Taxonomic rank
In biological taxonomy, a taxonomic rank determines the level that a group of organisms—either taxon or clade—occupies in a hierarchical system of classification, which is usually but not necessarily based on evolutionary relationships between those organisms. Some authors prefer to use the term nomenclatural rank,[1] contending that, according to some definitions, the hierarchical rank occupied by a group of organisms is more accurately described as concept of nomenclature rather than taxonomy. In general, the most inclusive taxa (or clades), such as the Eukarya and Animalia, are assigned the highest ranks in a particular system of taxonomy, 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 a pre-defined set of several descriptive terms (such as species, genus, tribe, family, order, class, phylum, kingdom, and domain) constitutes the set of allowable ranks for taxa; or "relative", where ranks are designated instead by an indented taxonomy in which the level of indentation reflects the rank. In a relative system, the rank of a taxon is not necessarily an inherent attribute of the taxon so much as an attribute of a particular indented/nested hierarchy defining the relations between different groups. Additionally, relative systems of taxonomic rank often allow indefinitely adding further nested taxa to allow finer distinctions to be captured as needed.
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.[2]
Example
[edit]Consider a particular species, the red fox, Vulpes vulpes, and its classification and ranking in context of the Zoological Code: the specific epithet vulpes (the second word, with small v) identifies the particular species vulpes (red fox) as found in the genus Vulpes (the first word, capital V); this genus groups all species of the "true" foxes at a rank assigned one level above that of the species vulpes. The closest relatives of the foxes are grouped in the family taxon Canidae, which also includes dogs, wolves, and jackals. The next higher taxon but one in the 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.
Carnivora are, in turn, one group of the hairy, warm-blooded, nursing members of the class Mammalia. Mammalia, 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 classification 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 and derivation from an ancestor common with other taxa, often reflected in traits or molecular features of the organism.
History
[edit]Older approaches to taxonomic classification were phenomenological, forming groups on the basis of similarities in appearance, organic structure and behavior.
In his landmark publications, such as the 10th edition of his Systema Naturae, Carl Linnaeus used and popularized a ranking scale limited to kingdom, class, order, genus, species, and one subspecific rank, variety.
A pattern of groups nested within groups was specified by Linnaeus' classifications of plants and animals, and these patterns began to be represented as tree diagrams of the animal and plant kingdoms toward the end of the 18th century, well before Charles Darwin's On the Origin of Species was published.[3] The classification structure used in Systema Naturae did not entail any generating or originating process such as evolution, but instead relied on observable characteristics and primarily served as a guide to quickly identify samples by species.
With Darwin's theory, it was soon generally accepted that a classification should reflect the Darwinian principle of common descent.[4]
One of the first modern groups tied to fossil ancestors was birds.[5] Using the then newly discovered fossils of Archaeopteryx and Hesperornis, Thomas Henry Huxley pronounced that they had evolved from dinosaurs, a group formally named by Richard Owen in 1842.[6][7] The resulting description, that of dinosaurs "giving rise to" or being "the ancestors of" birds, is the essential hallmark of evolutionary taxonomic thinking.
20th century onwards
[edit]With the modern evolutionary synthesis of the early 1940s, an essentially modern understanding of the evolution of the major groups was in place. As evolutionary taxonomy is based on Linnaean taxonomic ranks; the two terms are largely interchangeable in modern use.[8]
The cladistic method has emerged since the 1960s.[4] In 1958, Julian Huxley used the term clade, and later, in 1960, Cain and Harrison introduced the term cladistic.[9] The salient feature is the arrangement of taxa in a hierarchical evolutionary tree, with the desired objective of all named taxa being monophyletic (based on a common ancestor).[4] A taxon is called monophyletic if it includes all the descendants of an ancestral form.[10][11] Groups that have descendant groups removed from them are termed paraphyletic,[10] while groups representing more than one branch from the tree of life are called polyphyletic.[10][11]
Monophyletic groups are recognized and diagnosed on the basis of synapomorphies, shared derived character states.[12]
Today, the nomenclature is regulated by the nomenclature codes.
Cladistic classifications are compatible with traditional Linnean taxonomy and the Codes of Zoological and Botanical nomenclature, to a certain extent.[13] An alternative system of nomenclature, the International Code of Phylogenetic Nomenclature or PhyloCode, has been proposed, which regulates the formal naming of clades.[14][2][15] Linnaean ranks are optional and have no formal standing under the PhyloCode, which is intended to coexist with the current, rank-based codes.[2] While popularity of phylogenetic nomenclature has grown steadily in the last few decades,[15] it remains to be seen whether a majority of systematists will eventually adopt the PhyloCode or continue using the current systems of nomenclature that have been employed (and modified, but arguably not as much as some systematists wish)[16][17] for over 250 years.
