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 novel circumscriptions that do not address paraphyly. For example, the broadly circumscribed monophyletic moth 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]

See also

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<span class="mw-page-title-main">Liliales</span> 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.

<span class="mw-page-title-main">Malvales</span> 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 nine families. The order is placed in the eurosids II, which are part of the eudicots.

<span class="mw-page-title-main">Paraphyly</span> Type of taxonomic group

Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and most of its descendants, but excludes one or more subgroups. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping includes a common ancestor and all of its descendants.

<span class="mw-page-title-main">Dicotyledon</span> Historical grouping of flowering plants

The dicotyledons, also known as dicots, are one of the two groups into which all the flowering plants (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, typically each having one cotyledon. Historically, these two groups formed the two divisions of the flowering plants.

<span class="mw-page-title-main">Aceraceae</span> Family of maples

Aceraceae were recognized as a family of flowering plants also called the maple family. They contain two to four genera, depending upon the circumscription, of some 120 species of trees and shrubs. A common characteristic is that the leaves are opposite, and the fruit a schizocarp.

<span class="mw-page-title-main">Taxon</span> Grouping of biological populations

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, especially in the context of rank-based ("Linnaean") nomenclature. 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.

<span class="mw-page-title-main">Ape</span> Branch of primates

Apes are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia, which together with its sister group Cercopithecidae form the catarrhine clade, cladistically making them monkeys. Apes do not have tails due to a mutation of the TBXT gene. In traditional and non-scientific use, the term ape can include tailless primates taxonomically considered Cercopithecidae, and is thus not equivalent to the scientific taxon Hominoidea. There are two extant branches of the superfamily Hominoidea: the gibbons, or lesser apes; and the hominids, or great apes.

<span class="mw-page-title-main">Pongidae</span> Obsolete ape taxon

Pongidae, or the pongids is an obsolete primate taxon containing chimpanzees, gorillas and orangutans. By this definition pongids were also called "great apes". This taxon is not used today but is of historical significance. The great apes are currently classified as Hominidae. This entry addresses the old usage of pongid.

<span class="mw-page-title-main">Bombacaceae</span> Family of flowering plants

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.

<span class="mw-page-title-main">Hemerocallidoideae</span> Subfamily of flowering plants

Hemerocallidoideae is the a subfamily of flowering plants, part of the family Asphodelaceae sensu lato in the monocot order Asparagales according to the APG system of 2016. Earlier classification systems treated the group as a separate family, the Hemerocallidaceae. The name is derived from the generic name of the type genus, Hemerocallis. The largest genera in the group are Dianella, Hemerocallis (15), and Caesia (11).

<span class="mw-page-title-main">Asphodelaceae</span> 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.

<span class="mw-page-title-main">Thymelaeaceae</span> Family of flowering plants

The Thymelaeaceae are a cosmopolitan family of flowering plants composed of 50 genera and 898 species. It was established in 1789 by Antoine Laurent de Jussieu. The Thymelaeaceae are mostly trees and shrubs, with a few vines and herbaceous plants.

<span class="mw-page-title-main">Boraginales</span> Order of flowering plants within the lammiid clade of eudicots

Boraginales is an order of flowering plants in the asterid clade, with a total of about 125 genera and 2,700 species. Different taxonomic treatments either include only a single family, the Boraginaceae, or divide it into up to eleven families. Its herbs, shrubs, trees and lianas (vines) have a worldwide distribution.

<span class="mw-page-title-main">Tofieldiaceae</span> Family of flowering plants

Tofieldiaceae is a family of flowering plants in the monocot order Alismatales. The family is divided into four genera, which together comprise 28 known species. They are small, herbaceous plants, mostly of arctic and subarctic regions, but a few extend further south, and one genus is endemic to northern South America and Florida. Tofieldia pusilla is sometimes grown as an ornamental.

<span class="mw-page-title-main">Balanophoraceae</span> Family of flowering plants

The Balanophoraceae are a subtropical to tropical family of obligate parasitic flowering plants, notable for their unusual development and formerly obscure affinities. In the broadest circumscription, the family consists of 16 genera. Alternatively, three genera may be split off into the segregate family Mystropetalaceae.

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).

In phylogenetics, basal is the direction of the base of a rooted phylogenetic tree or cladogram. The term may be more strictly applied only to nodes adjacent to the root, or more loosely applied to nodes regarded as being close to the root. Note that extant taxa that lie on branches connecting directly to the root are not more closely related to the root than any other extant taxa.

<span class="mw-page-title-main">Cichorioideae</span> Subfamily of plants

The Cichorioideae are a subfamily of the family Asteraceae of flowering plants. Familiar members of Cichorioideae include lettuce, dandelions, chicory and Gazania species. The subfamily comprises about 240 genera and about 2900 species. It is heterogeneous and hard to characterize except with molecular characters.

<span class="mw-page-title-main">Hominidae</span> Family of primates

The Hominidae, whose members are known as the great apes or hominids, are a taxonomic family of primates that includes eight extant species in four genera: Pongo ; Gorilla ; Pan ; and Homo, of which only modern humans remain.

<span class="mw-page-title-main">Polygonoideae</span> Subfamily of the knotweed family of plants (Polygonaceae)

Polygonoideae is a subfamily of plants in the family Polygonaceae. It includes a number of plants that can be highly invasive, such as Japanese knotweed, Reynoutria japonica, and its hybrid with R. sachalinensis, R. × bohemica. Boundaries between the genera placed in the subfamily and their relationships have long been problematic, but a series of molecular phylogenetic studies have clarified some of them, resulting in the division of the subfamily into seven tribes.

References

  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. - www.pyraloidea.org
  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].