View source for Philosophy:Circumscription (taxonomy)

{{short description|Definition of a taxon}}
[[File:Anacardium_occidentale_-_Köhler–s_Medizinal-Pflanzen-010.jpg|thumb|right|alt=biological taxonomy, circumscription is the content of a taxon, that is, the delimitation of which subordinate taxa are parts of that taxon.]]
In [[Biology:Taxonomy|biological taxonomy]], '''circumscription''' is the content of a [[Biology:Taxon|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 [[Biology:International Code of Zoological Nomenclature|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 [[Biology:Molecular phylogenetics|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 [[Biology:Pongidae|Pongidae]] contained orangutans (''[[Biology:Pongo (genus)|Pongo]]''), chimpanzees (''[[Biology:Pan (genus)|Pan]]'') and gorillas (''Gorilla''), but not humans (''[[Biology:Homo|Homo]]''), which are placed in [[Biology:Hominidae|Hominidae]].  Once molecular phylogenetic data showed that [[Biology:Chimpanzee|chimpanzee]]s were more closely related to [[Biology:Human|human]]s than to [[Biology:Gorilla|gorilla]]s or [[Biology:Orangutan|orangutan]]s,<ref>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.</ref> it became clear that Pongidae is a paraphyletic group, and the circumscription of Hominidae was changed to include all four extant genera of great [[Biology:Ape|ape]]s.

Sometimes, systematists propose novel circumscriptions that do not address paraphyly. For example, the broadly circumscribed [[Biology:Monophyly|monophyletic]] moth superfamily [[Biology:Pyraloidea|Pyraloidea]] can be split into two families, [[Biology:Pyralidae|Pyralidae]] and [[Biology:Crambidae|Crambidae]], which are reciprocally monophyletic sister taxa.<ref>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</ref>

An example of a botanical group with unstable circumscription is [[Biology:Anacardiaceae|Anacardiaceae]], a family of [[Biology:Flowering plant|flowering plant]]s. Some experts favor a circumscription<ref>[http://delta-intkey.com/angio/www/anacardi.htm Anacardiaceae] {{webarchive |url=https://web.archive.org/web/20050315000926/http://delta-intkey.com/angio/www/anacardi.htm |date=March 15, 2005 }} in [http://delta-intkey.com/angio/ L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants.] {{webarchive |url=https://web.archive.org/web/20101213041459/http://delta-intkey.com/angio/ |date=December 13, 2010 }}</ref> in which this family includes the Blepharocaryaceae, Julianaceae, and Podoaceae, which are sometimes considered to be separate families.<ref>Stevens, P. F. (2001 onwards). [http://www.mobot.org/MOBOT/research/APweb/  Angiosperm Phylogeny Website]. Version 9, June 2008 [and more or less continuously updated since].</ref>

==See also==
* [[Biology:Glossary of scientific naming|Glossary of scientific naming]]
* [[Biology:Circumscriptional name|Circumscriptional name]]

==References==
{{reflist}}

[[Category:Logic]]
[[Category:Taxonomy (biology)]]

{{Sourceattribution|Circumscription (taxonomy)}}