cladistics

cladistics or phylogenetic systematics , an approach to the classification of living things in which organisms are defined and grouped by the possession of one or more shared characteristics (called characters) that are derived from a common ancestor and that were not present in any ancestral group (as envisioned by Charles Darwin 's idea of "descent with modification" ). Developed by Willi Hennig, a German entomologist, in the 1950s, it is a method of reconstructing evolutionary relationships that emphasizes the importance of descent and common ancestry rather than chronology.

Cladistics places species in a group, or clade, based on a shared character. Within a clade, species that share other characters unique to them are grouped together, and so on, until a cladogram (a branching diagram that resembles a family tree) is assembled. For example, all vertebrates make up a clade; all tetrapods (vertebrates that have four limbs with wrists, ankles, toes, and fingers) form their own clade within the vertebrate clade. In this example the vertebrate clade would be considered "primitive" and the tetrapod clade "derived" or "advanced." In living creatures genetic characters or behaviors as well as more obvious anatomical features might be considered in assembling a cladogram. In paleontology the characters are necessarily skeletal.

Cladistics is especially significant in paleontology, as it points out gaps in the fossil evidence. It is also felt to be more objective than fossil study, which of necessity extrapolates from a limited number of finds that may or may not be representative of the whole.

See also fossil ; dating .

cladistics The application of phylogenetic systematics to produce a taxonomic system that is applied to the study of evolutionary relationships. In the cladograms used to portray these relationships, cladogenesis always creates two equal sister groups: the branching is dichoto-mous. Thus each pair of sister groups constitutes a monophyletic group with a common stem taxon, unique to the group. Monophyletic groups are deduced by identifying synapomorphic character states.

cladistics A special taxonomic system applied to the study of evolutionary relationships. In the branching diagrams or ‘cladograms’ used to portray these relationships, cladogenesis, or splitting of an evolutionary lineage, always creates two equal sister taxa: the branching is dichotomous. Thus, each pair of sister taxa constitutes a monophyletic group with a common stem taxon, unique to the group. See PHYLOGENETIC SYSTEMATICS.