Cladistics

Cladistics (/kləˈdɪstɪks/, from Greek κλάδος, kládos, '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 a common ancestor and all its descendants are part of the clade. Importantly, all descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms animals, bilateria/worms, fishes/vertebrata, or monkeys/anthropoidea were used, these terms would include humans. Many of these terms are normally used paraphyletically, outside of cladistics, e.g. as a 'grade'. 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.) Cladistics is now the most commonly used method to classify organisms. 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 systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, and evolutionary taxonomy by Ernst Mayr.