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 in isolation: a species will
either be recognized as having no subspecies at all or two or
more (including any that are extinct), never just one. Organisms
that belong to different subspecies of the same species are
capable of interbreeding and producing fertile offspring, but
they often do not interbreed in nature due to geographic
isolation or other factors. The differences between subspecies
are usually less distinct than the differences between species.
The characteristics attributed to subspecies generally have
evolved as a result of geographical distribution or isolation.
In zoology, the International Code of Zoological Nomenclature
(4th edition, 1999) accepts only one rank below that of species,
namely the rank of subspecies. Other groupings,
"infrasubspecific entities" do not have names reg