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classification, in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and discussed. Modern classification also attempts to show the evolutionary relationships among organisms (see the table entitled Examples of Systematic Classification). A system based on categories that show such relationships is called a natural system of classification; one based on categories assigned only for convenience (e.g., a classification of flowers by color) is an artificial system.

Modern classification is part of the broader science of taxonomy, the study of the relationships of organisms, which includes collection, preservation, and study of specimens, and analysis of data provided by various areas of biological research. Nomenclature is the assigning of names to organisms and to the categories in which they are classified.

A modern branch of taxonomy, called numerical taxonomy, uses computers to compare very large numbers of traits without weighting any type of trait—in contrast to the traditional view that certain characteristics are more significant than others in showing relationships. For example, the structure of flower parts is considered more significant than the shape of the leaves in flowering plants because leaf shape appears to evolve much more quickly. Much of the science of taxonomy has been concerned with judging which traits are most significant. If new evidence reveals a better basis for subdividing a taxon than that previously used, the classification of the group in question may be revised. A considerable number of classification changes as well as insights in recent years have been the result of comparisons of nucleic acid (genetic material) sequences of organisms.

See also cladistics.

The Kingdoms

The broadest division of organisms has been into kingdoms. Traditionally there were two kingdoms, Animalia and Plantae, but many unicellular and simple multicellular organisms are not easily classified as either plants or animals. In 1866 the zoologist Ernst Heinrich Haeckel proposed a third kingdom, the Protista, to include all protozoans, algae, fungi, and bacteria. In the 20th cent. his proposal was refined, and a grouping became widely accepted that was made up of five kingdoms: animals; plants; Protista, including protozoans and some algae; Monera, comprising the prokaryotic bacteria and cyanobacteria (blue-green algae); and Fungi. Other groupings have been proposed from time to time.

Analysis of genetic sequences in various organisms has recently suggested placement of the Archaebacteria into a separate major group called the archaea. In this system, the second and third major groups are the other bacteria and the eukarya (or eukaryotes), organisms that have cell nuclei and include the fungi, plants, and animals.

The Lower Taxa

Kingdoms are divided into a hierarchical system of categories called taxa (sing. taxon). The taxa are, from most to least inclusive: phylum (usually called division in botany), class, order, family, genus, and species. Intermediate divisions, such as suborder and superfamily, are sometimes added to make needed distinctions. The lower a taxon is in the hierarchy, the more closely related are its members.

The species, the fundamental unit of classification, consists of populations of genetically similar interbreeding or potentially interbreeding individuals. If two populations of a species are completely isolated geographically and therefore evolve separately, they will be considered two species once they are no longer capable of mixing genetically if brought together. In a few cases interbreeding is possible between members of closely related species—for example, horses, asses, and zebras can all interbreed. The offspring of such crosses, however, are usually sterile, so the two groups are nonetheless kept separate by their genetic incompatibility. Populations within a species that show recognizable, inherited differences but are capable of interbreeding freely are called subspecies, races, or varieties.

The genus (pl. genera) is a grouping of similar, closely related species. For example, the domestic cat and the jungle cat are species of the genus Felis; dogs, wolves, and jackals belong to the genus Canis. Often the genus is an easily recognized grouping with a popular name; for example, the various oak species, such as black oak and live oak, form the oak genus (Quercus). Similarly, genera are grouped into families, families into orders, orders into classes, and classes into phyla or divisions.

Binomial Nomenclature

The present system of binomial nomenclature identifies each species by a scientific name of two words, Latin in form and usually derived from Greek or Latin roots. The first name (capitalized) is the genus of the organism, the second (not capitalized) is its species. The scientific name of the white oak is Quercus alba, while red oak is Quercus rubra. The first name applies to all species of the genus—Quercus is the name of all oaks—but the entire binomial applies only to a single species. Many scientific names describe some characteristic of the organism (alba=white; rubra=red); many are derived from the name of the discoverer or the geographic location of the organism. Genus and species names are always italicized when printed; the names of other taxa (families, etc.) are not. When a species (or several species of the same genus) is mentioned repeatedly, the genus may be abbreviated after its first mention, as in Q. alba. Subspecies are indicated by a trinomial; for example, the southern bald eagle is Haliaeetus leucocephalus leucocephalus, as distinguished from the northern bald eagle, H. leucocephalus washingtoniensis.

