Linnaean taxonomy

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Linnaean taxonomy classifies living things into a hierarchy, originally starting with kingdoms. Today, many biologists consider Domains to be a classification above Kingdoms. Kingdoms are divided into phyla (singular: phylum)—for animals; the term division, used for plants, is equivalent to the rank of phylum (and the current International Code of Botanical Nomenclature allows the use of either term). Phyla (or divisions) are divided into classes, and they, in turn, into orders, families, genera (singular: genus), and species (singular: species). Variety (varietas) and form (forma) are ranks below the level of subspecies that are unique to plant classification; "form" has largely fallen out of favor (although some botanists still cling to this rank), and many botanists now prefer to use "subspecies" instead of "variety" although the two are not, strictly speaking, of equivalent rank. Groups of organisms at any of these ranks are called taxa (singular: taxon), or phyla, or taxonomic groups.

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Taxonomic ranks

A summary of this scheme, from most general to most specific, would be: Image:Beetle collection.jpg

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Example classification: humans

As an example, consider the Linnaean classification for modern humans:

  • Kingdom: Animalia (with eukaryotic cells having cell membrane but lacking cell wall, multicellular, heterotrophic)
  • Phylum: Chordata (all animals with a notochord)
  • Class: Mammalia (vertebrates with mammary glands that in females secrete milk to nourish young, hair, warm-blooded, bears live young)
  • Order: Primates (collar bone, eyes face forward, grasping hands with fingers, two types of teeth: incisors and molars)
  • Family: Hominidae (upright posture, large brain, stereoscopic vision, flat face, hands and feet have different specializations)
  • Genus: Homo (s curved spine, "man")
  • Species: Homo sapiens (high forehead, well-developed chin, skull bones thin)
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Nomenclature

A strength of Linnaean taxonomy is that it can be used to develop a simple and practical system for organizing the different kinds of living organisms. The greatest innovation of Linnaeus, and still the most important aspect of this system, is the general use of binomial nomenclature, the combination of a genus name and a single specific epithet ("sapiens" in the example above), to uniquely identify each species of organism. In the example, humankind is uniquely identified by the binomial Homo sapiens. No other species of animal can have this binomial. In this way, every species is given a unique and stable name (compared with common names that are often neither unique nor consistent from place to place and language to language). This uniqueness and stability are, of course, a result of the acceptance by working systematists (biologists specializing in taxonomy); not merely of the binomial nomenclature in itself, but of much more complex codes of rules and procedures governing the use of these names.

These rules—or at least those governing the nomenclature and classification of plants and fungi—are contained in the International Code of Botanical Nomenclature, maintained by the International Association for Plant Taxonomy. The current code, the "Saint Louis Code" was adopted in 1999 and supersedes the "Tokyo code". Similar codes exist for animals and bacteria. Scientists follow these codes so that the names of organisms can be as clear and stable as possible.

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Later developments

Over time, our understanding of the relationships between living things has changed. The greatest change was the widespread acceptance of evolution as the mechanism of biological diversity and species formation. After this, it became generally understood that classifications ought to reflect the phylogeny of organisms, where each taxon should originate from a single ancestral form. Such taxa are designated as being monophyletic. In modern systems it is generally encouraged that taxa should be strictly monophyletic.

Originally, Linnaeus established three kingdoms in his scheme, namely Plantae, Animalia and an additional group for minerals, which has since been abandoned. Since then, various life forms have been moved into three new kingdoms: Monera, for prokaryotes (i.e., bacteria); Protista, for protozoans and most algae; and Fungi. This five kingdom scheme is still far from the phylogenetic ideal and has largely been supplanted in modern taxonomic work by a division into three domains: Bacteria and Archaea, which contain the prokaryotes, and Eukaryota, comprising the remaining forms. This change was precipitated by the discovery of the Archaea.

Reflecting truly evolutionary relationships, especially given the wide acceptance of cladistic methodology and numerous molecular phylogenies that have challenged long-accepted classifications, has proven problematic within the framework of Linnaean taxonomy, and some systematists have proposed a Phylocode to replace it.

See also: Evolutionary tree, which has further subdivisions and presents the most current taxonomic viewpoint.

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Quotations

  • "Taxonomy (the science of classification) is often undervalued as a glorified form of filing—with each species in its prescribed place in an album; but taxonomy is a fundamental and dynamic science, dedicated to exploring the causes of relationships and similarities among organisms. Classifications are theories about the basis of natural order, not dull catalogues compiled only to avoid chaos." Stephen Jay Gould (1990, p.98)
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References

  • Gould, S.J. (1990), Wonderful Life: The Burgess Shale and the Nature of History. Hutchinson Radius.
  • Dawkins, Richard The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution. Houghton Mifflin.
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External links

hu:Rendszertan (biológia) ru:Таксономия живой природы fi:Kasvien luokitus Linnén teennäisjärjestelmän mukaan sv:Linnés sexualsystem

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