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But why would you want them to become diverse - this could only lead to them *losing* the traits you want? Once you got a variety you want, you just stick to it and breed the hell of it.
Monoculture is a separate problem - it certainly bad for environment if you cut 100 km2 of forest and plant only soya or wheat there. But it's just as harmful if you plant organic, Bt or whatever else variety, as long as it's single plant.