Natural Selection Drives Antibiotic Resistance

Steps:

• Mutations arise randomly in bacterial populations, independent of antibiotics.
• Most mutations are neutral or harmful; only rare ones confer resistance.
• Antibiotics kill susceptible bacteria, allowing resistant ones to survive and reproduce.
• Over generations, the proportion of resistant bacteria increases only under antibiotic pressure.

Why D is correct:

• Natural selection favors resistant bacteria when antibiotics are present, increasing their relative proportion in the population (Darwin's theory of evolution by natural selection).

Why the others are wrong:

• A: Mutations are random and rarely lead to resistance; most do not.
• B: Antibiotics select for pre-existing resistant mutants but do not increase mutation rates.
• C: Mutations occur spontaneously at all times, not just during antibiotic treatment.

Final answer: D