These are crops that can expand out of Southwest Asia more easily than out of South America or North America. The rapid expansion of a crop can prevent the domestication of not only wild ancestors of the same plant elsewhere but also related wild species. If we have grown good bean plants, of course there is no reason to re-dominate from the beginning the wild ancestor of the same bean again, but even the related wild beans. The close proximity that farmers see is almost the same as that of domesticated beans, their domestication is useless. All of the founding crops of Southwest Asia prevented the domestication of any species close to them throughout the western part of Eurasia. In contrast, the New World has many instances of similar but closely related species that have been domesticated in both Central America and South America nonetheless.
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