User:Jaspergeli/Taxonomic rank

Taxonomic rank

A given rank subsumes under it less general categories, that is, more specific descriptions of life forms. Above it, each rank is classified within more general categories of organisms and groups of organisms related to each other through inheritance of traits or features from common ancestors. The rank of any species and the description of its genus is basic; which means that to identify a particular organism, it is usually not necessary to specify ranks other than these first two.

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)."

Main ranks

There are eight main taxonomic ranks: domain, kingdom, phylum or division, class, order, family, genus, and species.

A taxon is usually assigned a rank when it is given its formal name. The basic ranks are genus and species. 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.

Names of taxa

• A taxon above the rank of subgenus has a scientific name in one part.
• A taxon above the rank of species has a scientific name in two parts.
• A species has a name composed of two parts (a binomial name or binomen):
  generic name + specific name
  ; for example Canis lupus.
• A subspecies has a name composed of three parts (a trinomial name or trinomen): generic name + specific name + subspecific name; for example Canis lupus lupus'. As there is only one possible rank below that of species, no connecting term to indicate rank is needed or used.
• Actually, a scientific name with subgroups included is called quadrinomial or quadrinomen. For example, Canis Canis lupus lupus or a scientific name with subgroups included in the parentheses, Canis (Canis) lupus (lupus).

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:

• Domain or Empire
  • Subdomain (biology)
• Hyperkingdom
  • Superkingdom
    • Kingdom
      • Subkingdom
        • Infrakingdom
          • Parvkingdom
• Superphylum
  , or superdivision (in botany)
  • Phylum, or division (in botany)
    • Subphylum, or subdivision (in botany)
      • Infraphylum
        , or infradivision (in botany)
        • Microphylum
• Superclass
  • Class
    • Subclass
      • Infraclass
        • Parvclass
• Superdivision (in zoology)
  • Division (in zoology)
    • Subdivision (in zoology)
      • Infradivision (in zoology)
• Superlegion (in zoology)
  • Legion
    (in zoology)
    • Sublegion (in zoology)
      • Infralegion (in zoology)
• Supercohort (in zoology)
  • Cohort (in zoology)
    • Subcohort (in zoology)
      • Infracohort (in zoology)
• Gigaorder (in zoology)
  • Magnorder
    or megaorder (in zoology)
    • Grandorder or capaxorder (in zoology)
      • Mirorder or hyperorder (in zoology)
        • Superorder
          • Series (for fish)
            • Order
              • Parvorder
                (position in some zoological classifications)
                • Nanorder (in zoology)
                  • Hypoorder (in zoology)
                    • Minorder (in zoology)
                      • Suborder
                        • Infraorder
                          • Parvorder
                            (usual position), or microorder (in zoology)
• Section (in zoology)
  • Subsection (in zoology)
• Gigafamily (in zoology)
  • Megafamily (in zoology)
    • Grandfamily (in zoology)
      • Hyperfamily (in zoology)
        • Superfamily
          • Epifamily (in zoology)
            • Series (for Lepidoptera)
              • Group (for Lepidoptera)
                • Family
                  • Subfamily
                    • Infrafamily
• Supertribe
  • Tribe
    • Subtribe
      • Infratribe
• Genus
  • Subgenus
    • Section (in botany)
      • Subsection (in botany)
        • Series (in botany)
          • Subseries (in botany)
• Species-group
  • Species
    • Subspecies, or forma specialis (for fungi), or pathovar (for bacteria)
      • morph (in zoology) or aberration
        (in lepidopterology)
        • Subvariety (in botany)
          • Form or forma (in botany)
            • Subform (in botany)

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 more diverse than other species, but because they are 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 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-