The citrus family starts off with just three types of citrus: citrus medica, citrus maxima, and citrus reticulata, also known as citrons, pomelos, and mandarins. These three citrus species are native to South and East Asia, but in the last few million years or so, genes from these three types have combined and mutated and generally made their way across the globe resulting in thousands of types of wild citrus.

https://www.wideopencountry.com/citrus-hybrids/