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  Taxonomy (Greek, ‘naming groups’) is the science of biological classification and involves the organization of related species into hierarchical groups (taxa) that are related to one another, and which may themselves be organized along the same lines. The relationship between these species and groups need not be genetic, though most taxonomic systems are based on phylogeny and therefore attempt to reflect evolutionary relationships. The chief alternative to this phyletic taxonomy is a taximetric classification, which attempts to reduce subjectivity of judgement by measuring quantifiable taxonomic features and, using a computer to collate the data, to produce a classification based on total similarity. This is most effectively done by composing the DNA of separate species, though this strategy can only be applied where the DNA sequence has been ellucidated.

The first attempt at classification of living organisms was made by Aristotle (384 - 322 BCE) and his pupil Theophrastus ( (c. 371-287  BCE)) who described plants and animals in ranked groups based on their apparent complexity according to Aristotle\'s system of logic. The principles of this system endured until the 19th century when phylogenics appeared. During the Renaissance, interest grew in the idea of imposing order on man\'s knowledge of nature. Botanical gardens were being established and anatomy was an expanding field, providing much new material for classification. In the 17th century, John Ray determined that the species was the basic unit of taxonomy and summed up many of the previous attempts at classification, but the 18th-century, Swedish botanist Linnaeus, author of the Systema Naturae, is generally considered the founder of modern taxonomy. He developed a system which was important because it could be standardized. Species were given binomial names which indicate the genus (the group of immediate relatives) and the species (for example, Homo sapiens and his extinct but close relative Homo erectus) and these were arranged into hierarchies. He published many books which could be used to identify species because he used systematic, consistent taxonomic keys.

The Linnaean system is the basis for the taxonomic system in use today, which divides life into kingdoms (such as the animal kingdom) and each kingdom into hierarchically arranged groups (phylum, class, order, family and genus) according to perceived evolutionary closeness. The subjective nature of such a system means that it is in dynamic equilibrium and, as new techniques to investigate phylogeny become available (for example, immunology and molecular biology), the taxonomic positions of many species are continually being reinforced or undermined. However, the sine qua non of taxonomy is that it imposes order on living species: thus a classification system should be reliable and consistent above all else. RB

See also homology; morphology; palaeontology; speciation.Further reading Salvador Luris, A View of Life.



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