This article is talking about mutation. Two of them in fact. The E Coli adapted to a reduced food supply by mutations that allowed for additional food stuffs to be consumed. New food stuffs that were before unusable. At the same time it retained the ability to continue obtaining sustenence as it always had. It's absolutely fascinating. And even moreso that the same 2 mutations occured in 2 seperate colonies.
What evidence do they have that this is a mutation at all and not just an expression of a rare recessive trait that multiplied when the food supply was reduced? I'm guessing that they didn't test the entire population for the gene in question. Just because they haven't seen it before doesn't mean it wasn't there. It would be impossible to recognize until the dominant ones were removed.
They tested for this. They discovered that only specimens (from the line that produced citrate metabolizing E. Coli) existing after a certain generation (I think it was like generation 20,000) contained the necessary mutations, and were only put together in such a way as to allow for citrate metabolism about at generation 32,000. They saved "fossils" of generations from the beginning (at regular intervals) by freezing for later examination and comparison. These frozen "fossils" could also be fully resuscitated by thawing.
