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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. BC 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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Classification of Disease


The word "classification" can refer to either a "thing" or an "activity." As a thing, a classification is a set of categories (pigeonholes) into which may be placed all the objects in the universe for which it has been designed. As an activity, classification is the process of placing the objects into the categories. This article shall deal only with the first meaning: a framework for organizing information.

Two terms are used in describing a classification: "universe" and "axis." The universe is the totality of the objects that are to be classifiedall diseases, all automobiles, all causes of death, all reasons why people encounter the health system, all persons in a given population, and so on. An axis is an attribute or property shared by members of the universe. In health matters, there are many axesages of patients, causes of illness, disorders produced, physiological systems disrupted, reasons for encounters, and so on.

Every classification has basic attributes:

  1. It deals with a defined universe.
  2. It is designed for a specific purpose, which determines its scheme of organization.
  3. It groups the objects, using as few groups as consistent with its purpose. In public health and epidemiology, classifications are designed primarily for compilation of statistics.
  4. It uses a schema that depends on the logic of its author (which often is a committee).
  5. It must accommodate all the objects in its universe and as a result always has one or more categories termed other, which are often called wastebasket categories.

In public health, diseases are only one of the kinds of "objects" that cause death or disability, so it is rare to develop classifications for them alone. The earliest use of classifications in public health was for presenting "causes of death," which of course included injuries. Later, as classifications were expanded to include morbidity as well as mortality, their titles were expanded to "diseases, injuries, and causes of death." In the latest version of the standard classification used in public health, the International Statistical Classification of Diseases (ICD), the universe has become even broader, and the title of the tenth revision (1992) includes Related Health Problems.

In the early twenty-first century, only ICD is in widespread use. Its universe is all individuals who have (or should have) any contact with health servicesfor prevention, rehabilitation, acute care, long-term care, behavioral problems, investigation of abnormal findings, or for any other reasons. It is not surprising that, to handle this diversity, a number of different axes are found in the classification.

An early need for multiple axes involved trauma. Early mortality statistics showed deaths by external causes. But it was equally valid to tabulate the same deaths according to the injuries sustained. These are two different axes that are used by ICD.

Certain diseases are caused by infectious agents, and one chapter in the classification uses infectious agents as the organizing axis. Other chapters use physiological systems as their organizing axes respiratory and circulatory, for example. Conflict arises, of course, because a disease such as bacterial pneumonia is both infectious and respiratory. If it is classified both ways, it will almost certainly be counted twice in the statistics.

Largely because of the multiple axis nature of ICD, an extensive set of rules called conventions has been developed to instruct the classifier how to handle these and other conflicting demands. One convention (which the United States has resisted) is the use of dagger and asterisk coding in which a code marked with a dagger () indicates the underlying disease and that with an asterisk (*) indicates the manifestation.

It should be clear by this point that classifications in health care are not really classifications of diseases or injuries or causes of disability but are actually classifications of individuals who are of interest to the public health community. As a result, almost never can the classifying be done from a single factor, such as the diagnosis. Rather, the person's other attributes, such as age and other diagnoses, must at least be considered and taken into the classification decision when called for by the conventions.

Retrieval of information for making statistical tabulations or finding individual case records is done by referring to the codes that have been substituted for the labels of the categories; retrieval is code-dependent. It is essential, then, to know the limitations presented by this fact. Retrieval can never produce any more detailed information than the category levelthe code is equivalent to the category label. This is a serious limitation when, for example, an epidemic appears and it is a condition that is hidden in a wastebasket category. For example, in the 1970s, Guillain-Barré syndrome was lost in "Polyneuritis and polyradiculitis," where it could not be separated from the other miscellany.

