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Biological classification, or Scientific Classification in Biology, is a method to group and categorize organisms into groups such as genus or species. These groups are known as taxa (singular: taxon). Biological classification is part of scientific taxonomy.

Taxonomic Ranks Edit

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In biological classification, rank is the level (the relative position) in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class.

Each rank subsumes under it a number of less general categories. The rank of species, and specification of the genus to which the species belongs is basic, which means that it may not be necessary to specify ranks other than these.

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 Edit

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, which allow names divided into an indefinite number of ranks. 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.


Main zoological taxonomic ranks
Latin English
regio domain
regnum kingdom
phylum phylum
classis class
ordo order
familia family
genus genus
species species

A taxon is usually assigned a rank when it is given its formal name. The basic rank is that of species. The next most important rank is that of 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 third-most important rank, although it was not used by Linnaeus, is that of family.

The species name is sometimes 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 Edit

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:


Superfamily

Family
Subfamily
Tribe
Subtribe

Genus

Subgenus

Species

Subspecies

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 (small flies).

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).

All ranksEdit

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
    • Kingdom
      • Subkingdom
        • Branch
          • Infrakingdom
  • Superphylum
    • Phylum
      • Subphylum
        • Infraphylum
          • Microphylum
  • Superclass
    • Class
      • Subclass
        • Infraclass
          • Parvclass
  • Superdivision
    • Division
      • Subdivision
        • Infradivision
  • Superlegion
    • Legion
      • Sublegion
        • Infralegion
  • Supercohort
    • Cohort
      • Subcohort
        • Infracohort
  • Gigaorder
    • Megaorder
      • Capaxorder
        • Hyperorder
          • Superorder
            • Series (for fishes)
              • Order
                • Parvorder
                  • Nanorder
                    • Hypoorder
                      • Minorder
                        • Suborder
                          • Infraorder
                            • Microorder
  • Gigafamily
    • Megafamily
      • Grandfamily
        • Hyperfamily
          • Superfamily
            • Epifamily
              • Family
                • Subfamily
                  • Infrafamily
  • Supertribe
    • Tribe
      • Subtribe
        • Infratribe
  • Genus
    • Subgenus
  • Superspecies
    • Species
      • Subspecies
        • Form/Morph

Significance Edit

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.

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.

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