Answer:
The correct answer is "natural selection in Staphylococcus aureus populations".
Explanation:
Natural selection is a process at which populations of organisms develop advantageous traits under the premise of survival of the fittest. In this case, the continuous exposure to penicillin and other antibiotics since 1940s have a natural selection effect in Staphylococcus aureus populations, allowing an increase in antibiotic-resistant alleles in the bacteria, which acts as an advantageous trait for them. Antibiotic resistance is a serious medical concern that has been addressed by a culture of responsible use of antibiotics and development of novel treatments for infections.
