Taxon
From Freepedia
A taxon (plural taxa), or taxonomic unit, is an element of a taxonomy, most commonly used in the scientific classification in biology, where a taxon is a group of organisms that has been named. Taxa form a hierarchical scheme, each being broken down into sub-taxa. A taxon can then be placed at a particular level in the hierarchy.
In traditional Linnaean taxonomy, taxa are ranked as follows, with some of the less widely used ranks of taxa indented:
For example, Homo, the generic name of human, is a taxon at the genus level, and Homo sapiens is a taxon with the rank of species.
A prefix is used to indicate a ranking that falls between two taxa. The prefix super- indicates a rank above another, the prefix sub- indicates a rank below another, and the prefix infra- indicates a rank below sub-. For instance:
- Superclass
- Class
- Subclass
- Infraclass
In some groups of organism, phylogenetic relationships are not fully understood. For example, liverworts have been grouped, in different systems of classification, as a family, order, or a class. Such taxa with a high, but fluctuating, rank are sometimes informally referred to as "major groups".
It has become generally accepted that taxa should contain an ancestral form and all or some of its descendants, though the validity of the latter category is increasingly debated. A natural taxon is one such group that results from evolutionary events (monophyletic). On the other hand, an artificial taxon results from other manners of classification (such as superficial similarities due to convergent evolution), so it is polyphyletic or paraphyletic.
When studying a certain taxon in cladistics, the sister taxon (or sister group) is the closest relative to the taxon under study.
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