Synapomorphies

Synapomorphies are shared derived character states that diagnose a clade in phylogenetic analyses. They originate in the most recent common ancestor of the group and are inherited by all its descendants. In contrast, plesiomorphies are ancestral traits, and autapomorphies are derived traits found in a single lineage. Synapomorphies are the basis for defining monophyletic groups in cladistics.

Identifying synapomorphies involves comparing a set of taxa with an appropriate outgroup to determine derived versus ancestral states and to polarize character changes. Characters can be morphological, such as bone structure or plumage, or molecular, such as DNA sequence changes or gene arrangements. A character shared by two or more taxa due to common ancestry is a synapomorphy for the clade formed by those taxa and their most recent common ancestor.

Classic examples include hair and lactation in Mammalia, along with the presence of three middle ear bones, which together help define mammals. Feathers are a synapomorphy for birds (Aves) and their closest fossil relatives in many phylogenies. In molecular data, shared DNA substitutions or particular gene arrangements can likewise act as synapomorphies that diagnose a clade.

Synapomorphies are essential for reconstructing evolutionary relationships because they indicate shared ancestry. However, homoplasy—similar traits arising independently—can obscure true relationships, and some traits may be autapomorphies or reversals at certain nodes. Consequently, robust phylogenetic conclusions rely on multiple, independent synapomorphies across different data types.