Synapomorphy - Cladistic Analyses

Cladistic Analyses

Synapomorphies are used to establish phylogenies in cladistic analyses. As such they are empirical data which can support a certain hypothesis that terminal groups form a clade (monophyletic group) together to the exclusion of certain other groups – whereas character-states that are shared, but also shared by other terminal groups descending from an earlier common ancestor, cannot be used to exclude these other groups. The latter character-states can consist of symplesiomorphies ("primitive" character-states having originated in the earlier common ancestor) or homoplasies (superficially similar but independently evolved derived character-states).

The key problem is to identify the polarity of the transformation series to which several character-states belong, i.e. to tell which character-state is apomorphic and which is plesiomorphic. Various criteria were used to polarise the transformation series in earlier cladistics; however in the recent two decades pattern criteria based on outgroup comparison have dominated the field.

The concepts of apomorphy and plesiomorphy are relative to a certain level of generality. What counts as an apomorphy at one level of generality may well be a plesiomorphy at the other. For example, for rats and apes, the presence of mammary glands is a symplesiomorphy, but it is a synapomorphy for mammals in relation to tetrapods more broadly.

It is not essential to a synapomorphy that all members of a clade possess it; even if some would have secondarily lost the trait it could still be a synapomorphy of the clade as a whole. A character state that is a synapomorphy for a clade, but for lineages in this clade is a plesiomorphy that is altered in some lineages, is called underlying synapomorphy. If no crown group taxa are known, it is sometimes difficult to decide which character state is the underlying synapomorphy and which the autapomorphy that overlies it.

Clades are not defined by synapomorphies as such, though it is possible to define them by apomorphies in general.

Relative apparent synapomorphy analysis is a method used to determine whether a given character is shared between taxa due to shared ancestry or due to convergence.