Also, in twenty-first-century information systems, neither the category label nor the category content can be known with certainty, because there is no method for determining the source of the code, that is, the classification from which the code was taken and the version of that classification. For example, code 395 was for Meniere's disease in ICD-6 and ICD-7. With ICD-8 and ICD-9 it was used for diseases of the aortic valve. Especially in ICD 's derivatives, such as the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) in North America, changes are made annually to reflect new diseases and new knowledge. The result may be to add a new disease to an existing category or to move a disease from one category to another. In the mid-1980s, code 279.1 (deficiencies of cell-mediated immunity) was the category to which AIDS (acquired immunodeficiency syndrome) was assigned; after 1986, AIDS was supposed to go to an infectious disease category. Whether it did or not is moot279.1 looks just the same. This problem, the ambiguity of category labels and codes, will persist until the information systems are modified and standardized to tag each code with an unique source identifier.

In view of the requirement that a classification be designed with a purpose, it is no surprise that ICD is increasingly unsatisfactory. Beginning with the desire simply to tabulate mortality statistics, it has taken on the burden of trying to serve multiple purposes, to accommodate morbidity, health care reimbursement, quality review, epidemiological surveillance, evidence-based medicine, facility planning, public policy, and others. Developers of electronic medical records are expecting it to serve the needs for clinical care for individual patients as well. One classification cannot serve all masters equally well.

It will not be possible to have optimal classifications for public health and epidemiology, as well as for the other legitimate uses of health and health care information, until a simple but major modification of the information system is adopted. That modification is to capture and uniquely and permanently code the specific diagnoses (clinical entities) which go into the classification categories. When that is done, the entities can be distributed into a variety of classifications, each designed optimally for its intended purpose.

Vergil Slee

(see also: International Classification of Diseases )


Israel, R. A. (1978). "The International Classification of Diseases: Two Hundred Years of Development." Public Health Reports 93:150152.

L'Hours, A. G. P. (1990). An Overview of the Tenth Revision of the International Statistical Classification of Diseases and Related Health Problems (JCD-10). Geneva: World Health Organization.

Slee, V. N.; Slee, D. A.; and Schmidt, H. J. (2000). The Endangered Medical Record: Ensuring Its Integrity in the Age of Informatics. St. Paul, MN: Tringa Press.

White, K. L. (1985). "Restructuring the International Classification of Diseases: Need for a New Paradigm." The Journal of Family Practice 21:1720.

World Health Organization (1992). International Statistical Classification of Diseases and Related Health Problems, 10th revision (ICD-10), 3 vols. Geneva: Author.

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83. Classification

See also 288. NAMES ; 301. ORDER and DISORDER .

a name composed of two terms, a generic and a specific. binomial , adj.
biosystematy. biosystematic, biosystematical , adj.
the science of the classification of living things. Also called biosystematics . biosystematic , biosystematical , adj.
the area of taxonomy that uses cytological structures, as chromosomes, in classifying organisms.
division of material into two parts for the purpose of classification. dichotomist , n.
1. the science of method or orderly arrangement and classification.
2. any system created to impose order. See also 250. LOGIC . methodological , adj.
the investigation and classification of trivial matters. micrologist , n. micrologic, micrological , adj.
the enumeration and description of a museums collection. museographer, museographist , n.
a person who invents or assigns names, as in nomenclature. See also 53. BOOKS .
1. a system of names used in the classification of an art or science or other field or subject.
2. a naming system peculiar to a social group. See also 53. BOOKS .
Biology. a technical name, as one that forms part of a system of nomenclature or classification.
the application of onyms; classification or systematic nomenclature.
the nomenclature of organs. organonymal, organonymic , adj.
any of the basic divisions of the plant or animal kingdom.
the systematic classification and description of nature. See also 178. GEOGRAPHY ; 179. GEOLOGY . physiographer , n. physiographic, physiographical , adj.
an advocate of the quinary system of animal classification, which regarded all animal groups as being naturally divisible by five. quinarian, quinary , adj.
the condition or quality of being of the same type. syntypic , adj.
the study of classification and methods of classification. systematician, systematist , n.
the practice or act of systematizing.
the study or science of systematizing.
a botanical or zoological name in which the two terms, the generic name and the specific, are the same (a practice no longer approved by the International Code of Botanical Nomenclature). tautonymic, tautonymous , adj.
taxonomy, taxology
1. the technique or science of classification.
2. the scientific identification, naming, and classification of living things. Also called systematics . taxonomist , n. taxonomie, taxonomical , adj.
1. the terms of any branch of knowledge, field of activity, etc.
2. the classification of terms associated with a particular field; nomenclature.
3. Rare. the science of classification. terminologic, terminological , adj.
division into three parts, especially the theological division of mans nature into the body, the soul, and the spirit. trichotomic, trichtomous , adj.
the use of three terms or names in the classification of a species, genus, variety, etc. trinomial , adj.
a trinomial or name composed of three terms.
Rare. a universal system of nomenclature or classification.
zoological classification; the scientific classification of animals.

