In biology, a
genus (plural:
genera) is a
taxonomic unit (a
taxon) used in the classification of living and fossil
organisms. The term comes from Latin genus "descent, family, type, gender" (plurals: genera), cognate with –
genos, "race, stock, kin" (plurals: genera)..
In addition, genus (not countable, no plural) is a taxonomic rank in the hierarchy (thus "a genus" is a taxon at the rank of genus).
Other well-known taxonomic ranks are: kingdom, phylum, class, order, family, and species, with genus fitting between family and species. Like for the other well-known taxonomic ranks, mentioned above, there can be an immediately lower rank, indicated by the prefix sub-, in this case subgenus (plural subgenera). The most important taxonomic unit below the genus is the species, which is the basic rank.
The composition of each genus is determined by a
taxonomist, but often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing a genus, but see below for some rules of thumb.
Generic name
The scientific name of a genus may be called the
generic name: it is always capitalized. It plays a pivotal role in
binomial nomenclature, the system of biological nomenclature.
Binomial nomenclature
The rules for scientific names are laid down in the
Nomenclature Codes; depending on the kind of organism and the Kingdom it belongs to, a different Code may apply, with different rules, laid down in a different terminology. The advantages of scientific over common names are that they are accepted by speakers of all languages, and that each species has only one name. This reduces the confusion that may arise from the use of a common name to designate different things in different places (example elk), or from the existence of several common names for a single species.
It is possible for a genus to be assigned to a kingdom governed by one particular Nomenclature Code by one taxonomist, while other taxonomists assign it to a kingdom governed by a different Code, but this is the exception, not the rule.
Pivotal in binomial nomenclature
The generic name often is a component of the names of taxa of lower rank. For example,
Canis lupus is the scientific name of the
Gray wolf, a species, with
Canis the generic name for the
dog and its close relatives, and with
lupus particular (specific) for the wolf (
lupus is written in lower case). Similarly,
Canis lupus familiaris is the scientific name for the
domestic dog.
Taxonomic units in higher ranks often have a name that is based on a generic name, such as the family name
Canidae, which is based on
Canis. However, not all names in higher ranks are necessarily based on the name of a genus: for example,
Carnivora is the name for the order to which the dog belongs.
The problem of identical names used for different genera
A genus in one
kingdom is allowed to bear a scientific name that is in use as a generic name (or the name of a taxon in another rank) in a kingdom that is governed by a different Nomenclature Code. Although this is discouraged by both the
International Code of Zoological Nomenclature and the
International Code of Botanical Nomenclature, there are some five thousand such names that are in use in more than one kingdom. For instance,
Anura is the name of the
order of
frogs but also is the name of a genus of plants (although not current: it is a
synonym);
Aotus is the genus of
golden peas and
night monkeys;
Oenanthe is the genus of
wheatears and
water dropworts, and
Prunella is the genus of
accentors and
self-heal.
Within the same kingdom one generic name can apply to only one genus. This explains why the
platypus genus is named
Ornithorhynchus—
George Shaw named it
Platypus in 1799, but the name
Platypus had already been given to the
pinhole borer beetle by
Johann Friedrich Wilhelm Herbst in 1793. Names with the same form but applying to different taxa are called homonyms. Since beetles and platypuses are both members of the kingdom Animalia, the name
Platypus could not be used for both. Johann Friedrich Blumenbach published the replacement name
Ornithorhynchus in 1800.
Types and genera
Because of the rules of scientific naming, or "
binomial nomenclature", each genus should have a designated
type, although in practice there is a backlog of older names that may not yet have a type. In zoology this is the type species (see
Type (zoology)); the generic name is permanently associated with the
type specimen of its type species. Should this specimen turn out to be assignable to another genus, the generic name linked to it becomes a
junior synonym, and the remaining
taxa in the former genus need to be reassessed.
See
scientific classification and
Nomenclature Codes for more details of this system. Also see
type genus.
Guidelines
There are no hard and fast rules that a taxonomist has to follow in deciding what does and what does not belong in a particular genus. This does not mean that there is no common ground among taxonomists in what constitutes a "good" genus. For instance, some rules-of-thumb for delimiting a genus are outlined in Gill. According to these, a genus should fulfill three criteria to be descriptively useful:
- monophyly – all descendants of an ancestral taxon are grouped together;
- reasonable compactness – a genus should not be expanded needlessly; and
Nomenclature
None of the
Nomenclature Codes require such criteria for defining a genus, because these are concerned with the nomenclature rules, not with taxonomy. These regulate formal nomenclature, aiming for universal and stable scientific names.
See also
