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Classification
A Natural System
Difficulties in
Classifying Microbes

Molecular Phylogeny
Eucarya
Archaea
Bacteria
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Classifications

©2000 Gary Olsen, University of Illinois

In general, a classification is a method for organizing information. Human beings classify things spontaneously. For example, after seeing enough examples of chairs, we form a classification in which any given object is either a chair or a non-chair. Given a new object we do remarkably well at deciding whether it is a chair (in essence, whether or not it is practical and socially acceptable sit on it). Although there is little scientific value in this particular classification, it illustrates their ubiquity.

Classifications Group Similar Things Together

In general, classifications group similar things together. This definition is necessarily vague; there are many reasonable ways of defining similarity, and hence many alternative classifications for the same things. In the example of chairs, we considered the relationship (similarity) of objects to an abstract definition of chair. This definition was based on a potential function of the object. In choosing to classify by "chairishness", we assign less importance to other properties - color, hardness, age, etc. - that can vary without altering the utility of the object as a chair.

There is no "right way" to classify things. For example, words can be classified

  1. by their spelling (as in a dictionary);

  2. by the spelling of a foreign language synonym (as in a bilingual dictionary)

  3. by their meanings (as in a thesaurus).l

The intended use of a classification is an integral part of deciding what properties it should be based upon. Here are some other classifications that you have encountered: the Library of Congress book classifications (organized by subject); the fiction section in a library (usually organized by the author's last name); and a food store (organized primarily by storage temperature).

An early biological classification is implicit in the ancient Greeks' definition of the three kingdoms: animal, vegetable and mineral. This classification split animate matter into (what we would now call) plants and animals. More detailed classifications also existed. For example, Aristotle grouped humans and birds together, since they walk on two feet.

Classifications are often Hierarchical

A key property of classifications is that they can be nested within one another, creating an hierarchy. Thus, any group within a classification can be split in still greater detail. For example, objects that have been classified as chairs might be subdivided into stools, rocking chairs, recliners, etc. Similarly, listings in the "yellow pages" of a telephone directory are first classified by the product or service, and then within each of these categories they are further classified alphabetically.

There is no limit to the depth of a hierarchical classification. Most only have a few levels, but there are some that are quite deep. When alphabetical order is considered carefully, it is seen that the first level of classification deals only with the first letter of each word. Then all words starting with the same first letter (for example "c") are categorized by their second letter, adding a second level to the alphabetical classification. This process continues one letter at a time until all words have been distinguished.

Biological Classification and the Linnaean System

Linnaeus defined the biological classification system that we still use for plants and animals, and, with relatively minor modifications, for fungi and microorganisms. It is a hierarchical system that starts with a few categories at the highest level, and further subdivides them at each lower level. The levels in the hierarchy were given names that are already familiar to you 2:

Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Using this system, a fruit fly might be classified as:
Eucarya
Animalia
Arthropoda
Insecta
Diptera
Drosophilidae
Drosophila
melanogaster
This means that this fly is the species melanogaster in the genus Drosophila in the Family Drosophilidae in the Order Diptera in the Class Insecta in the Phylum Arthropoda in the Kingdom Animalia in the Domain Eucarya. In the Linnaean system, to uniquely name a species it is necessary to supply both the genus and species, Drosophila melanogaster.3

In the case of animals, Linnaean classifications often reflect our intuitive sense of similarity. Thus in the following classifications species 2 and 3 have more levels of classification in common than either has with species 1. This is a reflection of the greater similarity of the fly (Drosophila melanogaster) and the mosquito (Aedes aegypti), than of fly and lobster (Homarus americanus) or mosquito and lobster.


Classifications of Three Species
LevelSpecies
123
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Eucarya
Animalia
Arthropoda
Crustacea
Decapoda
Caridea
Homarus
americanus
Eucarya
Animalia
Arthropoda
Insecta
Diptera
Drosophilidae
Drosophila
melanogaster
Eucarya
Animalia
Arthropoda
Insecta
Diptera
Nematocera
Aedes
aegypti

The shared levels of classification can be accentuated by slightly reformatting the data in the table.
 

Classifications of Three Species
LevelSpecies
123
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Eucarya
Animalia
Arthropoda
Crustacea
Decapoda
Caridea
Homarus
americanus
Insecta
Diptera
Drosophilidae
Drosophila
melanogaster
Nematocera
Aedes
aegypti

Alternatively, the hierarchical nature of the classification can be represented as an outline:

Eucarya
Animalia
Arthropoda
Crustacea
Decapoda
Caridea
Homarus
americanus

Insecta
Diptera
Drosophilidae
Drosophila
melanogaster
Nematocera
Aedes
aegypti

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