This section also introduced the idea of informative and uninformative characters. Informative characters are those which are shared among some but not all members of a group in question. Uninformative characters are those which which are shared by all members of the group OR which are possessed only by single members. In neither case can they be used for inferring relationships. The parsimony principle works if evolutionary change is improbable.
Parsimony is an ancient guide for inference.
Essentials shall not be multiplied beyond necessity. Explanations should be kept as simple as possible When applied in the reconstruction of phylogeny, parsimony is usually taken to mean that the true tree will call for fewer evolutionary changes during phylogeny than would be needed to fit the same set of similarities and differences onto an incorrect tree. This would be fine if any evolutionary change occurred only once and was never reversed. Unfortunately, that is not the case. Therefore, rigid parsimony cannot be trusted absolutely.