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The Endosperm Balance Number (EBN) hypothesis can explain to a considerable degree the crossability between tuber-bearing Solanum species. It has been shown to be genetically controlled and is dosage dependent. There is a good correlation between EBN and the postulated evolution and present taxonomy of potatoes. The primitive white stellate-flowered species from Mexico are 1EBN, and this condition is also found in species from South America with flowers of the same colour and shape. The evolution of a rotate corolla seems to be correlated with 2EBN. It is postulated that the 2EBN state arose as a reproductive isolating mechanism in South America. The taxonomic and evolutionary implications of the EBN hypothesis are discussed.