence from a common ancestor. It contrasts with plesiomorphy, a primitive or ancestral trait that is retained from earlier ancestors. Apomorphies can take two forms: autapomorphies, which are unique to a single lineage, and synapomorphies, which are shared by two or more lineages and thus help define a clade.

In systematic biology, apomorphies are central to reconstructing evolutionary relationships. Synapomorphies diagnose monophyletic groups (clades) because they mark shared evolutionary innovations, while autapomorphies distinguish terminal taxa. The use of apomorphies relies on the assumption that similarities due to common descent are more informative than those arising from convergent evolution or reversals, a phenomenon known as homoplasy.

Examples help illustrate the concept. The amniotic egg is a synapomorphy for Amniota, distinguishing this lineage (reptiles, birds, and mammals) from amphibians. Mammalian lactation and hair are derived features that unite the Mammalia, functioning as synapomorphies for that group relative to other amniotes. Because the term apomorphy covers both autapomorphies and synapomorphies, its precise meaning depends on the taxonomic scope being studied.

Overall, apomorphies are the derived traits used to infer evolutionary relationships, with their interpretation shaped by context, data quality, and the potential influence of convergent evolution.