Circumscription (taxonomy)

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In biological taxonomy, circumscription is the content of a taxon, 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 ), which are placed in Hominidae. Once molecular phylogenetic data showed that chimpanzees were more closely related to humans than to gorillas or orangutans, [1] 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, which are reciprocally monophyletic sister taxa. [2]

An example of a botanical group with unstable circumscription is Anacardiaceae, a family of flowering plants. Some experts favor a circumscription [3] in which this family includes the Blepharocaryaceae, Julianaceae, and Podoaceae, which are sometimes considered to be separate families. [4]

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Related Research Articles

Apiales Order of eudicot flowering plants in the asterid group

The Apiales are an order of flowering plants. The families are those recognized in the APG III system. This is typical of the newer classifications, though there is some slight variation and in particular, the Torriceliaceae may be divided.

Liliales Order of monocot flowering plants, including lilies

Liliales is an order of monocotyledonous flowering plants in the Angiosperm Phylogeny Group and Angiosperm Phylogeny Web system, within the lilioid monocots. This order of necessity includes the family Liliaceae. The APG III system (2009) places this order in the monocot clade. In APG III, the family Luzuriagaceae is combined with the family Alstroemeriaceae and the family Petermanniaceae is recognized. Both the order Lililiales and the family Liliaceae have had a widely disputed history, with the circumscription varying greatly from one taxonomist to another. Previous members of this order, which at one stage included most monocots with conspicuous tepals and lacking starch in the endosperm are now distributed over three orders, Liliales, Dioscoreales and Asparagales, using predominantly molecular phylogenetics. The newly delimited Liliales is monophyletic, with ten families. Well known plants from the order include Lilium (lily), tulip, the North American wildflower Trillium, and greenbrier.

Lamiales Order of dicot flowering plants

The Lamiales are an order in the asterid group of dicotyledonous flowering plants. It includes about 23,810 species, 1,059 genera, and is divided into about 24 families. Well-known or economically important members of this order include lavender, lilac, olive, jasmine, the ash tree, teak, snapdragon, sesame, psyllium, garden sage, and a number of table herbs such as mint, basil, and rosemary.

Malvales Order of flowering plants

The Malvales are an order of flowering plants. As circumscribed by APG II-system, the order includes about 6000 species within 9 families. The order is placed in the eurosids II, which are part of the eudicots.

Dicotyledon Historical grouping of flowering plants

The dicotyledons, also known as dicots, are one of the two groups into which all the flowering plants or angiosperms were formerly divided. The name refers to one of the typical characteristics of the group, namely that the seed has two embryonic leaves or cotyledons. There are around 200,000 species within this group. The other group of flowering plants were called monocotyledons or monocots, typically having one cotyledon. Historically, these two groups formed the two divisions of the flowering plants.

Taxon Group of one or more populations of an organism or organisms which have distinguishing characteristics in common

In biology, a taxon 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 given a particular ranking, especially if and when it is accepted or becomes established. It is very common, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.

Celastrales Order of flowering plants, mostly fom tropics and subtropics

The Celastrales are an order of flowering plants found throughout the tropics and subtropics, with only a few species extending far into the temperate regions. The 1200 to 1350 species are in about 100 genera. All but seven of these genera are in the large family Celastraceae. Until recently, the composition of the order and its division into families varied greatly from one author to another.


Melanthiaceae, also called the bunchflower family, is a family of flowering herbaceous perennial plants native to the Northern Hemisphere. Along with many other lilioid monocots, early authors considered members of this family to belong to the family Liliaceae, in part because both their sepals and petals closely resemble each other and are often large and showy like those of lilies, while some more recent taxonomists have placed them in a family Trilliaceae. The most authoritative modern treatment, however, the APG III system of 2009, places the family in the order Liliales, in the clade monocots. Circumscribed in this way, the family includes up to 17 genera.


Pongidae, or the pongids, is an obsolete primate taxon containing gorillas, chimpanzees and orangutans. They are sometimes called "great apes". Pongidae is now known to be paraphyletic. Pongids split from Hominina around seven mya. The corresponding crown group for this taxon is Hominidae. Pongidae has seven extant member species. This taxon is rarely used today but is of historical significance.

