LINNAEAN HIERARCHY

The distinction between a hierarchy and a key, and consideration of the relationships between the two, are essential to understanding either one of them. Nevertheless, in FISH/BIOL 311 we will be concerned almost exclusively with classifications in the form of hierarchies (i.e., interested sets of taxonomic categories and names; see “Rules on Names and Naming”). Throughout the course, we shall be using a single form of hierarchy that has been adopted by general agreement for most all zoological classifications and which is the basis for most all zoological nomenclature. This hierarchy was developed mainly in the seventeenth and eighteenth centuries and reached nearly definitive form (at least for zoologists) in the tenth edition of the Systema Naturae published by Carolus Linnaeus in 1758, for which reason it is called the Linnaean hierarchy. Its basic feature is a sequence of seven levels:*

Kingdom

Phylum

Class

Order

Family

Genus

Species

The sequence from top to bottom and the customary indentations indicate decreasing scope or inclusiveness of the various levels. The number of kinds of organisms to be classified has now become so enormous that seven levels are rarely enough in practice. The deficiency has been made up, for the most part, by adding additional levels designated as super- lying above the various basic levels and as, successively, sub- and infra- lying below them. Numerous proposals to add to the seven basic levels have also been made, but these have not been standardized and are not in general use. It is unnecessary to list all of them, and the use by any particular taxonomist can be picked up readily enough in his or her work. Those in widest use are probably cohort, placed between class and order, and tribe, usually, but not always, placed between family and genus. An example of a complete hierarchy used in the classification of a large group of animals (e.g., mammals) is as follows:

Kingdom

Phylum

Subphylum

Superclass

Class

Subclass

Infraclass

Cohort

Superorder

Order

Suborder

Infraorder

Superfamily

Family

Subfamily

Tribe

Subtribe

Genus

Subgenus

Species

Subspecies

This example has twenty-one levels. Use of all possible super-, sub-, and infra- levels between kingdom and subspecies would give thirty-four, probably more than is ever really needed in practice.