The advantages of scientific over common names are that they are accepted by speakers of all languages, that each name applies only to one species, and that each species has only one name. This avoids the confusion that often arises from the use of a common name to designate different things in different places (for example, see elk), or from the existence of several common names for a single species. There are two international organizations for the determination of the rules of nomenclature and the recording of specific names, one for zoology and one for botany. According to the rules they have established, the first name to be published (from the work of Linnaeus on) is the correct name of any organism unless it is reclassified in such a way as to affect that name (for example, if it is moved from one genus to another). In such a case definite rules of priority also apply.


The earliest known system of classification is that of Aristotle, who attempted in the 4th cent. B.C. to group animals according to such criteria as mode of reproduction and possession or lack of red blood. Aristotle's pupil Theophrastus classified plants according to their uses and methods of cultivation. Little interest was shown in classification until the 17th and 18th cent., when botanists and zoologists began to devise the modern scheme of categories. The designation of groups was based almost entirely on superficial anatomical resemblances.

Before the idea of evolution there was no impetus to show more meaningful relationships among species; the species was thought to be uniquely created and fixed in character, the only real, or natural, taxon, while the higher taxa were regarded as artificial means of organizing information. However, since anatomical resemblance is an important indication of relationship, early classification efforts resulted in a system that often approximated a natural one and that—with much modification—is still used. The most extensive work was done in the mid-18th cent. by Carolus Linnaeus, who devised the presently used system of nomenclature. As biologists came to accept the work of Charles Darwin in the second half of the 19th cent., they began to stress the significance of evolutionary relationships for classification.

Although comparative anatomy remained of foremost importance, other evidence of relationship was sought as well. Paleontology provided fossil evidence of the common ancestry of various groups; embryology provided comparisons of early development in different species, an important clue to their relationships. In the 20th cent., evidence provided by genetics and physiology became increasingly important. Recently there has been much emphasis on the use of molecular genetics in taxonomy, as in the comparison of nucleic acid sequences in the genetic makeup of organisms. Computers are increasingly used to analyze data relevant to taxonomy.


See E. Mayr, Principles of Systematic Zoology (1969); T. Savory, Animal Taxonomy (1972); H. M. Hoenigswald and L. F. Wiener, eds., Biological Metaphor and Cladistic Classification (1987); F. A. Stafleu and R. S. Cown, Taxonomic Literature: A Selective Guide to Botanical Publications and Collections (1988); N. Eldredge, Fossils: The Evolution and Extinction of Species (1991).

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The arrangement or classification of objects according to certain criteria. Systematics is a broader term applied to all comparative biology, including taxonomy. For classifying plants and animals, where the term taxonomy is most often applied, the criteria are characters of structure and function.

A given character usually has two or more states. These variations are used as the basis of biological classification, grouping together like species (in which the majority of the character states are alike) and separating unlike species (in which many of the character states are different). Since the acceptance by biologists of the concept of organic evolution, more and more effort has been made to produce systems of classification that conform to phylogenetic (that is, evolutionary) relationships. Taxonomy is thus concerned with classification, but ultimately classification itself depends upon phylogeny—the amount, direction, and sequence of genetic changes. Scientists try to classify lines, or clusters of lines, of descent. This has not always been the case, and in the past various other criteria have been used, such as whether organisms were edible (ancient times) and whether flowers had five stamens or four or some other number (Linnaean times). Modern taxonomists generally agree that the patterns or clusters of diversity they observe in nature, such as the groups of primates, the rodents, and the bats, are the objective results of purely biological processes acting at different times and places in the past. At the least, animal and plant taxonomy provides a method of communication, a system of naming; at the most, taxonomy provides a framework for the embodiment of all comparative biological knowledge. See Animal systematics, Classification, biological, Numerical taxonomy, Organic evolution, Phylogeny, Plant taxonomy

