Taxonomic rank

Main ranks

In his landmark publications, such as the Systema Naturae, Carolus Linnaeus used a ranking scale limited to: kingdom, class, order, genus, species, and one rank below species. Today, nomenclature is regulated by the nomenclature codes. There are seven main taxonomic ranks: kingdom, phylum or division, class, order, family, genus, species. In addition, the domain (proposed by Carl Woese) is now widely used as one of the fundamental ranks, although it is not mentioned in any of the nomenclature codes. Also, this term represents a synonym for the category of dominion (lat. dominium), introduced by Moore in 1974. Unlike Moore, Whoese et al. (1990) did not suggest a Latin term for this category, which represents a further argument supporting the accurately introduced term dominion.

A taxon is usually assigned a rank when it is given its formal name. The basic ranks are species and genus. When an organism is given a species name it is assigned to a genus, and the genus name is part of the species name.

The species name is also called a binomial, that is, a two-term name. For example, the zoological name for the human species is Homo sapiens. This is usually italicized in print and underlined when italics are not available. In this case, Homo is the generic name and it is capitalized; sapiens indicates the species and it is not capitalized.

Ranks in zoology

There are definitions of the following taxonomic ranks in the International Code of Zoological Nomenclature: superfamily, family, subfamily, tribe, subtribe, genus, subgenus, species, subspecies.

The International Code of Zoological Nomenclature divides names into "family-group names", "genus-group names" and "species-group names". The Code explicitly mentions:

Family

Genus

Species

The rules in the Code apply to the ranks of superfamily to subspecies, and only to some extent to those above the rank of superfamily. In the "genus group" and "species group" no further ranks are allowed. Among zoologists, additional terms such as species group, species subgroup, species complex and superspecies are sometimes used for convenience as extra, but unofficial, ranks between the subgenus and species levels in taxa with many species (e.g. the genus Drosophila).

At higher ranks (family and above) a lower level may be denoted by adding the prefix "infra", meaning lower, to the rank. For example, infraorder (below suborder) or infrafamily (below subfamily).

Names of zoological taxa

Ranks in botany

According to Art 3.1 of the International Code of Nomenclature for algae, fungi, and plants (ICN) the most important ranks of taxa are: kingdom, division or phylum, class, order, family, genus, and species. According to Art 4.1 the secondary ranks of taxa are tribe, section, series, variety and form. There is an indeterminate number of ranks. The ICN explicitly mentions:

primary ranks

kingdom (regnum)

division or phylum (divisio, phylum)

class (classis)

order (ordo)

family (familia)

genus (genus)

species (species)

There are definitions of the following taxonomic categories in the International Code of Nomenclature for Cultivated Plants: cultivar group, cultivar, grex.

The rules in the ICN apply primarily to the ranks of family and below, and only to some extent to those above the rank of family. Also see descriptive botanical names.

Names of botanical taxa

Taxa at the rank of genus and above have a botanical name in one part (unitary name); those at the rank of species and above (but below genus) have a botanical name in two parts (binary name); all taxa below the rank of species have a botanical name in three parts (an infraspecific name). To indicate the rank of the infraspecific name, a "connecting term" is needed. Thus Poa secunda subsp. juncifolia, where "subsp." is an abbreviation for "subspecies", is the name of a subspecies of Poa secunda.

Hybrids can be specified either by a "hybrid formula" that specifies the parentage, or may be given a name. For hybrids getting a hybrid name, the same ranks apply, prefixed with notho (Greek: 'bastard'), with nothogenus as the highest permitted rank.

Outdated names for botanical ranks

If a different term for the rank was used in an old publication, but the intention is clear, botanical nomenclature specifies certain substitutions:

Examples

Classifications of five species follow: the fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans (Homo sapiens), the peas used by Gregor Mendel in his discovery of genetics (Pisum sativum), the "fly agaric" mushroom Amanita muscaria, and the bacterium Escherichia coli. The eight major ranks are given in bold; a selection of minor ranks are given as well.

Terminations of names

Taxa above the genus level are often given names based on the type genus, with a standard termination. The terminations used in forming these names depend on the kingdom (and sometimes the phylum and class) as set out in the table below.

Pronunciations given are the most Anglicized. More Latinate pronunciations are also common, particularly /ɑː/ rather than /eɪ/ for stressed a.

All ranks

There is an indeterminate number of ranks, as a taxonomist may invent a new rank at will, at any time, if they feel this is necessary. In doing so, there are some restrictions, which will vary with the nomenclature code which applies.

The following is an artificial synthesis, solely for purposes of demonstration of relative rank (but see notes), from most general to most specific:

Significance and problems

Ranks are assigned based on subjective dissimilarity, and do not fully reflect the gradational nature of variation within nature. In most cases, higher taxonomic groupings arise further back in time: not because the rate of diversification was higher in the past, but because each subsequent diversification event results in an increase of diversity and thus increases the taxonomic rank assigned by present-day taxonomists. Furthermore, some groups have many described species not because they are really very diverse, but due to the fact of being more easily sampled and studied than other groups.

Of these many ranks, the most basic is species. However, this is not to say that a taxon at any other rank may not be sharply defined, or that any species is guaranteed to be sharply defined. It varies from case to case. Ideally, a taxon is intended to represent a clade, that is, the phylogeny of the organisms under discussion, but this is not a requirement.

Classification, in which all taxa have formal ranks, cannot adequately reflect our knowledge about phylogeny; at the same time, if taxon names are dependent on ranks, rank-free taxa can't be supplied with names. This problem is dissolved in cladoendesis, where the specially elaborated rank-free nomenclatures are used.

There are no rules for how many species should make a genus, a family, or any other higher taxon (that is, a taxon in a category above the species level). It should be a natural group (that is, non-artificial, non-polyphyletic), as judged by a biologist, using all the information available to them. Equally ranked higher taxa in different phyla are not necessarily equivalent (e.g., it is incorrect to assume that families of insects are in some way evolutionarily comparable to families of mollusks). For animals, at least the phylum rank is usually associated with a certain body plan, which is also, however, an arbitrary criterion.