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(plural genera) In the of , the genus is the next major rank up from . It holds special significance in that it is the only rank other than species currently effectively mandated by the various codes of nomenclature. Genera are generally regarded as representing a cluster of species that are very similar to each other, though not so similar as members of the same species. It is not uncommon for genera rather than species to act as the common unit of everyday taxonomic communication, especially for groups where there is a high ratio of monotypic genera (such as ) or groups where differences between species may not be easily distinguished by non-specialists (such as many ).
Unlike species, and like all taxa at other supraspecific ranks, genera are artificial constructs, and it is effectively arbitrary what level of difference an author chooses as representing different genera. A biological genus concept has been suggested that defines a genus as a collection of species that can hybridise to form viable, but not necessarily fertile, offspring (such as horses and donkeys). This definition has not been widely accepted, as it suffers the same practical issues as the , and would probably only be applicable for a small subsection of the world's biodiversity.
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