Exactly. They need to prove that an actual change in DNA occurred not that a particular trait was favored but already existed.
Um, why? Is there some, "one-series-of-experiments-and-resulting-publication-must-prove-everything-all-at-once" rule that applies only to evolution? Whereas every other science (at least those that don't perturb biblical literalists) gets to follow the normal pattern of using experiments to ISOLATE different aspects of complex phenomena?
[cue "scraping" sound of goalposts being pushed waaaaaay back]