Cladistics

Cladistics is the method of classifying organisms into groups called clades.

A clade (Greek = branch) is a group of organisms with a common ancestor and all its descendants (and nothing else). Such a clade is monophyletic.Hennig, Willi 1979. Phylogenetic systematics. Urbana: University of Illinois Press. . The term 'clade' was coined by English biologist Julian Huxley.

Examples
Birds, dinosaurs, crocodiles, and all other descendants (living or extinct) of their most recent common ancestor form a clade.Glossary entry "clade"  Understanding Evolution. 2010. University of California Museum of Paleontology. In the terms of biological systematics, a clade is a single branch on the tree of life, a monophyletic group. Biological classification needs such a natural group of organisms to be put together and given a taxonomic name.

This brings classification in line with phylogeny (how living things evolved). In cladistics, clades are the only acceptable units.

Some versions of cladistics have been the subject of controversy.Mayr, Ernst 1982. The growth of biological thought: diversity, evolution and inheritance. Cambridge, MA: Harvard University Press. .p226Patterson, Colin 1982. Morphological characters and homology. In Joysey, Kenneth A. & Friday A.E. (eds) Problems in phylogenetic reconstruction. Systematics Association Special Volume 21, London: Academic Press. Ridley, Mark 1986. Evolution and classification: the reformation of cladism. Longman, London. 

History of cladistics
The term clade was introduced in 1958 by Julian Huxley, cladistic by Cain and Harrison in 1960, and cladist (for an adherent of Hennig's school) by Mayr in 1965.Dupuis, Claude 1984. Willi Hennig's impact on taxonomic thought. Annual Review of Ecology and Systematics 15: 1–24. ISSN 0066-4162 Hennig referred to his own approach as phylogenetic systematics. From the time of his original formulation until the end of the 1980s cladistics remained a minority approach to classification.

In the 1990s it rapidly became the dominant method of classification in evolutionary biology. Computers made it possible to process large quantities of data about organisms and their characteristics (traits). At about the same time the development of effective sequence analysis techniques made it possible to apply cladistic methods of analysis to biochemical and molecular features of organisms as well as to anatomical ones.Baron C. & Høeg J.T. 2005. Gould, Scharm and the paleontological perspective in evolutionary biology. In Koenemann S. & Jenner R.A. Crustacea and arthropod relationships. CRC Press, 3–14.   retrieved 2008-10-15

For some decades in the mid to late twentieth century, a commonly used methodology was numerical taxonomy.p221 This made no attempt to resolve phylogeny, only similarities. The weakness of this approach was that it left out the connection between classification and evolution.

Phylogenetic nomenclature
Phylogenetic nomenclature is a way of giving names to the groups (clades) that have been decided on by cladistic methods. It differs in many ways from Linnaean nomenclature. The paper to which he was responding is reprinted in Mayr, Ernst 1976. Evolution and the diversity of life: selected essays. Cambridge, MA: Harvard University Press,  Critics of phylogenetic nomenclature include Ashlock, 

  Mayr,, 

 ,
  and Williams.
Phylogenetic NomenclatureLinnaean NomenclatureHandles arbitrarily deep trees.Biased towards trees about 4 to 12 levels deep.Primary goal is to reflect the process of evolution, as currently understoodPrimary goal is to group organisms in a clear and useful wayAssumes that the shape of the tree will change frequently with new discoveriesNew discoveries may require releveling of Genera, Classes, Orders, and KingdomsLimited to entities related by evolution or ancestrySupports groupings when evolution or ancestry are not fully knownDoes not include a process for naming speciesIncludes a process for giving unique names to speciesIgnores established paraphyletic groups such as reptilesPermits well-known groups such as reptilesLimited to organisms that evolved by inherited traits; not applicable to hybrid organisms, or when lateral transfer has happenedApplicable to all organisms, regardless of evolutionary mechanism

Monophyly and paraphyly
Monophyly is defined differently in evolutionary biology and cladistics. In evolutionary biology, the term monophyletic refers to a group of organisms descended from its most recent common ancestor.Mayr E. & Ashlock P.D. 1991. Principles of systematic zoology. 2nd ed, McGraw-Hill. A monophyletic group may include all or only a part of the descendants of the common ancestor. The ancestor may be a taxon of various ranks.

In cladistics, by contrast, a monophyletic group is a group consisting of all the inferred descendants of an ancestral species.Judd W.S. et al 2002. Plant systematics; a phylogenetic approach. 2nd ed, Sunderland MA: Sinauer.

In cladistics, a group that includes only some of the descendants of the ancestral species is not monophyletic but paraphyletic. The objective of cladistics is to block out monophyletic clades. Therefore, the elements of a paraphyletic group must be rearranged so that they do form one or several clades. Degree of similarity is not a criterion in grouping organisms into clades.

 A distinction 
Cladograms show who's related to what, and evolutionary trees show which evolved from whom.

Related pages
 Genealogy
 Monophyly
 Paraphyly
 Polyphyly

References