Genus

In biological classification, a genus (plural: genera) sits above the species level and below the family level, and it applies to living organisms, fossils, and viruses. Under binomial nomenclature, the genus name comes first in the two-part scientific name for each species within that genus. For instance, the lion and the jaguar belong to the same genus, Panthera, which itself falls under the family Felidae. Taxonomists decide what makes up a genus, and the rules for defining one are not strictly written down, so different experts often end up with different classifications.

Still, some general guidelines exist: a useful new genus should be monophyletic (meaning all its members come from a single ancestral group and form a distinct evolutionary line), reasonably compact (not unnecessarily large), and distinct in terms of evolutionary factors like ecology, shape, or geographic distribution. DNA sequences are seen as a result of diverging lineages, not a requirement for defining them, unless those sequences directly block gene flow. Also, genera should be made up of evolutionary units similar to those in other genera.

Etymology

The word "genus" comes from Latin, related to a verb meaning to bear or give birth. The French botanist Joseph Pitton de Tournefort is credited with founding the modern idea of a genus, and Carl Linnaeus later made it widely used in his 1753 work Species Plantarum. The scientific name of a genus, also called the generic name, is always capitalized in modern style. It is a key part of binomial nomenclature, where it combines with a species name to give each organism a unique, Latinized, two-part label for eukaryotes and prokaryotes, unlike common names that vary by region and language.

Use in nomenclature

A species name follows the format of the generic name plus a specific epithet unique within that genus. For example, the gray wolf is Canis lupus, with Canis as the generic name and lupus as the specific name. A botanical example is Hibiscus arnottianus.

The specific name is written in lowercase and may include subspecies or infraspecific names. When the genus is clear from context, the generic name can be shortened to its first letter, like C. lupus. The generic name (or its abbreviation) always leads the scientific name, even in subspecies, and all Latinized parts are italicized. Genus names, like other scientific names, can include authority citations: in zoology, this is typically "author, year" (e.g., Canis Linnaeus, 1758), and in botany, a standard abbreviated author name (e.g., Hibiscus L.).

Each genus should have a designated type; in zoology, this is a type species, and the genus name is permanently tied to the type specimen of that species. If that specimen is later assigned to a different genus, the original genus name becomes a junior synonym, and the remaining species must be reassessed. In zoology, genus names are classified as "available" or "unavailable." Available names follow the International Code of Zoological Nomenclature, and the earliest such name for a taxon becomes the valid name.

Categories of generic name

More available names exist than valid ones at any time, depending on taxonomists combining or splitting groups. Unavailable names include those not published according to the Code, such as incorrect spellings, names from theses, or generic names after 1930 without a type species. Suppressed names remain available but cannot be used as valid names, though names from suppressed works are made unavailable by official opinions. Botany has similar concepts but with different details.