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Bacteria, Classification

Bacteria, Classification

The shapes of bacterial cells, often of keen interest to forensic investigators, are classified as spherical (coccus), rodlike (bacillus), spiral (spirochete), helical (spirilla), and comma-shaped (vibrio) cells. Many bacilli and vibrio bacteria have whiplike appendages (called flagella) protruding from the cell surface. Flagella are composed of tight, helical rotors made of chains of globular protein called flagellin, and act as tiny propellers, making the bacteria very mobile. On the surface of some bacteria are short, hairlike, proteinaceous projections that may arise at the ends of the cell or over the entire surface. These projections, called fimbriae, facilitate bacteria adherence to surfaces.

Other proteinaceous projections, called pili, occur singly or in pairs, and join pairs of bacteria together, facilitating transfer of DNA between them.

Oxygen may or may not be a requirement for a particular species of bacteria, depending on the type of metabolism used to extract energy from food (aerobic or anaerobic). Obligate aerobes must have oxygen in order to live. Facultative aerobes can also exist in the absence of oxygen by using fermentation or anaerobic respiration. Anaerobic respiration and fermentation occur in the absence of oxygen, and produce substantially less ATP than aerobic respiration.

During periods of harsh environmental conditions some bacteria can produce within themselves a dehydrated, thick-walled endospore. These endospores can survive extreme temperatures, dryness, and exposure to many toxic chemicals and to radiation. Endospores can remain dormant for long periods (hundreds of years in some cases) before being reactivated by the return of favorable conditions.

Pathogens are disease-causing bacteria that release toxins or poisons that interfere with some function of the host's body.

An understanding of the basic classification of bacteria found at crime scenes and taken from bodies at autopsy is critical to forensic investigators (including forensic epidemiologists) attempting to identify bacteria. The identification schemes of Bergey's Manual are based on morphology (e.g., coccus, bacillus), staining (gram-positive or negative), cell wall composition (e.g., presence or absence of peptidoglycan), oxygen requirements (e.g., aerobic, facultatively anaerobic) and biochemical tests (e.g., which sugars are aerobically metabolized or fermented).

Another important identification technique is based on the principles of antigenicitythe ability to stimulate the formation of antibodies by the immune system . Commercially available solutions of antibodies against specific bacteria (antisera) are used to identify unknown organisms in a procedure called a slide agglutination test. A sample of unknown bacteria in a drop of saline is mixed with antisera that has been raised against a known species of bacteria. If the antisera causes the unknown bacteria to clump (agglutinate), then the test positively identifies the bacteria as being identical to that against which the antisera was raised. The test can also be used to distinguish between strains of slightly different bacteria belonging to the same species.

see also Anthrax; Bacterial biology; Bacteria, growth and reproduction; Bacterial resistance and response to antibacterial agents; Biological weapons, genetic identification; Biosensor technologies; Bubonic plague; Decontamination methods.

