Answer:
Periodic selection.
Explanation:
Some bacterial strain has the ability to develop the resistance against the particular drug. This helps the bacteria to survive against the adverse environmental conditions.
The periodic selection may be defined as the natural selection that selects some strains of the bacteria that can survive in the particular environment. The periodic selection also favors the adaptive mutation that might occur due to the recombination in bacteria and removes the non selective species. The information given in the question provides the example of periodic selection.
Thus, the answer is periodic selection.
