Which statement best exemplifies natural selection leading to antibiotic resistance in bacteria?

Which statement best exemplifies natural selection leading to antibiotic resistance in bacteria?

• A. Bacteria with resistance alleles survive antibiotic exposure and reproduce, increasing allele frequency
• B. Antibiotics kill all bacteria regardless of genotype equally
• C. Bacteria acquire resistance only through horizontal gene transfer that affects other species
• D. Antibiotics cause random changes in all organisms equally

Answer: (A)

Natural selection under antibiotic pressure explains this evolution clearly. When an antibiotic is present, bacteria that carry resistance traits survive and reproduce, while susceptible ones die or stop reproducing. Because the resistance trait is heritable, offspring inherit it, so the resistant group grows in proportion over generations. That increase in resistance alleles in the population is exactly how natural selection leads to antibiotic resistance. If antibiotics killed all bacteria equally, there would be no differential survival to drive a change in allele frequencies. If resistance could only spread via horizontal gene transfer across species, that ignores the way selection operates within a population on heritable traits. And if antibiotics caused random changes in all organisms, the process would not reflect the non-random survival based on fitness that drives natural selection.

Natural selection under antibiotic pressure explains this evolution clearly. When an antibiotic is present, bacteria that carry resistance traits survive and reproduce, while susceptible ones die or stop reproducing. Because the resistance trait is heritable, offspring inherit it, so the resistant group grows in proportion over generations. That increase in resistance alleles in the population is exactly how natural selection leads to antibiotic resistance.

If antibiotics killed all bacteria equally, there would be no differential survival to drive a change in allele frequencies. If resistance could only spread via horizontal gene transfer across species, that ignores the way selection operates within a population on heritable traits. And if antibiotics caused random changes in all organisms, the process would not reflect the non-random survival based on fitness that drives natural selection.