of our foodstuffs
doesn't in fact seem to have any adverse effects
The problem is we can't know what we're doing. There have been problems in the past with transported species or hybridization that have developed unusually pervasive plant and animal populations in their non-native regions. GM takes this a step further by removing some natural controls over what organisms create and grow--it may be possible to create an organism with no natural enemies, that may be very hard to contain in a farm, that may kill other beneficial organisms, or change the ecosystem in non-insignificant ways. No, I'm not saying it definitely will happen--I'm not even saying it's likely. It's a case of simple risk management. How much of a chance are we willing to take when the consequences could be environmental disaster?
Also consider the GM corn modified to produce the Bt toxin, which is a toxin usually created by bacteria that are inoculated into fields. This bacteria kills most larvae and grubs that feed on things like corn. In the form produced by bacteria, it is safe for humans and animals to be around and to eat. However, once it is pervasive in plant tissues, strange things happen. Monarch butterflies have been killed by pollinating corn with the Bt toxin in it. The toxin that used to be only on the surface of plant matter is now inside of it as well. Sure, I can eat an ear of this corn and not keel over, but who knows what might happen in 20 years?
After all that, I'm not against GM research. I just think people aren't taking it seriously enough. I think much tighter controls need to be in place until long-term safety testing is done, and until we understand more about genetics. We're making great strides in the latter, and I think with continued careful research, we'll actually be able to say with more certainty what GM plants will do in the wild. Until then, I'm buying organic.