Taxon

Taxon ( listen^{? /i}, the, pl. : taxa; from ancient Greek τάξις táxis, order, rank) refers in the systematics of biology to a unit to which a group of living organisms is assigned according to certain criteria. In most cases, this systematics is expressed by a separate name for this group.

Basically, form taxa and true taxa can be distinguished.

Formtaxa show similar characteristics, but do not have to be directly related to each other phylogenetically (such as invertebrates, deciduous trees or protozoa). The corresponding traits have evolved independently of each other in different places (analogous evolution).

Genuine taxa, on the other hand, ideally form closed descent communities ("natural groups"), so that the corresponding systematics provides an image of the evolutionary kinship relationships. The relationships show a hierarchical, multi-branched tree structure ("phylogenetic tree"). Accordingly, the biological systematics is hierarchically structured: Most taxa are subtaxa of a higher-level taxon and contain subtaxa themselves. In traditional systematics, the levels of hierarchy, also known as ranks or ranks, are named. The highest rank is the domain (containing the taxa Bacteria, Archaea, and Eukaryotes). The next lower rank level is the phylum (within the bacteria and archaea) or the kingdom (within the eukaryotes), each of which in turn is subdivided into lower rank levels. In eukaryotes, these further ranks in descending order are traditionally referred to as tribes, classes, orders, families, genera, and species. These ranks are also often subdivided into super- and/or sub-ranges (e.g. superorders or subgenera). The actual taxa are the individually named units within a rank. With the growing importance of cladistics in biology, whose methods for determining relationships result in many times more potential hierarchical levels than traditional systematics, the use of traditional ranks is increasingly being abandoned.

Since viruses are not considered living organisms, virus taxonomy is outside the hierarchy discussed here.

The relationship of the taxa grouped into a superordinate taxon is expressed by at least one common feature that distinguishes these taxa from other taxa of the same rank. In cladistic systematics, these characteristics have, by definition, a common evolutionary origin, i.e. they are homologous to each other and were inherited from a parent species. While in cladistics the taxon defined by a particular parent species always includes all its descendants (i.e., all subtaxa of all ranks), in traditional systematics there are also taxa that do not contain all descendants of the parent species. For example, the class Reptilia does not include birds (but these form a separate class of terrestrial vertebrates), although birds are widely believed to be the closest living relatives of crocodilians (a traditional order of reptiles).

The rules for the assignment of scientific names to taxa are the content of the relevant nomenclatural framework (ICZN, ICBN, ICNB). The classification according to the degree of relationship in the corresponding hierarchical system is made by the academic discipline of taxonomy based on biological systematics. Different methodological approaches and/or feature interpretations can lead to alternative results in the determination of relationships and thus to divergent taxon concepts and often to differently named taxa. Traditionally, names derived from Latin or Greek are often assigned. A taxon named in this way is referred to as a nominal or formal taxon.

Formtaxa

Formtaxa are groups of organisms that have been recognized by systematics as not belonging together phylogenetically, but are nevertheless retained with their own names for practical reasons. Examples of such form taxa are:

• the invertebrates (Invertebrata), which are characterized by the fact that they do not have bones. This group, named by Jean-Baptiste de Lamarck, includes the vast majority of all animal species. However, invertebrates such as sponges (Porifera), insects (Insecta) or starfish (Asteroidea) are only very distantly related to each other and do not form a natural relationship group.
• the lichens (Mycophycophyta), which, as symbiotic communities of fungi and algae, are not independent organisms, but consist of two fundamentally different partners. Nevertheless, they are still partly listed as a separate taxon with associated subgroups such as genera and species.
• the Fungi imperfecti (Deuteromycota), which represent a collection group for all those fungi that either have no sexual reproductive structures or whose reproductive pathways are still not understood. In some cases, even asexual stages are included by fungi that are otherwise quite capable of sexual reproduction. Phylogenetically, many organisms of this group belong to other groups of fungi such as the tube (Ascomycota) or stand fungi (Basidiomycota). Here, anamorphs and teleomorphs are not infrequently listed under different taxa.
• the protists (Protista), defined as the totality of all organisms with a true nucleus (Eukaryota), without being animals (Animalia), fungi (Fungi) or plants (Plantae). They are often called unicellular organisms, although there are numerous multicellular organisms in this group, which, such as the brown algae (Phaeophyta), are among the largest living organisms on our planet.

While today it is largely undisputed that the above examples are form taxa, the question of what should be considered a true taxon is problematic and depends on fundamental considerations regarding systematics: what is a valid taxon from the perspective of one may be merely a formal group from the perspective of another.

Synonyms

The easiest to resolve are discrepancies that arise merely because of synonymous naming. For example, two biologists encountering the same species in different regions may initially assign different names to it, or two species originally considered separate may be recognized as subpopulations of a single one. In this way, for example, the brown hare (Lepus europaeus) has received forty-three different scientific names (synonyms) over time. Another possibility is that, because of new scientific knowledge, a grouping believed to be obsolete is again considered a true taxon after all, but is initially given a new name in ignorance of a designation that may date back centuries. In both cases, the conflict is resolved by giving the oldest available name precedence over the "younger synonyms" as the "valid" name according to the priority rule.

Only in rare cases are there exceptions, e.g. Equus ferus Boddaert, 1785 takes precedence over Equus caballus Linnaeus 1758, as a 2003 ICZN Commission decision gave precedence to the younger name over the older. Another exception is Tyrannosaurus rex Osborn, 1905, exceptionally given precedence over Manospondylus gigas Cope, 1892.

The reverse case, that two different taxa are assigned the same name, can also occur. Here, too, the name can only be used for the taxon named first (if at all), for the other taxon this name cannot be used.

Of greater theoretical importance for the validity of a taxon, however, are deeper systematic considerations on the question of which criteria are used to determine which group of organisms constitutes a true taxon and which does not. The most important systems at the beginning of the 21st century are classical and cladistic taxonomy. Their following comparison is idealized to highlight the differences; in biological practice, both systems are often used side by side or in combination.