Ulmaceae Family of flowering plants

The Ulmaceae are a family of flowering plant that includes the elms, and the zelkovas. Members of the family are widely distributed throughout the north temperate zone, and have a scattered distribution elsewhere except for Australasia.


Bombacaceae were long recognised as a family of flowering plants or Angiospermae. The family name was based on the type genus Bombax. As is true for many botanical names, circumscription and status of the taxon has varied with taxonomic point of view, and currently the preference is to transfer most of the erstwhile family Bombacaceae to the subfamily Bombacoideae within the family Malvaceae in the order Malvales. The rest of the family were transferred to other taxa, notably the new family Durionaceae. Irrespective of current taxonomic status, many of the species originally included in the Bombacaceae are of considerable ecological, historical, horticultural, and economic importance, such as balsa, kapok, baobab and durian.

Colchicaceae Family of monocot flowering plants, in order Liliales

Colchicaceae is a family of flowering plants that includes 15 genera with a total of about 285 known species according to Christenhusz and Byng in 2016.

Asphodelaceae Family of flowering plants in the order Asparagales

Asphodelaceae is a family of flowering plants in the order Asparagales. Such a family has been recognized by most taxonomists, but the circumscription has varied widely. In its current circumscription in the APG IV system, it includes about 40 genera and 900 known species. The type genus is Asphodelus.


Siparunaceae is a family of flowering plants in the magnoliid order Laurales. It consists of two genera of woody plants, with essential oils: Glossocalyx in West Africa and Siparuna in the neotropics. Glossocalyx is monospecific and Siparuna has about 74 known species.

Boraginales Order of flowering plants within the lammiid clade of eudicots

Boraginales is a valid taxonomic name at the rank of order for a group of flowering plants. It includes Boraginaceae and closely related asterid families. The Boraginales include about 125 genera, 2,700 species and its herbs, shrubs, trees and lianas (vines) have a worldwide distribution.

Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a type, which can be a specimen or a taxon of lower rank, and a description in words. Phylogenetic nomenclature is currently regulated by the International Code of Phylogenetic Nomenclature (PhyloCode).

Caryophyllales Order of flowering plants

Caryophyllales is a diverse and heterogeneous order of flowering plants that includes the cacti, carnations, amaranths, ice plants, beets, and many carnivorous plants. Many members are succulent, having fleshy stems or leaves. The betalain pigments are unique in plants of this order and occur in all its families with the exception of Caryophyllaceae and Molluginaceae.

Hominidae Family of mammals

The Hominidae, whose members are known as great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo, the Bornean, Sumatran and Tapanuli orangutan; Gorilla, the eastern and western gorilla; Pan, the common chimpanzee and the bonobo; and Homo, of which only modern humans remain.

When the APG II system of plant classification was published in April 2003, fifteen genera and three families were placed incertae sedis in the angiosperms, and were listed in a section of the appendix entitled "Taxa of uncertain position".


Vittarioideae is a subfamily of the fern family Pteridaceae, in the order Polypodiales. The subfamily includes the previous families Adiantaceae and Vittariaceae.


  1. Perelman, Polina, Warren E. Johnson, Christian Roos, Hector N. Seuánez, Julie E. Horvath, Miguel AM Moreira, Bailey Kessing et al. "A molecular phylogeny of living primates." PLoS Genet 7, no. 3 (2011): e1001342.
  2. Nuss, M., B. Landry, R. Mally, F. Vegliante, A. Tränkner, F. Bauer, J. Hayden, A. Segerer, R. Schouten, H. Li, T. Trofimova, M. A. Solis, J. De Prins & W. Speidel 2003–2020: Global Information System on Pyraloidea. -
  3. Anacardiaceae Archived March 15, 2005, at the Wayback Machine in L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants. Archived December 13, 2010, at the Wayback Machine
  4. Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 [and more or less continuously updated since].