The common wisdom is that crops are most diverse in their centres (or secondary centres) of domestication, because that’s where people have been playing with them longest. Wild species, too, are often less diverse when they have moved to a new area. That’s down to the founder effect; a small bunch of founding individuals will have a less diversity than the population as a whole and is also more subject to random fluctuations that can change things from the original population. But on the Invasive Species Blog (via this month’s Mendel’s Garden) I recently read that reed canary grass (Phalaris arundinacea) is much more diverse in North America, where it is a recent arrival, than in Europe, its home. Genes from all over the old world are mixed up within single individuals in North America, whereas they are never found together in Europe. The reason, apparently, is that the species has been introduced many times, presumably from many places, and this has brought widely separated populations together and given the opportunity to mingle their genomes.
I wonder whether the same is true for some of the crops that have really travelled around, like tomatoes or peppers.