This is a very interesting question to which we would like to add one additional factor. Evolution and adaptation can be one response to change; extinction can be another. As highly evolved beings we are fortunate that we are not among the greater than 99% of all species that have gone extinct! The plants that we farm descended from wild plant ancestors but they were extensively genetically modified over the years by a process called domestication. The ancestor of modern corn, for example, looks like a grassy weed – most people never guess that it’s related to corn. The ancestor of carrots looks like a bunch of inedible hairy roots. At this point few of our crops resemble their wild ancestors and virtually none can survive in the wild. They must be planted and cultivated by humans to survive. It is fair to say crop plants are human made and genetically modified, not natural, and to conclude that we have been tampering with nature for a very long time. One might view the whole of human existence, houses, cities, cars, the internet, clothing, and just about everything else we do as tampering with nature. Of course we might also ask why humans separate themselves from all other specials and view what they do as unnatural and tampering with nature? At the end of the day one cannot deny that any form of agriculture is tampering with nature. Producing more food than we can find in nature is actually the whole point of agriculture.
Viewed in light of the fact that GM technology is the least disruptive, best understood, and most precise technology we have every developed for breeding new plants there is a significant scientific consensus that GM crops pose no new or different risks than other methods of breeding. GM crops are extensively tested prior to their approval to ensure that they have no adverse impact on wild ancestors, other crops, or unrelated wild plants. They are also monitored in the field after they are released to ensure that they don't have any adverse impacts. Although there have been many wild and unfounded claims to the contrary, GM crops have been planted for 17 years and to date no adverse effects on wild plants has been observed. That doesn’t mean, however, that GM crops are perfect, For example, farmers and scientists are carefully monitoring the emergence of herbicide-resistant weeds that have occurred in some places in and near fields of herbicide-resistant crops.
It’s worth noting that considerable damage has been done through the human introduction of exotic species (think kudzu to the US, rabbits to Australia, etc.). That’s why we have learned to be very careful about introduction of exotic species. GM crops are almost the exact biological opposite to exotic species in that they are virtually identical to the seeds they replace with the exception of the addition of a single well-studied and well-understood trait. From a scientific risk perspective this is a much improved way to cope with the need to increase productivity and sustainability of agriculture at the same time. And yes, a significant amount of data on the potential impact of GM foods on humans, animals, and the environment is required before they receive approval by government regulators. The pre-market safety testing procedures can be found at http://www.fda.gov/food/guidanceregulation/guidancedocumentsregulatoryinformation/biotechnology/default.htm, http://www.epa.gov/oppbppd1/biopesticides/pips/index.htm, http://www.aphis.usda.gov/biotechnology/index.shtml. The real advantage of GM technology is that it takes a minimalist approach to introducing beneficial changes to our crops. It's a pity that so much negative misinformation has been spread about a technology that is both more powerful than other methods of breeding and is at the same time the most precise and minimalist.