Inbiological taxonomy,circumscription is the content of ataxon, that is, the delimitation of which subordinate taxa are parts of that taxon. For example, if it is determined that species X, Y, and Z belong in genus A, and species T, U, V, and W belong in genus B, those are the 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 ofzoological orbotanical 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 anatural 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 inmolecular phylogenetics. New evidence may suggest that a traditional circumscription should be revised, particularly if the old circumscription is shown to beparaphyletic (a group containing some but not all of the descendants of the common ancestor).
For example, the familyPongidae contained orangutans (Pongo), chimpanzees (Pan) and gorillas (Gorilla), but not humans (Homo), which are placed inHominidae. Once molecular phylogenetic data showed thatchimpanzees were more closely related tohumans than togorillas ororangutans,[1] it became clear that Pongidae is a paraphyletic group, and the circumscription of Hominidae was changed to include all four extant genera of greatapes.
Sometimes, systematists propose novel circumscriptions that do not address paraphyly. For example, the broadly circumscribedmonophyletic moth superfamilyPyraloidea can be split into two families,Pyralidae andCrambidae, which are reciprocally monophyletic sister taxa.[2]
An example of a botanical group with unstable circumscription isAnacardiaceae, afamily offlowering plants. Some experts favor a circumscription[3] in which this family includes theBlepharocaryaceae,Julianaceae, andPodoaceae, which are sometimes considered to be separate families.[4]