Citrus taxonomy
All of these hybrids have in turn been bred back with their parent stocks or with other pure or hybrid citrus to form a broad array of fruits. Naming of these is inconsistent, with some bearing a variant of the name of one of the parents or simply another citrus with superficially-similar fruit, a distinct name, or a portmanteau of ancestral species. The Ponderosa lemon ("Citrus limon" × "medica") and Florentine citron ("Citrus x limonimedica") are both true lemon/citron hybrids, the Bergamot orange is a sweet orange/lemon hybrid and the Oroblanco is a grapefruit/pomelo mix, while tangelos are tangerine (mandarin)/pomelo or mandarin/grapefruit hybrids, orangelos result from grapefruit backcrossed with sweet orange, and a sweet orange backcrossed with a tangerine gives the tangor. One lumia, a member of the sweet lemons, is the product of crossing a lemon with a pomelo/citron hybrid, though another lumia variety, the Pomme d'Adam, is a micrantha/citron cross, like the Key lime. The most common and commercially popular 'limes', the Persian limes, are Key lime/lemon hybrids that combine the genetic lineages of four ancestral citrus species: mandarin, pomelo, citron and micrantha.
Citrus taxonomy
All of the wild 'pure' citrus species trace to a common ancestor that lived in the Himalayan foothills, where a late-Miocene citrus fossil, "Citrus linczangensis", has been found. At that time, a lessening of the monsoons and resultant drier climate in the region allowed the citrus ancestor to expand across south and east Asia in a rapid genetic radiation. After the plant crossed the Wallace line a second radiation took place in the early Pliocene (about 4 million years ago) to give rise to the Australian species. Most modern cultivars are actually hybrids derived from a small number of 'pure' original species. Though hundreds of species names have been assigned, a recent genomic study by Wu, "et al." identified just ten ancestral species of citrus among those studied. Of these, seven were native to Asia: pomelo ("Citrus maxima"), the 'pure' mandarins ("C. reticulata" - most mandarin cultivars were hybrids of this species with pomelo), citrons ("C. medica"), micranthas ("C. micrantha"), the Ichang papeda ("C. ichangensis"), the mangshanyegan ("C. mangshanensis"), and the oval (Nagami) kumquat ("Fortunella margarita" or "C. japonica" var. "margarita"). In Australia three were identified: the desert lime ("C. glauca"), round lime ("C. australis") and the finger lime ("C. australasica"). Many other cultivars previously identified as species were found to be closely related variants (subspecies or varieties) or hybrids of these species, though not all cultivars were evaluated.