The concept of evolutionary biology encompasses various mechanisms, including selection pressures that drive the evolution of species. A particularly interesting case involves the evolution of antibiotic resistance in bacteria. Consider a population of bacteria exposed to an antibiotic. Not all bacteria will be equally affected; some may possess mutations that confer resistance. This leads to a scenario where the normal population declines, but the resistant bacteria may thrive and reproduce.
Which of the following describes the evolutionary mechanism primarily responsible for the increased prevalence of antibiotic-resistant bacteria in their environment?