Under antibiotic pressure, natural selection means what?

• A. The environment was created in which the tuberculosis bacterium carrying the antibiotic-resistant genes had an increased likelihood of survival and reproduction.
• B. The tuberculosis genes mutated to avoid being killed by the antibiotic.
• C. The antibiotic becomes weaker due to host metabolism.
• D. The population becomes entirely resistant to all antibiotics.

Answer: (A)

Natural selection under antibiotic pressure works because the antibiotic creates an environment where only bacteria with resistance traits tend to survive and reproduce. The tuberculosis bacteria that carry antibiotic-resistant genes are more likely to live through the drug treatment and pass those resistance traits to their offspring. Since these traits are heritable, their frequency in the population increases over generations, so the population becomes more resistant to that antibiotic.

Mutations can contribute to resistance, but the key idea is differential survival based on existing variation in the population. It’s not about the antibiotic physically becoming weaker inside the host, and it’s not realistic to expect the entire population to become resistant to all antibiotics, since resistance is specific to each drug and involves particular mechanisms.