I was always under the impression that if you remove a selective mechanism from an organism’s environment, that organism will no longer evolve in response to that mechanism.
If bacteria don’t have to defend against attack by streptomycin, for instance, it’s certain a fair amount of that strain will mutate into a non-resistant strain and survive along with the rest of their brethren. What’s unknown at this point is, does the lack of resistance give those that have it an edge over those that don’t?
I think the cite Alphagene brought up might not take into account all the streptomycin derivitives that have been developed over the last 30 years - those bacteria have resistance to all but the latest forms of that molecule. Kind of a molecular “backwards compatible” aspect. The streptomycin base is still acting as a selective mechanism in our world.