Subspecies are a rank of classification that is lower than a species. Related subspecies are less distinct than species of the same genus. Generally speaking, two related subspecies can successfully interbreed but two species cannot. If a subspecies can be shown to be different enough, it can be elevated to species status. There are several subspecies of tiger (Panthera tigris) including the Bengal tiger (Panthera tigris tigris), Sumatran tiger (Panthera tigris sumatrae) and Siberian tiger (Panthera tigris altaica).
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories. The rank of species, and specification of the genus to which the species belongs is basic, which means that it may not be necessary to specify ranks other than these.