Biological classification

For other uses, see Biological classification (disambiguation).

Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. Modern classification has its roots in the system of Carolus Linnaeus, who grouped species according to shared physical characteristics. These groupings have been revised since Linnaeus to improve consistency with the Darwinian principle of common descent. Molecular systematics, which uses genomic DNA analysis, has driven many recent revisions and is likely to continue to do so. Scientific classification belongs to the science of taxonomy or biological systematics.

Early systems

The earliest known system of classifying forms of life comes from the Greek philosopher Aristotle, who classified animals based on their means of transportation (air, land, or water).

In 1172 Ibn Rushd (Averroes), who was a judge (Qaadi) in Seville, translated and abridged Aristotle's book de Anima (On Animals) into Arabic. His original commentary is now lost, but its translation into Latin by Michael Scot survives.

An important advance was made by the Swiss professor, Conrad von Gesner (1516–1565). Gesner's work was a critical compilation of life known at the time.

The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. Advances in using this knowledge to classify living beings bear a debt to the research of medical anatomists, such as Fabricius (1537–1619), Petrus Severinus (1580–1656), William Harvey (1578–1657), and Edward Tyson (1649–1708). Advances in classification due to the work of entomologists and the first microscopists is due to the research of people like Marcello Malpighi (1628–1694), Jan Swammerdam (1637–1680), and Robert Hooke (1635–1702).

John Ray (1627–1705) was an English naturalist who published important works on plants, animals, and natural theology. The approach he took to the classification of plants in his Historia Plantarum was an important step towards modern taxonomy. Ray rejected the system of dichotomous division by which species were classified according to a pre-conceived, either/or type system, and instead classified plants according to similarities and differences that emerged from observation.

Linnaeus

Two years after John Ray's death, Carolus Linnaeus (1707–1778) was born. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735). In this work nature was divided into three realms: mineral, vegetable and animal. Linnaeus used four ranks: class, order, genus, and species.

Linnaeus is best known for his introduction of the method still used to formulate the scientific name of every species. Before Linnaeus, long many-worded names had been used, but as these names gave a description of the species, they were not fixed. By consistently using a two-word Latin name — the genus name followed by the specific epithet — Linnaeus separated nomenclature from taxonomy. This convention for naming species is referred to as binomial nomenclature.

Today, nomenclature is regulated by Nomenclature Codes, which allows names divided into ranks: see rank (botany) and rank (zoology).

Modern developments

Where Linnaeus classified for ease of identification, it is now generally accepted that classification should reflect the Darwinian principle of common descent.

Since the 1960s a trend called cladistic taxonomy or cladism has emerged, arranging taxa in an evolutionary tree. If a taxon includes all the descendants of some ancestral form, it is called monophyletic, as opposed to paraphyletic. Other groups are called polyphyletic.

A new formal code of nomenclature, the PhyloCode, is currently under development, intended to deal with clades rather than taxa. It is unclear, should this be implemented, how the different codes will coexist.

Domains are a relatively new grouping. The three-domain system was first invented in 1990, but not generally accepted until later. Now, the majority of biologists accept the domain system, but a large minority use the five-kingdom method. One main characteristic of the three-domain method is the seperation of Archaea and Bacteria, previously grouped into the single kingdom Bacteria (sometimes Monera). A small minority of scientists add Archaea as a sixth kingdom but do not accept the domain method.

Examples

The usual classifications of five species follow: the fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans (Homo sapiens), the peas used by Gregor Mendel in his discovery of genetics (Pisum sativum), the fly agaric mushroom Amanita muscaria, and the bacterium Escherichia coli. The eight major ranks are given in bold; a selection of minor ranks are given as well.

