taxa

A taxon (plural taxa; from Greek taxis, arrangement) is a group of one or more populations of organisms that are considered to form a unit in biological classification. Taxa are named and defined by taxonomists and can be placed at various levels of the taxonomic hierarchy.

The major ranks include species, genus, family, order, class, phylum (or division in plants), kingdom, and domain. A taxon is named at a particular rank, and nomenclatural codes regulate names at each rank to maintain consistency across scientific disciplines.

Taxa are defined by shared characteristics and, increasingly, by genetic relationships. A taxon’s circumscription specifies which organisms belong to it, while its diagnostic features distinguish it from other taxa. Ideally, taxa reflect evolutionary history, with many taxonomic practices aiming to recognize monophyletic groups.

Names of taxa are governed by nomenclature codes: the International Code of Zoological Nomenclature (ICZN) for animals and the International Code of Nomenclature for algae, fungi, and plants (ICN) for plants, algae, and fungi. These codes regulate naming procedures, priority, type concepts, and the standing of published names. Higher-rank names often rely on the concept of a type genus to anchor the name.

In practice, taxa are subject to revision as new data become available. Revisions may alter circumscription, rank, or naming, and synonyms or homonyms can arise as classifications change. Taxonomy and systematics work together to organize biological diversity and reflect our understanding of evolutionary relationships.