Taxonomic rank

In biological taxonomy, a taxonomic rank denotes the level that of a group of organisms—either taxon or clade—occupies in a hierarchical system of classification, which is based on evolutionary relationships. Some authors prefer to use the term nomenclatural rank, contending that, according to some definitions, the ranking of organisms is more accurately described under nomenclature rather than that of taxonomy. Thus, the most inclusive taxa (or clades), such as the Eukarya and Animalia, are assigned the highest ranks of classification, whereas the least inclusive ones, such as Homo sapiens, Bufo bufo, Tyrannosaurus rex, and Vulpes vulpes, are given the lowest ranks.

Ranks can be either "absolute", in which several descriptive terms (such as species, genus, tribe, family, order, class, phylum, kingdom, and domain) are ranks themselves; or "relative", where ranks are designated instead by an indented taxonomy in which the level of indentation reflects the rank. This page emphasizes absolute ranks.

Rank-based codes (such as the Zoological Code, the Botanical Code, the Code for Cultivated Plants, the Prokaryotic Code, and the Code for Viruses) all require absolute ranks, but absolute ranks are not required in all nomenclatural systems—the PhyloCode (of phylogenetic nomenclature), for instance, does not require absolute ranks.

Consider a particular species, the red fox, Vulpes vulpes, and its classification and ranking in context of the Zoological Code: the specific epithet vulpes (small v) identifies the particular species vulpes (red fox) as found in the genus Vulpes (capital V); which genus groups all species of the "true" foxes at a rank assigned one level above that of the specific epithet vulpes. The closest relatives of the foxes are grouped in the family taxon Canidae, which includes dogs, wolves, and jackals. The next higher taxon in hierarchy is the order Carnivora, which includes the suborder caniforms: all those mentioned above plus bears, seals, weasels, skunks, and raccoons; and the suborder feliforms: cats, civets, hyenas, mongooses. Carnivorans are but one group of the hairy, warm-blooded, nursing members of the class Mammalia, which all in turn are included among the animals with notochords in the phylum Chordata. And all those listed above are collected among all animals in the kingdom Animalia. Finally at the highest rank, all the above are grouped together with all other organisms possessing cell nuclei in the domain taxon Eukarya.

More generally, taxa describe and portray the hierarchical grouping of organisms resulting from the classifications process; and the assigned ranks of the taxa indicate the relative positions of such groups within the hierarchy. High-ranking taxa (e.g., domains or kingdoms) contain more sub-taxa groups than lower-ranking taxa, such as phyla or genera—as illustrated by the inverted pyramid graphic of taxonomic rank. And species, or any subspecies, contain the least number of sub-taxa groups, or none at all. The ranking of a given taxon reflects the evolutionary inheritance of traits or molecular features from an ancestor common with other taxa.

The binomial name is basic; which means that to identify a given organism, it is usually not necessary to specify the names of ranks other than the first two—genus and species (i.e., genus and specific epithet)—within a taxonomy comprising a rank-based code. This is not true globally because most rank-based codes are independent of each other, and there are many inter-code homonyms (i.e., the same name used for different organisms among the several codes)—typically for an animal or for a taxon covered by a given rank-based code. For this reason, attempts were made at creating a BioCode that would regulate all taxon names; but this mission has failed thus far due largely to firmly entrenched naming traditions among the communities.

The International Code of Zoological Nomenclature defines rank as: "The level, for nomenclatural purposes, of a taxon in a taxonomic hierarchy (e.g. all families are for nomenclatural purposes at the same rank, which lies between superfamily and subfamily)." Note that discussions on this page generally assume that taxa are clades (monophyletic groups of organisms), but such is not required by either the Zoological Code or the Botanical Code, (i.e., the International Code of Nomenclature for algae, fungi, and plants); and some experts on biological nomenclature hold that it should not be required. And in that case, the hierarchy of taxa—hence, their ranks—would not necessarily reflect the hierarchy of clades.