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In biological taxonomy, circumscription is the definition of the limits of a taxonomic group of organisms, a taxon. For every taxon the circumscription is based on a set of attributes that characterise every member of the taxon, and exclude every other organism.
One goal of biological taxonomy is to achieve a stable circumscription for every taxon. Achieving stability is not yet a certainty in most taxa, and many that had been regarded as stable for decades are in upheaval in the light of rapid developments in molecular phylogenetics. In essence new discoveries may invalidate the application of irrelevant attributes used in established or obsolete circumscriptions, or present new attributes useful in cladistic taxonomy.
An example of a taxonomic group with unstable circumscription is Anacardiaceae, a family of flowering plants. Some experts favor a circumscription in which this family includes the Blepharocaryaceae, Julianaceae, and Podoaceae, which are sometimes considered to be separate families.
- Anacardiaceae in L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants.
- Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 [and more or less continuously updated since].
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