Nomenclature has to do with the naming of things, with the legalistics, so to speak. The zoologist is bound by the "International Code of Zoological Nomenclature." (Plants, bacteria, and viruses likewise have codes of nomenclature that apply to those taxa; although somewhat similar, they differ in detail.)

Historically, it was found necessary to introduce a standard common to all workers in a field so as to promote stability and communication. One of the problems is obvious—common names are useless on other than a regional basis. The fledermaus of German is the ratón volador of Spanish and the "bat" of English. This was incentive to go to the common language of science during the 18th Century, which was Latin. A latin name applied to a taxon could be understood everywhere as belonging to a specific group—or so it was thought.

New problems of communication arose, however. Not infrequently, the same taxon was named by workers in different places (and, of course, with different names), usually due to ignorance of earlier names. Once used in a particular region, there was local reluctance to abandon a name for another. Thus it became necessary to erect a set of rules governing which name would serve. The rule of priority states that the first name applied according to the rules of nomenclature is the legitimate name (there are provisions for suppressing obscure "long-lost" names in favor of widely used ones, however). Names that were applied according to the rules but that are invalid for any of several reasons are synonyms.

Another problem was the nature of the name itself. Before treated by Linnaeus in 1758, the "name" of a species was a descriptive phrase in Latin. Linnaeus, in the 1758 10th edition of Systema Naturae, stabilized the name of a species as consisting of two words (the binomial system of nomenclature), a noun and an adjective (in latin, of course). The first word is also the name of the genus that the species belongs to.

Thus the real name of humankind is Homo sapiens, and the genus to which we belong is Homo. Note that the generic part of the species name is always capitalized and that the specific portion is, in animals, never capitalized. The species to which we belong is not sapiens (this is akin to trying to call a red barn a "red")—both parts of the name are necessary (when no confusion can occur, it is legitimate to abbreviate, usually by the first letter, the generic portion; e.g., H. sapiens).

The rules also call for setting apart generic names, species names, and subspecies names in different type from that used otherwise; by tradition, this is done by use of italics (or, in manuscript, by underlining, which is the printer's symbol for italics). You may have noticed, however, in the italized figure captions in these web pages, the species names are not italized so as to separate them from the rest of the caption.

Get into the habit of following the rules on capitalization, underlining, etc. Although by some criteria these are minor points, misuse immediately informs others that you are ignorant in this area (which can be a disaster in a job application interview). (Also remember that the plural of genus is genera and both the singular and plural of species is species.)

Another problem treated by the rules relates to tying the name and the taxon together. Many of the early descriptions were so poor that the animal being described was later unrecognizable. In other cases, a name was based on a series of specimens that eventually was found to include several different species (or even higher taxa). Today, there is a set of rules governing the requirements for a legitimate species (or generic or subspecific) description. Among other things, including the requirements concerning publication, a single specimen, the holotype, must be named so as to tie the name to a particular taxon. Originally, the "type" referred to the ideal essence of a species; variation in a species was considered essentially a series of imperfections, of deviations from the essence (reportedly, in the early days, if a specimen thought to better reflect the essence was found, a new holotype was substituted for the old). As taxonomy matured and it was realized that variation is a property of a species, it was accepted that no single specimen can adequately represent it. Thus, this concept was dropped (though it still lingers on subconsciously in many cases). Today, the type (holotype) is merely a name bearer. Whatever taxon is actually represented by the holotypic specimen is the taxon that the name is associated with forever.

There are seven ranks of the taxonomic hierarchy which are considered minimal to classify a taxon—this is the obligate taxonomic hierarchy:


To minimally classify an animal, it must be placed within each of these seven ranks—that is, assigned to a kingdom, a phylum, etc. In addition to these obligate levels, however, is a large number of finer divisions, such as subkingdom, superphylum, infraclass, etc. (there are more than 30 in fairly common use). The principle in each case is the same—if we haven't made errors, all entities within a given taxon are more closely related to one another than any is to an entity in a different taxon; e.g., members of a subgenus are more closely related to each other than to any member of a different subgenus of the same genus. Table 1 gives a complete hierarchy as recognized by McKenna and Bell (1997).


Last Update: 11 Jan 2007

Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso