Apomorphy

Definition

In phylogenetic systematics, a derived character state that has evolved from an ancestral condition (plesiomorphy) and distinguishes a lineage from its ancestors. An apomorphy may be unique to a single taxon (autapomorphy) or shared among multiple taxa (synapomorphy), the latter providing evidence of common ancestry. The concept is fundamental to cladistic analysis, where hierarchies of nested apomorphies define monophyletic groups.

Etymology

From Greek apo- ('away from') + morphe ('form'), reflecting departure from ancestral form.

Example

The haltere—a reduced, knob-like hindwing modified for gyroscopic balance—is an apomorphy of Diptera, distinguishing true flies from all other insect orders; among flies, the sponging mouthparts of Muscomorpha represent a further nested apomorphy.

Synonyms

• derived character
• derived trait

Related Terms

• synapomorphy
• Plesiomorphy
• autapomorphy
• Cladistics
• Homoplasy
• character state

Usage Notes

Contrast with plesiomorphy (ancestral state). Specialists distinguish autapomorphies (unique to one terminal taxon, uninformative for grouping) from synapomorphies (shared derived states, the basis for clade recognition). Apomorphy is relative: a trait is derived only with respect to a specified ancestral state. The term carries no implication of functional superiority or evolutionary 'advancement'.