There are many definitions of what kind of unit a species is (or should be). A common definition is that of a group of organisms capable of interbreeding and producing fertile offspring of both genders, and separated from other such groups with which interbreeding does not (normally) happen. Other definitions may focus on similarity of DNA or morphology. Some species are further subdivided into subspecies Subspecies in biological classification, is 1) a taxonomic rank subordinate to species, or 2) a taxonomic unit, a taxon in that rank (plural: subspecies). A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one, and here also there is no close agreement on the criteria to be used.
