The Linnaean taxonomy is a system of classification widely used in the biological sciences. It was first developed by Carolus Linnaeus in the 18th century during the great expansion of natural history knowledge. Linnaean taxonomy classifies living things into a hierarchy, starting with domains or kingdoms. Kingdoms are divided into phyla (singular: phylum)—for animals; the term divisions is used for plants. Phyla are divided into classes, and they, in turn, into orders, families, genera (singular: genus), and species (singular: species). Groups of organisms at any of these ranks are called taxa, or phyla, or taxonomic groups.
A summary of this scheme, from most general to most specific, would be:
Mnemonics for remembering the major divisions of this list in order via common intials, include:
King Phillip called out for good soup
Kings play chess on fat green stools.
Kings play cards on fairly good soft velvet (with "v" standing for "variety")
King Philip came over from Germany swimming.
Kenneth, please close our front gate soon.
Keep plates clean or family gets sick.
Killing people causes outbursts from general society.
Klingon phasers charge on fast gray ships.
As an example, consider the classification of a Butterfly weed, a species of plant native to roadsides and fields of eastern North America. The Linnean classification for this organism is:
Kingdom: Plantae (all "plants" ~ see, however, plants)
Division: Magnoliophyta (all flowering plants)
Class: Magnoliopsida (the dicotyledons)
Order: Gentianales (all plants with flowers having united petals and elaborate stigmas)
Family:Asclepiadaceae (all plants having an elaborate structure of fused stamens and stigmas in the flowers)
Genus:Asclepias (most milkweeds)
Species:Asclepias tuberosa (the milkweed distinguished by its tuberous root and orange-red flowers)
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 most important aspect of this is the general use of binomial nomenclature, the combination of a genus name and a specific epithet ('tuberosa', in the example above), to uniquely identify each species of organism. In the example, the Butterfly weed is uniquely identified by the binomial Asclepias tuberosa. No other species of plant 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.
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. In some systems it is generally encouraged that taxa should be strictly monophyletic, but this idea is controversial.
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.
See also: Evolutionary tree, which has further subdivisions and presents the most current taxonomic viewpoint.