Forklore: Loose genes
Vavilov's Principle (named for botanist Nikolai Vavilov, who died in a Soviet prison camp in 1943 after Stalin declared his scientific views politically incorrect) goes like this: The area where a plant was first domesticated is where the greatest number of strains of the plant are found.
Why should that be? After all, many a traveler takes a liking to an unfamiliar fruit or vegetable and brings some seeds home, where it could end up being even more popular than where it was domesticated. You might think enthusiasts in the new home would develop plenty of new strains.
They may, but never as many as exist in its place of origin. Plants are domesticated by selection from a multitude of wild varieties; and in the native habitat, all those wild cousins are still on hand for cross-pollination. There are always a lot of strains there because of all the genetic material floating around. Meanwhile, it never occurred to anybody until the last 100 years or so to collect seeds for every cultivated strain of a particular fruit or vegetable. Travelers would take home only a small fraction of what was being grown.
Take the carrot, said Vavilov. The existence of white and purple Afghan carrots shows conclusively that the carrot was domesticated in Afghanistan. Nobody else wanted the white or purple varieties (though these days some restaurants might give them a whirl).