Glossary

[Contains content from Species File Glossary link]

The taxonomic hierarchy

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:

• Kingdom
• Phylum (plural phyla)
• Class
• Order
• Family
• Genus (plural genera)
• Species

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
• Superorder
• Order
• Suborder
• Infraorder
• Superfamily group
• Superfamily
• Family
• Subfamily group
• Subfamily
• Tribe
• Subtribe
• Genus
• Genus group
• Subgenus
• Species series
• Species group
• Species subgroup
• Species
• Subspecies

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.

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 rules of nomenclature

The International Code of Zoological Nomenclature (ICZN)

It is important that scientists working in different parts of the world and speaking different languages must nevertheless be able to share results of their research without confusion as to what organisms they are talking about. In other words each species must have a name that is the same throughout the world. There are many cases where multiple names have been proposed for the same species. To resolve such matters, it is necessary to have a set of rules that is accepted by taxonomists everywhere. What we have is the International Code of Zoological Nomenclature. This complex set of rules was developed by the ICZN and adopted by the International Union of Biological Sciences. In addition to rules for determining which name should be used for a species, there are rules for determining whether a new name has been published in an acceptable way and whether the name is eligible to be used. There are also rules defining circumstances under which an established name will continue in useage even though a different name would be used if we strictly followed priority to use the oldest eligible name. This is necessary to avoid confusing persons who use the names. The explanation given here is greatly simplified. For full details see International Code of Zoological Nomenclature, Fourth Edition, published in 1999 by The International Trust for Zoological Nomenclature, c/o The Natural History Museum, Cromwell Road, London SW7 5BD, UK.

The type system

A central concept of the International Code is use of types as "name bearers." When scientists describe new species of Support, they designate one individual for each species as the holotype or name bearer for that species. This solves two problems:

1. The published description of a new species is often insufficient to distinguish it from other species that exist in nature but may be unknown to the scientist. A person studying the matter at a later time can examine the holotype and observe details not mentioned in the original published description.
2. It often happens that the scientist who describes a new species was looking at several specimens thought to belong to a single species. Subsequently, more extensive study reveals that the specimens actually belong to multiple species. The identity of the holotype determines which species keeps the name.

Just as specimens are designated as holotypes for species, species are designated as type species for genera and subgenera. At one time the type species was often called the genotype, but when genotype became widely used in genetics the taxonomists abandoned "genotype" and now use "type species." The type system also applies at higher levels. Taxa at ranks above genus up to the rank of superfamily have a type genus. There is a standard naming convention for these family-group names. The first step is to determine the "stem" of the type genus (see Article 29 in the ICZN).

• To name a superfamily, add "oidea" to the stem of the type genus name.
• To name a family, add "idae" to the stem of the type genus name.
• To name a subfamily, add "inae" to the stem of the type genus name.
• To name a tribe, add "ini" to the stem of the type genus name.
• To name a subtribe, add "ina" to the stem of the type genus name.

These standard endings are required for superfamilies, families and subfamilies. Adoption at the levels of tribe and subtribe has been slower, and names formed in different ways are also used.

Kinds of types

Various names are used to identify types of different kinds:

• Type genus, the "name bearer" for family-group taxa (taxa with a rank higher than genus up to superfamily).
• Type species, the "name bearer" for genus-group taxa (genus or subgenus).
• Type specimen
  • Primary types, the "name bearers" for species names
    • Holotype, the single specimen designated as holotype when the species was first described.
    • Syntypes (also known as cotypes), multiple specimens identified by the author when the species was first described without designating a specific holotype.
    • Lectotype, the specimen designated by a subsequent author and selected from the syntypes.
    • Neotype, a specimen designated by a subsequent author as the namebearer when the original primary type(s) have been lost or destroyed.
  • Secondary types, not recognized as "name bearers" for species names
    • Allotype, a specimen of the opposite sex from the holotype and designated as allotype when the species was first described.
    • Paratype, additional specimens examined when the species was first described, but not designated as holotype or allotype.
    • Paralectotype, a specimen that was once a syntype, but is not the specimen later designated as lectotype.