Rank	Fruit fly	Human	Pea	Fly Agaric	E. coli
Domain	Eukarya	Eukarya	Eukarya	Eukarya	Bacteria
Kingdom	Animalia	Animalia	Plantae	Fungi	Bacteria
Phylum or Division	Arthropoda	Chordata	Magnoliophyta	Basidiomycota	Proteobacteria
Subphylum or subdivision	Hexapoda	Vertebrata	Magnoliophytina	Hymenomycotina
Class	Insecta	Mammalia	Magnoliopsida	Homobasidiomycetae	γ-Proteobacteria
Subclass	Pterygota	Placentalia	Magnoliidae	Hymenomycetes
Order	Diptera	Primates	Fabales	Agaricales	Enterobacteriales
Suborder	Brachycera	Haplorrhini	Fabineae	Agaricineae
Family	Drosophilidae	Hominidae	Fabaceae	Amanitaceae	Enterobacteriaceae
Subfamily	Drosophilinae	Homininae	Faboideae	Amanitoideae
Genus	Drosophila	Homo	Pisum	Amanita	Escherichia
Species	D. melanogaster	H. sapiens	P. sativum	A. muscaria	E. coli

Notes:

• Botanists and mycologists use systematic naming conventions for higher taxa, using the Latin stem of the type genus for that taxon, plus a standard ending. (See below for a table of standard endings.) For example, the pea family Fabaceae is named after the stem "Fab-" of the type genus Faba plus the standard ending "-aceae" for a family (though to confuse matters, all plants in the genus Faba are now classified in Vicia, and the old name for the pea family, Leguminosae, is still widely used).
• Zoologists use similar conventions for higher taxa, but only up to the rank of superfamily.
• Higher taxa and especially intermediate taxa are prone to revision as new information about relationships is discovered. For example, the traditional classification of primates (class Mammalia — subclass Theria — infraclass Eutheria — order Primates) is challenged by new classifications such as McKenna and Bell (class Mammalia — subclass Theriformes — infraclass Holotheria — order Primates). See mammal classification for a discussion. These differences arise because there are only a small number of ranks available and a large number of branching points in the fossil record.
• Species may be further classified. Animals may be classified into subspecies (for example, Homo sapiens sapiens, modern humans). Plants may be classified into subspecies (for example, Pisum sativum subsp. sativum, the garden pea) or varieties (for example, Pisum sativum subsp. sativum var. macrocarpon, snow pea), with commercial "races" being called cultivars (for example, Pisum sativum sativum var. macrocarpon 'Snowbird', the common snow pea cultivar 'Snowbird'). Bacteria may be classified by strains (for example Escherichia coli O157:H7, a strain that can cause food poisoning).

Group suffixes

Taxa above the genus level are often given names derived from the Latin (or Latinized) stem of the type genus, plus a standard suffix. The suffixes used to form these names depend on the kingdom, and sometimes the phylum and class, as set out in the table below.

Rank	Plants	Algae	Fungi	Animals
Division/Phylum	-phyta	-phyta	-mycota
Subdivision/Subphylum	-phytina	-phytina	-mycotina
Class	-opsida	-phyceae	-mycetes
Subclass	-idae	-phycidae	-mycetidae
Superorder	-anae
Order	-ales
Suborder	-ineae
Infraorder	-aria
Superfamily	-acea			-oidea
Family	-aceae			-idae
Subfamily	-oideae			-inae
Tribe	-eae			-ini
Subtribe	-inae			-ina

Notes

• The stem of a word may not be straightforward to deduce from the nominative form as it appears in the name of the genus. For example, Latin "homo" (human) has stem "homin-", thus Hominidae, not "Homidae".
• For animals, there are standard suffixes for taxa only up to the rank of superfamily (ICZN article 27.2).

Related topics

• Binomial nomenclature
• Trinomial nomenclature
• Taxonomy
• International Code of Botanical Nomenclature
• International Code of Zoological Nomenclature
• List of chordate orders
• List of Latin and Greek words commonly used in systematic names
• Phylogenetic tree
• Virus classification

External links

• Phylocode
• Classification of all living things, with many pictures
• For a cladistic approach to animal classification: Classification of living things
• Wikispecies: a directory of life