A method of classification in which the relationships between organisms are represented by a diagram, or * cladogram , based on selected shared characteristics. It is basically a phyloge-netic system since the selected characteristics are those that are assumed to have been derived from a common ancestor. However some practitioners of cladistics, the 'transformed cladists' believe that cladograms are simply summaries of patterns of shared characteristics that provide a logical basis for classifications but do not have to be linked with and explained by the evolutionary process. Cladistics differs from traditional phylo-genetic approaches in the methods used to determine evolutionary relationships and in its definition of a true natural group. Cladistics assumes the closeness of relationship depends on the recentness of common ancestry, which itself is decided by the number and distribution of shared 'derived' character states (synapomorphies) in the organisms in question. Synapomorphies are those character states that can be traced back to the same character states in a recent common ancestor. They do not include shared derived characters that have arisen by convergent evolution. 'Primitive' character states (symplesiomorphies), inherited from more remote common ancestors, are ignored as misleading because some may have been inherited by one descendant but not by another, closely related, descendant. The same character can be both derived and primitive, depending at what level in the taxonomic hierarchy one is working. Thus a derived character at the class level will probably be a primitive character at the species level. In constructing a clado-gram, branches are based upon the simplest explanation (often called the most parsimonious) of the distribution of shared derived characters. Cladistics also differs from some traditional phylogenetic methods in regarding the only true natural groups as those that contain all the descendants of a common ancestor (see monophyletic, paraphyletic). Cladists also believe that those groups that are each others closest relative - sister groups - should be given equal taxonomic rank. |
