While sick with a cold recently and drinking lots of orange juice I decide to look up orange in Wikipedia and to my surprise it turns out that the common orange is technically called the "sweet orange" and a hybrid of the mandarin and the pomelo, which looks like a pear on the outside.
|not a hybrid|
In fact, it turns out that all known citrus fruits are hybrids of only four original species: the mandarin, the pomelo, the citron and the papeda. So for other examples, the lemon is a cross between the bitter orange (itself a mandarin x pomelo cross like the sweet orange) and the citron, while the key lime is a papeda x citron cross.
This really makes the phrase "like apples and oranges" even more apt than before, since apples are an original species while oranges are a sub-species. That's like instead of "they fought like cats and dogs" saying "they fought like cats and poodles."