Cladistics (from Greek κλάδος, klados, i.e., "branch") is an approach to biological classification in which organisms are categorized in groups ("clades") based on the most recent common ancestor. Hypothesized relationships are typically based on shared derived characteristics (synapomorphies) that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors. A key feature of a clade is that all descendants stay in their overarching ancestral clade. Radiation results in the generation of new subclades by bifurcation.
The techniques and nomenclature of cladistics have been applied to other disciplines. (See phylogenetic nomenclature.)
The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics (also the title of his 1966 book); the terms "cladistics" and "clade" were popularized by other researchers. Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field.
What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard (for insects) in 1921, and W. Zimmermann (for plants) in 1943. The term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Cain and Harrison in 1960, "cladist" (for an adherent of Hennig's school) by Mayr in 1965, and "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to phylogenetic inference with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal and the evolutionary taxonomist Ernst Mayr.
Originally conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966 (Lewin 1997). Today, cladistics is the most popular method for constructing phylogenies from morphological and molecular data. Unlike phenetics, cladistics is specifically aimed at reconstructing evolutionary histories.
In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, as well as to anatomical ones, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics rapidly became the dominant set of methods of phylogenetics in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics.
The way for computational phylogenetics was paved by phenetics, a set of methods commonly used from the 1950s to 1980s and to some degree later. Phenetics did not try to reconstruct phylogenetic trees; rather, it tried to build dendrograms from similarity data; its algorithms required less computer power than phylogenetic ones.