Oranges, lemons and grapefruit are mostly remixes of just three wild fruits
Botanists once counted 870 citrus species. Genetic studies now suggest most fruits in the supermarket descend from crosses between only three ancestors: the citron, the pomelo and the mandarin. A sweet orange and a grapefruit turn out to be different mixes of the same two parents.
All the wild citrus species trace back to one ancestor in the Himalayan foothills, where a fossil from the late Miocene, Citrus linczangensis, has been found. Between roughly 12 and 5 million years ago the monsoons weakened, the climate dried, and the ancestral plant spread quickly across south and east Asia. After it crossed the Wallace line, a second burst around 4 million years ago produced the Australian species. The speed of that first radiation probably explains why family trees built from nuclear DNA and chloroplast DNA disagree.
The big three evolved apart: citrons in northern Indochina, pomelos in the Malay Archipelago, mandarins in Vietnam, southern China and Japan. Despite being genetically distinct, they interbreed freely, yielding oranges, grapefruit, lemons and many limes and tangerines. The Key lime came from a citron crossing with a papeda called micrantha. Kumquats flower at different times and rarely mix naturally, though the calamansi is one such hybrid. Seeds from hybrids usually scramble the parents' traits, so most varieties survive only through grafting or cuttings, and a few clone themselves through seeds by a process called apomixis. Two fruits that look alike can have unrelated ancestry, while two that look different may differ only by a single bud mutation.
Classifying all this has been a battle of philosophies. Walter Tennyson Swingle recognised just 16 species and split kumquats and Australian limes into their own genera. Chozaburo Tanaka, in 1954, gave each cultivar its own name and ended up with 159. A 1969 compromise by Hodgson settled on 36. Genomics later showed several of Swingle's separate genera actually sit inside Citrus.
A 2020 scheme by Ollitrault, Curk and Krueger names hybrids by ancestry. Every mandarin-pomelo cross is Citrus x aurantium, with the sweet orange, grapefruit and clementine as varieties, while a cross of all three founders, the lemon, is Citrus x limon.
Source: Citrus taxonomy
