Apomorphy and synapomorphy

In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have evolved in their most recent common ancestor.

In cladistics, synapomorphy implies homology.

Examples of apomorphy are the presence of erectgait, fur, the evolution of three middle ear bones, and mammary glands in mammals but not in other vertebrateanimals, such as amphibians or reptiles, which have retained their ancestral traits of a sprawling gait and lack of fur. Thus, these derived traits are also synapomorphies of mammals in general as they are not shared by other vertebrate animals.