In biological classification, subspecies (abbreviated "subsp." or "ssp."; plural: "subspecies") is either a taxonomic rank subordinate to species, or a taxonomic unit in that rank. A subspecies cannot be recognized independently: a species will either be recognized as having no subspecies at all or at least two (including any that are extinct). In zoology, under the International Code of Zoological Nomenclature, the subspecies is the only taxonomic rank below that of species that can receive a name. In botany and mycology, under the International Code of Nomenclature for algae, fungi, and plants, other infraspecific ranks, such as variety, may be named. In bacteriology and virology, under standard bacterial nomenclature and virus nomenclature, there are recommendations but not strict requirements for recognizing other important infraspecific ranks. A taxonomist decides whether to recognize a subspecies or not. A common way to decide is that organisms belonging to different subspecies of the same species are capable of interbreeding and producing fertile offspring, but they do not usually interbreed in nature due to geographic isolation, sexual selection, or other factors. The differences between subspecies are usually less distinct than the differences between species.