Cladistics

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A clade in tree form: Progoneata is a clade of millipides including Symphyla, Pauropoda and Diplopoda.

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.[1] The term 'clade' was coined by English biologist Julian Huxley.

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

Examples[change | edit source]

Birds, dinosaurs, crocodiles, and all other descendants (living or extinct) of their most recent common ancestor form a clade.[2] 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.

Cladogram (family tree) of a biological group. The red and blue boxes represent clades, i.e. complete branches. The green box is not a clade, but rather represent an evolutionary grade, an incomplete group: the blue clade descends from the same ancestor but is not included in it.

Some versions of cladistics have been the subject of controversy.[3]p226[4][5]

History of cladistics[change | edit source]

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.[6] Hennig referred to his own approach as phylogenetic systematics.[1] 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.[7]

For some decades in the mid to late twentieth century, a commonly used methodology was numerical taxonomy.[3]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.[5]

Phylogenetic nomenclature[change | edit source]

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.[8] Critics of phylogenetic nomenclature include Ashlock,[9] Mayr,[10] and Williams.[11]

Phylogenetic Nomenclature Linnaean Nomenclature
Handles arbitrarily deep trees. Biased towards trees about 4 to 12 levels deep.
Primary goal is to reflect the process of evolution, as currently understood Primary goal is to group organisms in a clear and useful way
Assumes that the shape of the tree will change frequently with new discoveries New discoveries may require releveling of Genera, Classes, Orders, and Kingdoms
Limited to entities related by evolution or ancestry Supports groupings when evolution or ancestry are not fully known
Does not include a process for naming species Includes a process for giving unique names to species
Ignores established paraphyletic groups such as reptiles Permits well-known groups such as reptiles
Limited to organisms that evolved by inherited traits; not applicable to hybrid organisms, or when lateral transfer has happened Applicable to all organisms, regardless of evolutionary mechanism

Relevant pages[change | edit source]

References[change | edit source]

  1. 1.0 1.1 Hennig, Willi 1979. Phylogenetic systematics. Urbana: University of Illinois Press. ISBN 0-252-06814-9.
  2. Glossary entry "clade" Understanding Evolution. 2010. University of California Museum of Paleontology.
  3. 3.0 3.1 Mayr, Ernst 1982. The growth of biological thought: diversity, evolution and inheritance. Cambridge, MA: Harvard University Press. ISBN 0-674-36446-5.
  4. Patterson, 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. ISBN 0-12-391250-4
  5. 5.0 5.1 Ridley, Mark 1986. Evolution and classification: the reformation of cladism. Longman, London. ISBN 0-582-44497-7
  6. Dupuis, Claude 1984. Willi Hennig's impact on taxonomic thought. Annual Review of Ecology and Systematics 15: 1–24. ISSN 0066-4162
  7. 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. ISBN 978-0-8493-3498-6 [1] retrieved 2008-10-15
  8. Hennig, Willi (1975), "'Cladistic analysis or cladistic classification': a reply to Ernst Mayr", Systematic Zoology 24 (2): 244–256, doi:10.2307/2412765, JSTOR 2412765. 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, ISBN 0-674-27105-X
  9. Ashlock, Peter D. (1971), "Monophyly and associated terms", Systematic Zoology 20 (1): 63–69, doi:10.2307/2412223, JSTOR 2412223.
  10. Mayr, Ernst (1974), "Cladistic analysis or cladistic classification?", Zeitschrift fűr Zoologische Systematik und Evolutionforschung 12: 94–128, http://courses.cit.cornell.edu/jdv55/teaching/systematics/mayr%2074%20-%20cladistic%20analysis%20or%20cladistic%20classification.pdf, retrieved 2010-12-14,
    • Mayr, Ernst (1978), "Origin and history of some terms in systematic and evolutionary biology", Systematic Zoology 27 (1): 83–88, doi:10.2307/2412818, JSTOR 2412818.,
    • Mayr, E & Bock, WJ (2002), "Classifications and other ordering systems", Journal of Zoological Systematics and Evolutionary Research 40 (4): 169–194, doi:10.1046/j.1439-0469.2002.00211.x
  11. Williams, P.A. (1992), "Confusion in cladism", Synthese 01: 135–132