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between forms of taxation is between progressive forms (those proportional to income or wealth, e.g. income tax) and regressive forms (those levied at a flat rate, e.g. a poll tax). Historically, struggles by governments to increase state revenues, especially to fund WARFARE, were a major factor in state formation. As well as raising revenue, modern governments have also used changes in taxation (‘fiscal policy’) as an instrument of control over the economy or to inhibit undesirable social activities (e.g. smoking). In modern times, taxation has been frequently used as a main means of redistribution of income and wealth. However, the extent to which redistribution actually occurs as the result of taxation is debated. In the UK, for example, it is clear that some elements of redistribution through taxation benefit the better-off more than they do poorer sections of the community (e.g. tax relief on mortgages, or educational grants). Overall, even with redistribution, the poor pay a greater proportion of their incomes in taxes than other social groups. See also FISCAL CRISIS IN THE CAPITALIST STATE, STATE EXPENDITURES.


the theory and practice of CLASSIFICATION. As a scientific procedure taxonomy has been especially prominent in biology (e.g. hierarchical formal classificatory systems such as that of Linnaeus). Some proposed classifications of societies in sociology have used such classificatory systems as models (e.g. the work of Herbert SPENCER and W. G. RUNCIMAN). Although argument has raged in biology and elsewhere as to whether taxonomies should be seen as ‘natural’ or ‘imposed’, the only answer that can be given is that taxonomies are theory-relative, that when theories change taxonomies will also change, as in the move from pre- to post-Darwinian biology
Collins Dictionary of Sociology, 3rd ed. © HarperCollins Publishers 2000
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the theory of the classification and systematization of complexly organized spheres of reality, which usually have a hierarchical structure, such as the organic world or the subjects studied in geography, geology, linguistics, or ethnography. The concept of taxonomy first arose in biology; the term itself was first proposed in 1813 by the Swiss botanist A. P. de Candolle, who developed a classification of plants.

For a long time, the term “taxonomy” was usually used in biology as a synonym for “systematics.” In the 1960’s and 1970’s, there arose a tendency to define biological systematics more broadly as the science of the diversity of living organisms and of the kinship relationships between them; taxonomy was defined as a narrower discipline, or a division of systematics, concerned with the principles, methods, and rules of the classification of organisms. This point of view is advocated by the American taxon-omists G. G. Simpson and E. Mayr and the Soviet botanist A. L. Takhtadzhian. Thus, systematics deals with actual groups of organisms (taxons), and biological taxonomy is concerned primarily with the creation of a science of taxonomic categories and of the system of such categories, which would permit construction of a more informative, less contradictory, and more convenient classification with a maximum correspondence to the natural system of organisms.

The system of the organic world is exceptionally complex in structure, and serious difficulties are encountered in constructing a theory of the system; for example, in many cases there are no clear boundaries between taxa, and, consequently, it is necessary to operate with very large numbers of characters and properties. This complexity and the resultant difficulties have stimulated numerous attempts to give a theoretical (in some instances, a formal) substantiation of taxonomy and the basic taxonomic categories (numerical taxonomy). These attempts have made it possible to apply modern mathematical methods to taxonomy, but they have not yet led to generally accepted fundamental results.

In the second half of the 20th century, problems in taxonomy have come to play a significant role in biology and many other sciences that deal with large numbers of hierarchically organized discrete objects. This reflects a general tendency in modern science to assign greater importance to typology in scientific thought. Furthermore, not just the natural differences between given groupings of taxonomic categories, but even the fundamental concepts and goals of taxonomy are interpreted differently in different fields of knowledge. In linguistics, for example, taxonomy rests on the delineation of linguistic units in a text and the study of the units’ properties through analysis of their order and distribution. Accordingly, linguistic taxonomy operates by means of categories of a class of elements and of the type of relationship between the elements and classes. In linguistics, taxonomy is sometimes interpreted as the grouping of similar grammatical categories found in various languages into a single, systematized category, such as the passive voice or perfective aspect. Thus, the general principles of taxonomy as a theoretical discipline are still in the process of being established.


Mayr, E. Printsipy zoologicheskoi sistematiki. Moscow, 1971. (Translated from English.)
Liubishchev, A. A. “Okriteriiakh real’nosti v taksonomii.” In Informatsionnye voprosy semiotiki, lingvistiki i avtomaticheskogoperevoda, fasc. 1. Moscow, 1971.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A study aimed at producing a hierarchical system of classification of organisms which best reflects the totality of similarities and differences.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a. the branch of biology concerned with the classification of organisms into groups based on similarities of structure, origin, etc.
b. the practice of arranging organisms in this way
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


From the Greek words "taxis" and "nomos," which mean "division" and "law," a taxonomy classifies formally defined items in a hierarchical structure. See ontology.
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