What is "synapomorphy"

n. (context cladistics English) A derived trait that is shared by two or more taxon of shared ancestry

In phylogenetics, a synapomorphy is a shared derived character or trait state that distinguishes a clade from other organisms. In other words, it is an apomorphy shared by members of a monophyletic group, and thus assumed to be present in their most recent common ancestor. The word synapomorphy, coined by German entomologist Willi Hennig, is derived from the Greek words , syn = shared; , apo = away from; and , morphe = shape.

A derived character is a character that is derived from an ancestral character over evolutionary history. Derived characters are used in determining phylogenetic relationships. In phylogenetics, a derived character of a clade that has been inherited from a common ancestor and distinguishes the clade from other potentially related organisms is called a "synapomorphy," a shared ("syn") character that sets the clade apart ("apo").

The concept of synapomorphy is relative to a given clade in the tree of life. What counts as a synapomorphy for one clade may well be a primitive character or plesiomorphy at a less inclusive or nested clade. For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a plesiomorphy for mammals in relation to one another: rodents and primates.