A citrus hybrid is the result of crossing any two species of citrus plants. Species within the citrus genus can hybridize easily, and many common citrus fruits are actually citrus hybrids. It's even thought that some citrus species that propagate naturally in the wild could actually be hybrids. This ease of hybridization makes it unclear exactly how many natural species there were originally, and new hybrids are still being created today. Varieties such as oranges, tangelos and limes date to antiquity, while grapefruits and orangelos are more recent examples of citrus hybrids.
There may have been as few as four original species of citrus, with the rest being natural or man-made hybrids. Oranges are the result of an ancient hybridization, possibly a blend of pomelo and mandarin, while the lemon has been genetically linked to the sour orange and citron. The grapefruit is a more recent hybrid of pomelo and sweet orange that dates to the 19th century. The most common cultivars of tangelo resulted from the crossing of tangerine and grapefruit and date to the early 20th century. The orangelo may have hybridized naturally, with the species first being recognized in the 1950s.
Several Australian and New Zealand fruits, such as the kumquat, are now thought to belong to the citrus genus. The position of these fruits within the citrus genus has resulted in hybrids such as the sunquat, a cross between lemons and kumquats, and the calamondin from tangerines and kumquats. Kumquats are sometimes placed in their own genus, but they readily hybridize with citrus fruits and have resulted in a range of citrus hybrid fruits.
The ease of hybridization also means that the seeds from a citrus fruit may grow to bear fruit that bears little resemblance to its parent. This has led all commercial citrus growers to utilize a method of grafting the desired citrus cultivar onto root stock. This can ensure that the desired fruit will be grown while giving the added benefit of using well-established root stock.
The citrus hybrid has allowed four basic species to become the dozens of different citrus fruit varieties that people enjoy today. Without the citrus hybrid there would be no sweet orange, lemon or many of the other citrus fruits that have become common foods around the world. The ease with which citrus hybridizes may result in some difficulties for commercial growers, but the sheer variety of citrus fruit would never have been possible otherwise.