Let me give an example. Antibiotics have resulted in more resistant bacteria. This effect is due to the changes within the environment caused by the antibiotics. The antibiotics will kill off the good bacteria, with the resistant bacteria persisting. These resistant bacteria, already had the DNA tools, from day one, or else they would have died off in day one like the rest. All we did was create a Yellowstone scenario, or a micro-scale forest fire, with the resistant migrating. This is not Galapagos. It did not takes thousands of years but happened in decades with a wide swing in terms of the average bacterial DNA. But the explanation will remain Galapagos biased and will tend to fixate on genetic changes not migration of bacteria that already had the DNA for survival.