Phenetics versus cladistics The methodology of phylogenetic work rests on two approaches at present: numerical taxonomy (phenetics) and phylogenetic systematics (cladistics). The most-direct difference between the two methods is that phenetics classifies species by using as many characteristics as possible and arranges them by similarity regardless of any evolutionary relationships. The phenetics approach, which arose in the 1950s, based classification strictly on similarities between organisms and emphasized numerical analyses of a set of phenotypic characteristics (that is, biological characters that are observable). In the name of objectivity, one simply counted common characters without respect to ancestry, and divisions were made on the basis of totals: the more characters in common, the closer the classification. However, in phenetics, the shared history between one organism and another, such as the archosaur ancestry of both crocodiles and birds, was simply irrelevant. Other problems, such as the precise definition of a “character” (that is, an observable feature, or trait, of an organism), required subjective decisions, thereby reducing the credibility of phenetics as an objective approach to phylogeny. Although the tabulation of common characters remains very important in developing a species’s phylogeny, taxonomists have moved away from phenetics as a viable approach. Cladistics, on the other hand, does not base species classification on an assemblage of all shared characters between one species and another. It bases the classification of a group of species solely on their most-recent common ancestor. Taken from work by German zoologist Willi Hennig in the 1950s, cladistics uses only shared derived characters—that is, selected characteristics that infer monophyly (descent from a single species) or those that are expressed in all descendants of a common ancestor. Such uniquely shared historical (or derived) characteristics are called synapomorphies. For example, suppose that there is an original species marked by character A, and from that three species eventually evolve. The original species first breaks into two successor groups, in one of which A evolves into the character a; that successor group then breaks into two daughter groups, both of which have a. The other original successor group retains A throughout, with no further division. In that case, a is a synapomorphy, since the two species with a evolved from an ancestral species that had a uniquely. Therefore, the possessors of a must be classified more closely to each other than to the third species. Consequently, by means of cladistics, crocodiles and birds are classified together, before they can be jointly linked to lizards.