Wiley, Edward O., III Natural History Museum, University of Kansas, Lawrence, Kansas.
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Formal ranks used for the organization of organisms in a Linnaean biological classification system. Biological classifications are orderly arrangements of organisms in which the order specifies some relationship. Taxonomic classifications are usually hierarchical and comprise nested groups of organisms. The actual groups are termed taxa. In the hierarchy, a higher taxon may include one or more lower taxa. As a result, the relationships among taxa are expressed as a divergent hierarchy that is formally represented by so-called tree diagrams. In Linnaean classifications (that is, biological classifications as set forth by Carolus Linnaeus in the 1700s), taxonomic categories are devices that provide structure to the hierarchy of taxa without the use of tree diagrams. By agreement, there is a hierarchy of categorical ranks for each major group of organisms, beginning with the categories of highest rank and ending with categories of lowest rank (see illustration). Although it is not necessary to use all the available categories, they must be used in the correct order. See also: Biological classification; Plant nomenclature; Plant taxonomy; Systematics; Taxonomy; Zoological nomenclature
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