Let's say you want to create a strain of bacteria that is resistant to heat. You expose all bacteria to high heat and those who have mutated to survive this heat will be the survivors. Then you allow this group to reproduce and continue to gradually raise the heat until you are left with a population of highly heat-resistant bacteria.
Let's say you wanted to do the same but instead of selecting heat resistance you want to select for the production of a certain enzyme, for example amylase. How do you manipulate the environment so that only those with higher production would survive?
One thing that comes to my mind would be introducing all the microbes to an environment where the substrate they need to break down to survive is scarce. So the specimens that would create the most enzymes to break it down would survive. But then all of the other specimens would benefit too because the enzyme from the high-producing specimen would also give them the substrate they need to survive - so it would not be isolated but rather would 'carry the entire population on its back' (I have no idea whether this is how it would work, this is an idea)