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1. Any scheme for structuring data that is used to group individuals. In ecological and taxonomic studies numerical classification schemes have been devised, but various hierarchical or non-hierarchical classificatory strategies have also been used. In taxonomy, the fundamental unit is the species. Among living forms species are groups of individuals that look alike and can interbreed, but cannot interbreed with other species. In palaeontology, where breeding capability cannot be determined, species are defined according to morphological similarities. In formal nomenclature, taxonomists follow the binomial system devised by Linnaeus. In this system each species is defined by two names: the generic (referring to the genus) and the specific (referring to the species). Thus various related species may share a common generic name. Genera (sing. genus) may be combined with others to form families, and related families combined into an order. Orders may be combined into classes, and classes into phyla (sing. phylum) or divisions in the case of Metaphyta. For example, the brachiopods comprise some 11 orders split between two classes and these two classes are the major subdivisions of the phylum Brachiopoda. The basic groupings, the phyla, are combined together into kingdoms, e.g. Plantae (the plants) and Animalia (animals). Some workers have tackled the uncertainties arising from subjectivity in classification by using numerical methods. In their view, if enough characters were measured and represented by cluster statistics, the distances between clusters could be used as a measure of difference. Even so, the worker has to decide (subjectively) how best to analyse the measurements, and so objectivity is lost. Other workers emphasize those features shared by organisms that show a hierarchical pattern (see CLADISTICS).

2. In remote sensing, the computer-assisted recognition of surface materials. The process assigns individual pixels of an image to categories (e.g. vegetation, road) based on spectral characteristics compared to spectral characteristics of known parts of an image (training areas). Assignation of pixels is not always possible when the parameter space of different training areas overlaps. In such cases a principal component analysis prior to classification may be used to allow better separation of training areas by increasing the overall parameter space. See also BOX CLASSIFICATION; MINIMUM-DISTANCE-TO-MEANS CLASSIFICATION; and MAXIMUM-LIKELIHOOD CLASSIFICATION.

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classification The arrangement of organisms into a series of groups based on physiological, biochemical, anatomical, or other relationships. An artificial classification is based on one or a few characters simply for ease of identification or for a specific purpose; for example, birds are often arranged according to habit and habitat (seabirds, songbirds, birds of prey, etc.) while fungi may be classified as edible or poisonous. Such systems do not reflect evolutionary relationships. A natural classification is based on resemblances and is a hierarchical arrangement. The smallest group commonly used is the species. Species are grouped into genera (see genus), the hierarchy continuing up through tribes, families, orders, classes, and phyla (see phylum) to kingdoms and, in some systems, to domains. In traditional systems of plant classification the phylum was replaced by the division. Higher up in the hierarchy the similarities between members of a group become fewer. Present-day natural classifications try to take into account as many features as possible and in so doing aim to reflect evolutionary relationships (see cladistics). Natural classifications are also predictive. Thus if an organism is placed in a particular genus because it shows certain features characteristic of the genus, then it can be assumed it is very likely to possess most (if not all) of the other features of that genus. See also binomial nomenclature; taxonomy.

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classification Any scheme for structuring data that is used to group individuals or sometimes attributes. In ecological and taxonomic studies especially, quite sophisticated numerical classification schemes have been devised, and the methods developed in these disciplines are being applied increasingly in other fields, notably pedology and palaeontology. Various classificatory strategies may be used (e.g. hierarchical or non-hierarchical; and where hierarchical schemes are used, these may be divisive or agglomerative, and monothetic or polythetic, with the divisive polythetic approach being considered the optimum strategy). Compare ordination methods.

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classification Organization of organisms into categories based on appearance, structure, genetic sequence or evolution. The categories, from the most inclusive to the most exclusive are kingdom, phylum, class, order, family, genus, species, and sometimes variety. For example, the domestic dog is classified as kingdom Animalia, phylum Chordata, class Mammalia, order Carnivora, family Canidae, genus Canis, species Canis familiaris.

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classification (clustering)

classification (clustering) See CLUSTER ANALYSIS.

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classification See TAXONOMY.

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