Taxon
A taxon (plural taxa), taxonomic unit or taxonomic group, is a term designating a group of organisms that is recognised by a taxonomist as distinct from other such groups. A taxon may be given a name and may be assigned a rank: however in itself this is not a requirement.
A broad scheme of ranks in hierarchical order:
- Domain
- Kingdom
- Phylum (animals or plants) or Division (plants)
- Class
- Order
- Family
- Genus
- Species
- Subspecies
A simple mnemonic phrase to remember the order is "Dignified Kings Play Chess On Fine Green Silk"; another, highly expedient example is "King Philip's Class Orders the Family Genus to Speak".
A prefix is used to indicate a ranking of lesser importance. The prefix super- indicates a rank above, the prefix sub- indicates a rank below. In zoology the prefix infra- indicates a rank below sub-. For instance:
- Superclass
- Class
- Subclass
- Infraclass
Do note that rank is relative, and restricted to a particular systematic schema. For example, liverworts have been grouped, in various systems of classification, as a family, order, or a class. The use of a narrow set of ranks is challenged by users of cladistics; for example, the mere 10 ranks traditionally used between animal families (governed by the ICZN) and animal phyla (usually the highest relevant rank in taxonomic work) often cannot adequately represent the evolutionary history as more about a lineage's phylogeny becomes known. In addition, the class rank is quite often not an evolutionary but a phenetical and paraphyletic group and as opposed to those ranks governed by the ICZN, can usually not be made monophyletic by exchanging the taxa contained therein. This has given rise to phylogenetic taxonomy and the ongoing development of the PhyloCode, which is to govern the application of taxa to clades.