The results, too, are more worrisome than those of mere mechanical tinkering, because unlike mechanical contrivances, genetically modified organisms reproduce, genes spread, and mistakes literally take on a life of their own. Herbicide-resistance genes may escape to make "superweeds." Insecticide-making genes may kill beyond their intended targets. Both these problems have already occurred; their ecological effects are not yet known. Among other recent unpleasant surprises, spliced genes seem unusually likely to spread to other organisms. Canola pollen can waft spliced genes more than a mile, and common crops can hybridize with completely unrelated weeds. Gene-spliced Bt insecticide in corn pollen kills monarch butterflies; that insecticide, unlike its natural forebear, can build up in soil; and corn borers' resistance to it is apparently a dominant trait, so planned anti-resistance procedures won't work.
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