A clade (Greek = branch) is a group of organisms with a common ancestor and all its descendents (and nothing else). Such a clade is monophyletic. The term 'clade' was coined by English biologist Julian Huxley.
Cladistics is the method of classifying species of organisms into groups called clades.
Birds, dinosaurs, crocodiles, and all other descendants (living or extinct) of their most recent common ancestor form a clade. 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.
History of cladistics [change]
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. 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.
For some decades in the mid to late twentieth century, a commonly used methodology was numerical taxonomy.p221 This made made no attempt to resolve phylogeny, only similarities. The weakness of this approach was that it left out the connection between classification and evolution.
Pros and cons [change]
Phylogenetic nomenclature is classification using cladistics to show evolutionary relationships. It is now the most usual type of classification used for groups where there is good evidence of how they evolved.
Summary of advantages of phylogenetic nomenclature [change]
Proponents of phylogenetic nomenclature enumerate key distinctions between phylogenetic nomenclature and Linnaean taxonomy as follows:
|Phylogenetic Nomenclature||Linnaean Taxonomy|
|Handles arbitrarily deep trees.||Often must invent new level names (such as superorder, suborder, infraorder, parvorder, magnorder) to accommodate new discoveries. Biased towards trees about 4 to 12 levels deep.|
|Discourages naming or use of groups that are not monophyletic||Acceptable to name and use paraphyletic groups|
|Primary goal is to reflect actual process of evolution||Primary goal is to group species based on morphological similarities|
|Assumes that the shape of the tree will change frequently with new discoveries||New discoveries often require renaming or releveling of Classes, Orders, and Kingdoms|
Summary of criticisms of phylogenetic nomenclature [change]
|Phylogenetic Nomenclature||Linnaean Taxonomy|
|Limited to entities related by evolution or ancestry||Supports groupings without reference to evolution or ancestry|
|Does not include a process for naming species||Includes a process for giving unique names to species|
|Clade definitions emphasize ancestry at the expense of descriptive characteristics||Taxa definitions based on tangible characteristics|
|Ignores estalished paraphyletic groups such as reptiles||Permits well-known groups such as reptiles|
|Difficult to determine if a given species is in a clade or not (e.g. if clade X is defined as "most recent common ancestor of A and B along with its descendants", then the only way to determine if species Y is in the clade is to perform a complex evolutionary analysis)||Straightforward process to determine if a given species is in a taxon or not|
|Limited to organisms that evolved by inherited traits; not applicable to organisms that evolved via complex gene sharing or lateral transfer||Applicable to all organisms, regardless of evolutionary mechanism|
Relevant pages [change]
- Hennig, Willi 1979. Phylogenetic systematics. Urbana: University of Illinois Press. ISBN 0-252-06814-9.
- Glossary entry "clade" Understanding Evolution. 2010. University of California Museum of Paleontology.
- Mayr, Ernst 1982. The growth of biological thought: diversity, evolution and inheritance. Cambridge, MA: Harvard University Press. ISBN 0-674-36446-5.
- 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
- Ridley, Mark 1986. Evolution and classification: the reformation of cladism. Longman, London. ISBN 0-582-44497-7
- Dupuis, Claude 1984. Willi Hennig's impact on taxonomic thought. Annual Review of Ecology and Systematics 15: 1–24. ISSN 0066-4162
- 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  retrieved 2008-10-15
- 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
- Ashlock, Peter D. (1971), "Monophyly and associated terms", Systematic Zoology 20 (1): 63–69, doi:10.2307/2412223, JSTOR 2412223.
- Ashlock, Peter D. (1972), "Monophyly again", Systematic Zoology 21 (4): 430–438, doi:10.2307/2412435, JSTOR 2412435.
- Ashlock, Peter D. (1974), "The uses of cladistics", Annual Review of Ecology and Systematics 5: 81–99, doi:10.1146/annurev.es.05.110174.000501, ISSN 0066-4162.
- Ashlock, Peter D. (1979), "An evolutionary systematist’s view of classification", Systematic Zoology 28 (4): 441–450, doi:10.2307/2412559, JSTOR 2412559.
- 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
- Williams, P.A. (1992), "Confusion in cladism", Synthese 01: 135–132