In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade – characters or traits that are derived from ancestral characters over evolutionary history. An apomorphy is a character that is different from the form found in an ancestor, i.e., an innovation, that sets the clade apart ("apo") from other clades. A synapomorphy is a shared ("syn") apomorphy 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 σύν, syn = shared; ἀπό, apo = away from; and μορφή, morphe = shape.
As an example, in most groups of mammals, the vertebral column is highly conserved, with the same number of vertebrae found in the neck of a giraffe, for example, as in mammals with shorter necks. However, in the Afrotheria clade, which includes elephant shrews, golden moles and elephants, there is an increase in the number of thoracolumbar vertebrae. This is a synapomorphy of the clade: a shared feature considered to be derived from a common ancestor.
Phylogenetic similarities: These phylogenetic terms are used to describe different patterns of ancestral and derived character or trait states as stated in the above diagram in association with synapomorphies.
Apomorphy – specialized trait or character that is unique to a group or species
Plesiomorphy – the ancestral trait state, usually in reference to a derived trait state
Analysis: A new method of measuring phylogenetic characteristics is the use of Relative Apparent Synapomorphy Analysis (RASA). The objective of analysis is to determine if a given characteristic is common between taxa as a result of either shared ancestors or the process of convergence. This method allows for several advantages such as computational efficiency and it also administers an unbiased and reliable measure of phylogenetic signal.
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 symplesiomorphy for mammals in relation to one another, rodents and primates, for example. So the concept can be understood as well in terms of "a character newer than" (autapomorphy) and "a character older than" (plesiomorphy) the apomorphy: mammary glands are evolutionarily newer than vertebral column, so mammary glands are an autapomorphy if vertebral column is an apomorphy, but if mammary glands are the apomorphy being considered then vertebral column is a plesiomorphy.
Cladogram comprehension: Cladograms are diagrams that depict evolutionary relationships within groups of taxa. These illustrations are accurate predictive device in modern genetics. They are usually depicted in either tree or ladder form. Synapomorphies then create evidence for historical relationships and their associated hierarchical structure. Evolutionarily, a synapomorphy is the marker for the most recent common ancestor of the monophyletic group consisting of a set of taxa in a cladogram.