cladistics

Method of biological classification that uses a formal step-by-step procedure for objectively assessing the extent to which organisms share particular characteristics, and for assigning them to taxonomic groups called clades. Clades comprise all the species descended from a known or inferred common ancestor plus the ancestor itself, and may be large - consisting of a hierarchy of other clades.

The cladistic method of classification was proposed by German teacher and entomologist Willi Hennig (1913-1976) in 1950, though his ideas did not gain currency until his work was translated into English in 1966.

Cladistics assumes, controversially, that a new species arises when an existing species splits into two descendent species (with the ancestor disappearing after the splitting event). It also assumes that characteristics change over time, and differentiates between ‘primitive’ (or plesiomorphic) characteristics, which arose early in the lineage of a group of species, and ‘derived’ (or apomorphic) characteristics, which are evolutionary novelties that arose more recently. For example, the presence of a vertebral column is a primitive characteristic for bird species, while their possession of feathers is a derived one. The sharing of derived characteristics implies that the species concerned share a common evolutionary history and are more closely related than those that possess fewer or none of these characteristics.

When charting the relationship between species, cladists select a number of derived characteristics for consideration and then use computers to record whether those characteristics are present or absent, and to analyse any underlying pattern of relationship between the species. The results are usually presented graphically as a branching family tree, or cladogram, in which each fork, or node, has two branches and in which each organism under consideration appears at the tip of a branch. The cladogram may then be converted into a classification, with the organisms being grouped into clades, or monophyletic groups, consisting of a node (the extinct ancestral species) and all the species descending from that node.

Because there are a million or more species of organism, it follows that the cladistic method could, potentially, generate an enormous number of clades, each one consisting of either two species or a hierarchy of other clades. Cladists cannot, therefore, assign names to each clade - usually only the more significant ones and their sister clades are named - nor can they rank clades according to the traditional hierarchy of families, orders, and classes.

Many of the groupings arising from cladistic classification overturn conventional groupings. For example, the reptile group - which comprises a distinct class under conventional systems of classification - is not recognized as a clade because it does not include all the descendants of its members' most recent common ancestor (the missing descendants are the birds).