O.K., I'm re-reading the article spanks posted the link to, and I'm not sure where the destruction of genetic diversity comes in... Wild crops cross-pollinate all the time. So far they haven't melded into one omni-crop.
The *real* danger as I see it is two-fold:
1) Crops modified to produce pharmeceuticals cross-pollinate with food crops, and Jr. gets unprescribed meds with his morning corn flakes. This risk seems managable. But so did Jurassic Park.
2) Crops modified to produce sterile seed cross-pollinate with normal crops and reduce the viability of the next generation. The 2nd generation sterile seed thing is one that some corps like Monsato have tried in order to bleed customers in perpetuity (or maintain control of thier product, depending how you look at it). The big head-scratcher here is why anyone would want to buy seed that doesn't function as seed in the first place. Add in fears of contaminating viable crops, and I think this marketing strategy is pretty well doomed.
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I was gonna send a robot back in time, but I got high.