Linnaean taxonomy

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
   * Phylum
        o Subphylum
        o Superclass
   * Class
        o Subclass
        o Superorder
   * Order
        o Suborder
        o Superfamily
   * Family
        o Subfamily
   * Genus
        o Subgenus
   * Species
        o Subspecies

An acronym mnemonic for remembering this list in order is: "King Phillip
called out for good soup". Another is "Kings play chess on fat green
stools".

As an example, consider a classification of butterfly weed, a species of
plants native to the roadsides and fields of eastern North America. The
Linnean classification for this plant is:

   * Kingdom: Plantae (all plants)
   * Division: Magnoliophyta (all flowering plants)
   * Class: Magnoliopsida (dicotyledons)

   * Order: Gentianales (all flowers that have united petals and elaborate
     stigmas)
   * Family: Asclepiadaceae (all plants that have an elaborate structure of
     fused stamens and stigmas in the flowers)
   * Genus: Asclepias (milkweeds)
   * Species: Asclepias tuberosa (distinguished by its tuberous roots 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 this example), to uniquely identify species of organisms. In
the example above, the butterfly weeds are uniquely identified by the
binomial Asclepias tuberosa. No other species of plant can have this
binomial. In this way, every species can be given a unique, stable name.

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. 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 some further subdivisions and presents
the current taxonomic view.