The Linnaean taxonomy is widely used in the biological sciences. It was first developed by Carolus Linnaeus in the 18th century during the great expansion of natural history. Linnaean taxonomy classifies living things into a hierarchy, starting with domains or kingdoms. Kingdoms are divided phyla (singular: phylum) or divisions for plants. Phyla are divided into classes, then orders, families, genera (singular: genus) and 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:
- 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)
Rules for the proper naming and classification for all types of living organisms under the Linnaean system have been adopted by professional biologists. The rules 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 of 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 is controversial.
Originally Linnaeus had three kingdoms in his scheme, namely Plantae, Animalia and an additional group for minerals, which has since been abandoned. Since then various forms have been moved into three new kingdoms - Monera, for prokaryotes, Protista, for protozoans and algae, and Fungi. This scheme is still far from the phylogenetic ideal and the five kingdom view 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 was precipitated by the discovery of the Archaea.
See also: Evolutionary tree, which has further subdivisions and presents the most current taxonomic viewpoint.