genomics that are categorized under the umbrella of conventional breeding are also novel approaches for introducing new qualities into crops.

Transgenic crops bear many similarities to conventionally bred crop varieties because they are both selected by the same conventional methods; crop varieties therefore bear some similarities. It is this confounding of parts of the breeding process with the whole breeding process that has created some of the scientific confusion.

At a deeper level, however, the general comparison of environmental risk of transgenic and conventional crop varieties probably cannot be resolved scientifically at the present. The unresolved scientific challenge is to determine if the range of genetically engineered crop varieties has a similar degree of environmental risk as the range of conventionally produced crop varieties. Theoretically, the most quantitatively robust estimate of environmental risk for transgenic crops would compare the probability of environmental damage associated with transgenic crops to the probability of environmental damage for all crops. This would allow the risk from commercialization of transgenic crops to be expressed as a conditional probability given the presence of transgenes. Such comparison is impossible. There has never been a systematic attempt to measure the probability that a randomly selected agricultural crop will cause environmental damage; hence, the baseline data that would be needed to derive a conditional probability estimate simply do not exist. In addition, because the range of possibilities attainable by both conventional and transgenic methods is expanding rapidly, any scientific determination would be provisional and rapidly outdated. As argued in Chapter 1, while our long experience with some conventional crop breeding techniques indicates that for the most part it has been safe for humans and the environment, there are cases where it has led to environmental harm.

Most inquiries into this issue have focused on one of two narrow questions. First, does a commercial transgenic crop variety with a particular trait have similar ecological risk as an isogenic commercial variety with the same functional trait produced by a conventional method? APHIS expands this argument to assert that the risk of the transgene is not greater than the risk of a comparable conventional variety that is neither isogenic nor possessing the same trait (see Chapter 4). There are no published scientific studies that demonstrate the similarity of risk at either of these levels of comparison (see BOX 1.2).

Second, does a commercial transgenic crop variety with a particular trait have similar ecological risk as an isogenic conventional commercial variety without the trait? Crawley et al. (2001) provide some evidence to this point, although it is not clear if the comparisons were isogenic ones. For the several crop-trait-environment combinations that were tested, no difference in weediness was found within the pairs. Crawley et al. (2001)



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