In
biological taxonomy, circumscription is the content of a
taxon
In biology, a taxon ( back-formation from '' taxonomy''; plural 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 n ...
, that is, the delimitation of which subordinate taxa are parts of that taxon. 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. New evidence may suggest that a traditional circumscription should be revised, particularly if the old circumscription is shown to be
paraphyletic (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 the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' (modern humans), plus several extinct species classified as either ancestral to or closely relat ...
''), which are placed in
Hominidae. Once molecular phylogenetic data showed that
chimpanzees were more closely related to
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
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 circumscriptions that do not rectify problems with paraphyly. For example, the broadly circumscribed moth family
Pyralidae (or superfamily
Pyraloidea) can be split into two families, Pyralidae and
Crambidae
The Crambidae are the grass moth family of lepidopterans. They are variable in appearance, the nominal subfamily Crambinae (grass moths) taking up closely folded postures on grass stems where they are inconspicuous, while other subfamilies includ ...
, which are reciprocally
monophyletic sister taxa.
An example of a botanical group with unstable circumscription is
Anacardiaceae
The Anacardiaceae, commonly known as the cashew family or sumac family, are a family of flowering plants, including about 83 genera with about 860 known species. Members of the Anacardiaceae bear fruits that are drupes and in some cases produce ...
, a
family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
of
flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. They include all forbs (flowering plants without a woody stem), grasses and grass-like plants, a vast majority of ...
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
See also
*
Glossary of scientific naming
This is a list of terms and symbols used in scientific names for organisms, and in describing the names. For proper parts of the names themselves, see List of Latin and Greek words commonly used in systematic names. Note that many of the abbrevia ...
*
Circumscriptional name
References
{{reflist
Logic
Taxonomy (biology)