Two important new methods, developed in the second half of the 20th century, drastically changed taxonomic practice:
- The advent of cladistics, which stemmed from the works of the German entomologist Willi Hennig.[18] Cladistics is a method of classification of life forms according to the proportion of characteristics that they have in common (called synapomorphies). It is assumed that the higher the proportion of characteristics that two organisms share, the more recently they diverged from a common ancestor.
- Molecular systematics, based on genetic analysis, which can provide much additional data that prove especially useful when few phenotypic characters can resolve relationships, as, for instance, in many viruses, bacteria[19] and archaea, or to resolve relationships between taxa that arose in a fast evolutionary radiation that occurred long ago, such as the main taxa of placental mammals.[20]
Domains are a relatively new grouping, so named by Carl Woese in 1990 as a formal way to systematise the separation of Archaea from Bacteria in the three-domain system. In the 2020s, the domain is now widely used as a fundamental rank. Both regio and dominium have been proposed as the Latin form, and it was adopted into the International Code of Nomenclature of Prokaryotes in 2023 with the Latin form dominium.[21]
Relative systems of taxonomic rank
[edit]In systems of relative taxonomic rank, ranks are designated instead an indented taxonomy in which the level of indentation reflects the rank. In a relative system, the rank of a taxon is not necessarily an inherent attribute of the taxon so much as an attribute of a particular indented/nested hierarchy defining the relations between different groups. Additionally, relative systems of taxonomical rank often allow indefinitely adding further nested taxa to allow finer distinctions to be captured as needed.
The PhyloCode, which governs phylogenetic nomenclature, does not assign absolute ranks to the taxa defined thereunder.[2] Instead, taxa above the species-level are defined cladistically, based on their evolutionary relationships to known species, and these defined clades naturally fall into a relative hierarchy, allowing for distinctions between groups to be made on as precise a level as desired, and for those groups to be indefinitely nested, rather than being limited to a pre-defined list of absolute taxonomic ranks.
Absolute systems of taxonomic rank
[edit]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)."[22] 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.[23] And in that case, the hierarchy of taxa—hence, their ranks—would not necessarily reflect the hierarchy of clades.
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 thus 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 or 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. When a subspecific epithet is used, it is placed after the specific name, and uncapitalised and italicised like the specific epithet; the resulting subspecific name is sometimes called a trinomen. For instance, modern humans are Homo sapiens sapiens, or H. sapiens sapiens.
In zoological nomenclature, higher taxon names are normally not italicized, but the Botanical Code, the Prokaryotic Code, the Code for Viruses, the draft BioCode[24] and the PhyloCode[2] all recommend italicizing all taxon names (of all ranks).
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.[25] 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;[24] but this mission has failed thus far[26] due largely to firmly entrenched naming traditions among the communities.[15]
Fundamental ranks
[edit]There are 7 (or sometimes 8) ranks common to almost every popular modern system of absolute taxonomic ranks. These include seven main taxonomic ranks: kingdom, phylum or division, class, order, family, genus, and species; these seven ranks are often preceded by an eighth fundamental rank, that of domain, which was proposed by Carl Woese in 1990;[27] both regio and dominium have been proposed for the corresponding Latin form, and the International Code of Nomenclature of Prokaryotes adopted the rank with a Latin form of dominium in 2023. The Latin plurals have been added for ease of understanding.
| Latin | Latin (plural) | English |
|---|---|---|
| dominium/regio (unofficial) | dominia, regiones | domain |
| regnum | regna | kingdom |
| phylum | phyla | phylum / division (in botany) |
| classis | classes | class |
| ordo | ordines | order |
| familia | familiae | family |
| genus | genera | genus |
| species | species | species |
Mnemonics
[edit]There are several mnemonics intended to help memorise the taxonomic hierarchy in English. One common example is "Dear King Phillip Came Over For Good Soup", standing for the initial letters/sounds in Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. This particular example has many popular variations that substitute "good" for "great", or "soup" for a different food item starting with "S", such as "spaghetti" or "sandwiches".
Intermediate and alternative ranks
[edit]Because evolutionary relationships are empirical, taxonomists occasionally need to introduce new ranks. Different nomenclature codes have different rules pertaining to this.[citation needed] Below is a list of terms used in various fields of study for various intermediate ranks, based on which main rank they are derived from/associated with.[α]
| Rank | Domain Empire Realm[a][28] |
Kingdom | Phylum Division[b] |
Class | Division[c][β] | Legion[c] |
|---|---|---|---|---|---|---|
| Subrank |
|
|
|
|
| Rank | Cohort[c][γ] | Order[δ] | Section[c] | Family | Tribe | Genus |
|---|---|---|---|---|---|---|
| Subrank |
|
| Rank | Section[b] | Series | Species | Variety/Varietas[b] "Form"/" |
|---|