What is a taxon?
A taxon (plural taxa) is a taxonomic unit. It is a population or group of populations of organisms, usually inferred to be phylogenetically related. The individuals in a taxon share characteristics that differentiate them from individuals in other taxa. A taxon can be at any rank in the hierarchy. A taxon encompasses all included taxa of lower rank and the individuals in those taxa. Note that the taxon is the population of individuals. It exists or did exist in nature before humans gave it a name. The name given to the taxon is separate from the taxon even though this distinction is often ignored.
The Linnaean hierarchy
The variety of life forms on earth is far too complex for the human mind to comprehend without some type of classification. Biologists have adopted and expanded the system initially devised by Linnaeus in the eighteenth century. The system is a hierarchy because all organisms are assigned to smaller and smaller groups as you move from higher levels down to lower levels. Each group at lower level is placed in one, and only one, group at a higher level. For zoologists, the official starting point is the tenth edition of Systema Naturae per Regna tria naturae, published in 1758 (in which Linnaeus described three species of phasmids). Before the time of Darwin, the classification was used for convenience without implying any relationship other than degree of similarity. As evolution became accepted by most biologists, there has been a major effort to make the classification correspond to the phylogeny (the evolutionary relationships among the groups). All animals are placed in a hierarchy that contains the following ranks:
- Phylum (plural phyla)
- Genus (plural genera)
Hierarchy used in Species Files
The top of the hierarchy is called the apex taxon. The apex taxon can be of any rank with family, tribe, genus, and species as the main ranks at lower levels. (The words "rank" and "level" have the same meaning in this context.) Tribe is added as an additional rank between family and genus, but its use is optional. It is used in large families where is useful to place certain genera (plural of genus) together in groups at a rank lower than family (or subfamily). However, family, tribe, genus, and species do not provide enough levels. "Super" can be added as prefix to indicate a rank higher. "Sub" can be added as a prefix to indicate a rank lower. "Infra" can be added as a prefix indicating a rank underneath that shown by "Sub." Even then we sometimes need still more levels, so we insert "group" and "series" when needed. The full hierarchy used in this website is:
- Superorder group
- Superfamily group
- Subfamily group
- Genus group
- Species series
- Species group
- Species subgroup
Significance of rank in taxonomy
The assignment of a particular group of organisms to a specific rank in the hierarchy is arbitrary. Some taxonomists (the splitters) would place the group at a higher level while others (the lumpers) would put it at a lower level. The important information is the relationships to other groups of organisms placed nearby in the hierarchy. There is, however, one level that is more objective than any other. That is the rank of species. A species includes those individuals that would interbreed if they had the opportunity to interact in a natural setting. This simplistic definition ignores a lot of complications such as change over evolutionary time and parthenogenetic species (females reproduce without males). It also ignores a practical problem: How can a taxonomist know what individuals would interbreed if given the opportunity? Nevertheless, it is an ideal concept that most taxonomists apply as best they can.