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In biology, a genus (plural: genera) is a taxonomic unit (a taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a...

) 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
Binomial nomenclature
The formal system of naming species is called binominal nomenclature , binary nomenclature , or the binomial classification system...

, the system of biological nomenclature.

Binomial nomenclature


The rules for scientific names are laid down in the Nomenclature Codes
Nomenclature Codes
The Nomenclature Codes are the various rulebooks that govern biological nomenclature, each in their own area...

; 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
Gray Wolf
The grey wolf or gray wolf , also known as simply wolf, is the largest wild member of the Canidae family. It is an ice age survivor originating during the Late Pleistocene around 300,000 years ago. DNA sequencing and genetic drift studies reaffirm that the gray wolf shares a common ancestry with...

, a species, with Canis
Canis
Canis is a genus containing 7 to 10 extant species, including dogs, wolves, coyotes, and jackals, and many extinct species.-Wolves and dogs:...

the generic name for the dog
Dog
The dog is a domesticated form of the Gray Wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The domestic dog has been one of the most widely kept working and companion animals in human history...

 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
Canidae
Canidae is the biological family of carnivorous and omnivorous mammals that includes the wolves, foxes, jackals, coyotes, and the domestic dog; a member of this family is called a canid . The Canidae family is divided into the "wolf-like" and "dog-like" animals of the tribe Canini and the "foxes"...

, which is based on Canis. However, not all names in higher ranks are necessarily based on the name of a genus: for example, Carnivora
Carnivora
The diverse order Carnivora includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...

 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
Kingdom (biology)
In biological taxonomy, kingdom and/or regnum is a taxonomic rank in either the highest rank, or the rank below domain. Each kingdom is divided into smaller groups called phyla...

 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
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is both a book containing a set of rules and recommendations on the formal naming of animals, and that set itself. Among zoologists it is often referred to simply as "the Code"...

 and the International Code of Botanical Nomenclature
International Code of Botanical Nomenclature
The International Code of Botanical Nomenclature is the set of rules and recommendations dealing with the formal botanical names that are given to plants. Its intent is that each taxonomic group of plants has only one correct name that is accepted worldwide...

, 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
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by long hind legs, a short body, webbed digits , protruding eyes and the absence of a tail...

s but also is the name of a genus of plants (although not current: it is a synonym); Aotus
Aotus
Aotus may refer to:* Aotus , one of the plant genera commonly known as golden peas in the family Fabaceae .* Aotus , the genus of night monkeys in the family Aotidae....

is the genus of golden peas
Aotus (pea)
Aotus is an Australian genus of flowering plants, within the legume family Fabaceae. Aotus species, together with other species of the tribe Mirbelieae, are often called Golden Peas because of their distinctive small yellow flowers....

 and night monkey
Night monkey
The Night monkeys, also known as the Owl monkeys or Douroucoulis, are the members of the genus Aotus of New World monkeys . They are widely distributed in the forests of Central and South America, from Panama south to Paraguay and northern Argentina...

s; Oenanthe
Oenanthe
Oenanthe is the name of two genera:*the wheatear genus of birds*the water dropwort genus of plants...

is the genus of wheatear
Wheatear
The wheatears are birds of the genus Oenanthe. They were formerly considered to be members of the thrush family Turdidae, but are now more commonly placed in the flycatcher family Muscicapidae...

s and water dropwort
Water dropwort
The water dropworts, Oenanthe , are a genus of plants in the family Apiaceae. Most of the species grow in damp ground, in marshes or in water....

s, and Prunella
Prunella
Prunella can mean:* Prunella , also known as accentors or dunnocks* Prunella , also known as self-heal* Prunella Montrachet, David's professional partner during Season 7 of Dancing With The Stars...

is the genus of accentor
Accentor
The accentors are in the only bird family, the Prunellidae, which is completely endemic to the Palearctic. This small group of closely related passerines are all in a single genus Prunella...

s and self-heal.

