Phylogenetic analysis is a means of establishing evolutionary relationships.
Synapomorphy is a shared ("syn") character that is different from the form found in an ancestor that distinguishes a clade (monophyletic group)from other organisms
The absence of a trait can be used as a synapomorphy in phylogenetic analysis. For example, the loss of a trait, such as the loss of legs in snakes, can be a valuable synapomorphy for a clade.
