To say bacteria somehow selects due to its environment is not purely natural. It is resistant or it is not resistant that is all.
Which part do you imagine you are spinning?
Let's take this step by step:
1) Put a single, non-resistant bacterium in a clean petri dish.
2) Add a mutagen to the petri dish.
3) Allow the bacterium to grow into a large colony in the presence of the mutagen.
4) Add Antibiotic X to the colony.
5) Observe that the mortality rate of bacterium is less than 100%.
6) Allow the surviving members of the colony to repopulate.
7) Add another dose of Antibiotic X to the colony.
8) Observe that the mortality rate is less than in step 5.
Now, what can you conclude?