These sorts of studies can help researchers determine the relative importance of nurture and nature in determining a trait. Nature here would refer to genetics passed down to an individual from its parents, whereas nurture refers to the environment the individual is exposed to.
If you do not have variations to be selected by natural or sexual selection on their superior abilities to survive and reproduce successfully then nothing can be naturally selected against the immediate environment.
There may be some strains of bacteria that are naturally resistant to these antibiotics, or there may be some that mutate to become resistant. This means that as antibiotics are used more and more, those that can be killed using the antibiotics die out, but those that are resistant will remain, and will reproduce. Over time, it will be the resistant species that predominate.
