Almost all citruses - the orange , lemon , grapefruit , clementine , etc. - are hybrids of three original citrus species: the pomelo , mandarin , and citron . Most often, we represent genealogy as a tree, but the complex hybridization process that led to modern citrus would render more of a complex, incestual web. It does, however, lend itself to another representation: a ternary plot. Each corner represents one of the original species, and the closer the fruit is to that corner, the higher percentage it is of that species - press on any individual node to see its exact genetic distribution.
The original species diverged five or more million years ago, when a sudden climatic event caused the genus’ evolutionary radiation into these core species - pomelo , mandarin , and citron - among others, such as kumquats and kaffir limes, though these have not been hybridized to the same extent.
Historical movement and cultivation amplified that blending. After early domestication in Asia, citrus spread through Austronesian movement and later westward trade routes into the Mediterranean, where additional selection and crossing produced familiar market groups. That is why sweet orange , grapefruit , lemon , and clementine are better understood as overlapping recombination clusters than as isolated branches. Over time, human breeding tended toward sweeter, more mandarin-rich market varieties - as can be seen in the path from pomelo to mandarin , which is largely unidirectional.
The ternary framing is helpful, but not entirely complete. Another original citrus variety, the samuyao ( C. micrantha ), entered the genetic citrus lottery through key limes , a citron and samuyao hybrid. Then, other varieties like the Persian lime , a hybrid of the key lime and lemon , further complicate the ternary representation.
The chart is not a representation of chronology; that’s what makes it interesting. It reveals visually information that would be latent in a typical chronological representation. Clustering shows how various combinations of bitterness and sweetness complement human taste, how historical happenstance, like the development of the lemon - an accidental dot in the ternary plot - dictates the varieties we may see in the supermarket: what we consider to be normal. The crowded line from the pomelo to the mandarin defines a hidden directionality within otherwise directionless data. Citrus is made in the image of the ternary plot - the ternary plot, in the image of man.