A taxon /tak.sɔ̃/ is a conceptual entity that groups together all living organisms that share certain well-defined taxonomic or diagnostic characteristics. The term derives from the term taxonomy by truncation, and not directly from the Greek τάξις / táxis, "placement", "ordering".
The species constitutes the basic taxon of the systematic classification. The higher the rank of the taxon, the lower the degree of resemblance of the individuals concerned, i.e. the lower the number of characters that plants, animals, fungi, bacteria or chromists have in common between them, and vice versa.
In the Linnean classifications, the characters of a taxon are deemed to be homogeneous according to their taxonomic rank, their relative taxonomic value being left to the appreciation of the expertise of the systematists.
In cladistic phylogenetic classifications, the notion of taxon merges more or less with that of clade, which has the consequence of eliminating paraphyletic taxa, that is to say those which do not include all the descendants of a last common ancestor (like reptiles, from which birds are descended). On the contrary, in evolutionary classifications the notion of taxon is more or less superimposed on that of evolutionary grade, thus recognizing in them more or less important stages of evolutionary history.
Taxon, taxonomy and taxonomy
The term taxonomy does not derive from the word taxon, because the latter is a concept that appeared much more recently: Augustin Pyrame de Candolle would have coined the term taxonomy in 1813 (corrected later by Littré in taxonomy according to the Greek etymon taxis), while that another botanist, Herman Johannes Lam, created the word taxon in 1948. Thus, taxonomy is not the study of taxa but the laws on order, therefore the rules of classification.
In 1962, the Academy of Sciences adopted the spelling taxum (in the plural taxums) to designate a systematic unit.
The common forms “taxon” or “taxum”, in the singular, “taxons”, “taxums” or “taxa” in the plural, are mentioned in French dictionaries.
However, the Petit Larousse does not indicate “taxa” as the plural of “taxon” or “taxum”. English speakers use a Latinizing plural "taxa" whereas the correct Greek plural of τάξις / táxis would be τάξεις / táxeis.
The plural "taxa" is preferred to "taxa" by some botanists.
The spelling "taxin", which would however be more consistent with the etymon, is extremely rare. It is only found in a few specialists in entomology or phytosociology.
The German term Taxon was proposed in theoretical biology by the natural philosopher Adolf Meyer in 1926 and taken up taxon, internationally in English, by the Dutch botanist Lam in 1948.
Bearing in mind the concept of Meyer (1926), which he had used in 1936 to reference the use of the expression "taxonomic unit" in a scientific article promoting phylogenetics, Lam coined the word taxon in the sense of "taxonomic group" at the scientific congress organized by the International Union of Biological Sciences in Utrecht in June 1948.
After being officially adopted by the VII International Botanical Congress in Stockholm in July 1950, and having given the name in September 1951 to a new scientific journal Taxon devoted to systematics (in particular that of plants), the term became common in the 1950s.
With regard to animal systematics, the term does not appear in Simpson's (1945) Classification of Mammals, completed earlier, but is used instead of "taxonomic category" in the work of Mayr, Linsley and Usinger ( 1953),.
According to Simpson (1961), a taxon