Within the same kingdom one generic name can apply to only one genus. This explains why the platypus
Platypus
The Platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

 genus is named OrnithorhynchusGeorge Shaw
George Shaw
George Shaw was an English botanist and zoologist.Shaw was born at Bierton, Buckinghamshire and was educated at Magdalen Hall, Oxford, receiving his M.A. in 1772. He took up the profession of medical practitioner. In 1786 he became the assistant lecturer in botany at Oxford University...

 named it Platypus in 1799, but the name Platypus had already been given to the pinhole borer beetle
Beetle
Beetles are the group of insects with the largest number of known species. They are classified in the order Coleoptera , which contains more described species than in any other order in the animal kingdom, constituting about 25% of all known life-forms...

 by Johann Friedrich Wilhelm Herbst
Johann Friedrich Wilhelm Herbst
Johann Friedrich Wilhelm Herbst was a German naturalist and entomologist from Petershagen, Minden-Ravensberg....

 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
Binomial nomenclature
The formal system of naming species is called binominal nomenclature , binary nomenclature , or the binomial classification system...

", each genus should have a designated type
Biological type
In biology, a type is that which fixes a name to a taxon. Depending on the nomenclature code which is applied to the organism in question, a type may be a specimen, culture, illustration, description or taxon....

, 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
Nomenclature Codes
The Nomenclature Codes are the various rulebooks that govern biological nomenclature, each in their own area...

 for more details of this system. Also see type genus
Type genus
In biology, the phrase type genus is used differently depending on the nomenclatural Code that applies:* In zoological nomenclature, a type genus is "The nominal genus that is the name-bearing type of a nominal family-group taxon."...

.

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 et al. (2005). According to these, a genus should fulfill three criteria to be descriptively useful:
  1. monophyly
    Monophyly
    In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly...

     – all descendants of an ancestral taxon are grouped together;
  2. reasonable compactness – a genus should not be expanded needlessly; and
  3. distinctness – in regards of evolutionarily relevant criteria, i.e. ecology
    Ecology
    Ecology is the interdisciplinary scientific study of the interactions between organisms and the interactions of these organisms with their environment....

    , morphology
    Morphology (biology)
    In biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...

    , or biogeography
    Biogeography
    Biogeography is the study of the distribution of biodiversity over space and time. It aims to reveal where organisms live, and at what abundance....

    ; note that DNA sequences are a consequence rather than a condition of diverging evolutionarily lineages except in cases where they directly inhibit gene flow
    Gene flow
    In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...

     (e.g. postzygotic barriers).

Modern phylogenetic classification


These three criteria are often fulfilable for a given clade. However, an example where they cannot be met all three, at the same time, is the dabbling ducks in the genus Anas
Anas
Anas is a genus of dabbling ducks. It includes mallards, wigeons, teals, pintails and shovelers in a number of subgenera. Some authorities prefer to elevate the subgenera to genus rank...

. This group appears paraphyletic, when taking into account the distinct fossil species moa-nalo
Moa-nalo
Moa-nalo are a group of extinct aberrant, goose-like ducks that formerly lived on the Hawaiian Islands in the Pacific. They were the major herbivores on most of these islands for the last 3 million years or so, until they became extinct after human settlement.-Description:The moa-nalo were unknown...

. Considering these as distinct genera would violate criterion 1, including all species in the Anas genus would violate criteria 2 and 3, and splitting the genus so that the mallard
Mallard
The Mallard , probably the best-known and most recognizable of all ducks, is a dabbling duck which breeds throughout the temperate and sub-tropical areas of North America, Europe, Asia, Africa, New Zealand , and Australia...

 duck and the American black duck
American Black Duck
The American Black Duck is a large dabbling duck.The adult male has a yellow bill, a dark body, lighter head and neck, orange legs and dark eyes. The adult female has a similar appearance but is slightly lighter overall and has a greenish-gray bill. Both sexes have a shiny purple-blue wing patch,...

 would be in separate genera would violate criterion 3.

Nomenclature



None of the Nomenclature Codes
Nomenclature Codes
The Nomenclature Codes are the various rulebooks that govern biological nomenclature, each in their own area...

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.



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