synapomorphic

Synapomorphy is a term used in evolutionary biology to denote a character state that is derived and shared by two or more taxa, signaling a common ancestry. In cladistics, a synapomorphy defines a clade and serves as evidence that its members belong to a single evolutionary lineage.

By contrast, a plesiomorphy is an ancestral trait that predates the last common ancestor of a group and may be shared with distant relatives; an autapomorphy is a derived trait unique to a single lineage; a symplesiomorphy is a shared ancestral trait that is not informative for resolving relationships within a group; and homoplasy refers to similarity that arises independently in separate lineages (convergence or reversal).

Synapomorphies are identified by comparing characters across taxa and using an outgroup to polarize character states. In maximum parsimony analyses, the most likely tree is the one requiring the fewest changes in synapomorphies. The presence of synapomorphies helps define monophyletic groups (clades).

The term and concept were developed by Willi Hennig in the mid-20th century as part of phylogenetic systematics. Classic mammalian synapomorphies include hair, mammary glands, and three middle ear bones (malleus, incus, and stapes). Traits can be lost or arise in parallel (homoplasy), so synapomorphies are assessed across multiple characters to reliably infer evolutionary relationships.