In biological taxonomy, circumscription is the content of a

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

, that is, the delimitation of which subordinate taxa are parts of that taxon. For example, if we determine that species X, Y, and Z belong in genus A, and species T, U, V, and W belong in genus B, those are our circumscriptions of those two genera. Another systematist might determine that T, U, V, W, X, Y, and Z all belong in genus A. Agreement on circumscriptions is not governed by the Codes of Zoological or Botanical Nomenclature, and must be reached by scientific consensus. A goal of biological taxonomy is to achieve a stable circumscription for every taxon. This goal conflicts, at times, with the goal of achieving a natural classification that reflects the evolutionary history of divergence of groups of organisms. Balancing these two goals is a work in progress, and the circumscriptions of many taxa that had been regarded as stable for decades are in upheaval in the light of rapid developments in

molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

. New evidence may suggest that a traditional circumscription should be revised, particularly if the old circumscription is shown to be

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

(a group containing some but not all of the descendants of the common ancestor). For example, the family Pongidae contained orangutans ('' Pongo''), chimpanzees ('' Pan'') and gorillas ('' Gorilla''), but not humans (''

Homo ''Homo'' () is a genus of great ape (family Hominidae) that emerged from the genus ''Australopithecus'' and encompasses only a single extant species, ''Homo sapiens'' (modern humans), along with a number of extinct species (collectively called ...

''), which are placed in

Hominidae The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic Family (biology), family of primates that includes eight Neontology#Extant taxa versus extinct taxa, extant species in four Genus, genera: ''Orangutan ...

. Once molecular phylogenetic data showed that

chimpanzee The chimpanzee (; ''Pan troglodytes''), also simply known as the chimp, is a species of Hominidae, great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close rel ...

s were more closely related to

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s than to gorillas or orangutans, it became clear that Pongidae is a paraphyletic group, and the circumscription of Hominidae was changed to include all four extant genera of great apes. Sometimes, systematists propose novel circumscriptions that do not address paraphyly. For example, the broadly circumscribed

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

moth superfamily Pyraloidea can be split into two families,

Pyralidae The Pyralidae, commonly called pyralid moths, snout moths or grass moths, are a family of Lepidoptera in the ditrysian superfamily Pyraloidea. In many (particularly older) classifications, the grass moths (Crambidae) are included in the Pyr ...

and

Crambidae Crambidae comprises the grass moth family of lepidopterans. They are variable in appearance, with the nominal subfamily Crambinae (grass moths) taking up closely folded postures on grass stems where they are inconspicuous, while other subfamilies ...

, which are reciprocally monophyletic sister taxa. An example of a botanical group with unstable circumscription is Anacardiaceae, a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Ancient Greek, Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed with ...

s. Some experts favor a circumscription in which this family includes the Blepharocaryaceae, Julianaceae, and Podoaceae, which are sometimes considered to be separate families.Stevens, P. F. (2001 onwards)
Angiosperm Phylogeny Website
Version 9, June 2008 nd more or less continuously updated since

References

{{reflist Logic Taxonomy (biology)

See also

References