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For other uses of the word, please see Genus (disambiguation).
In the binomial nomenclature used worldwide, the name of an organism is composed of two parts: its genus name (always capitalized) and a species modifier (known as the "epithet"). An example is Homo sapiens, the name for the human species which belongs to the genus Homo. See scientific classification and nomenclature Codes for more details of this system. Also see type genus.
Each genus must have a designated type species (see Type (zoology)). The generic name is permanently associated with the type specimen of its type species.
The boundaries between genera are historically subjective, but with the advent of phylogenetics, it is increasingly common for all taxonomic ranks above the species level to be restricted to demonstrably monophyletic groupings; that is, all descendants of an ancestral taxon are grouped together, even if some of them do not share all of the visible characteristics of the others. This has never been a requirement of either the Zoological or Botanical Codes, and it is an ongoing controversy in the field of systematics, as to whether classifications should be based upon the hierarchical relationships among taxa (cladistics) or their visible traits, regardless of relationships (phenetics). At present, most of the classifications based on phenetics are being gradually replaced by new ones based on cladistics (e.g., Reptilia and Amphibia are no longer in use).
Many genera are divided into subgenera (singular subgenus).
A genus in one kingdom is allowed to bear a name that is in use as a genus name or other taxon name in another kingdom. 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); and 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. Obviously, within the same kingdom one particular generic name can apply to only one genus. This explains why the platypus genus is named Ornithorhynchus — it was indeed given the name Platypus, by George Shaw in 1799, but by then that name 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.