In drug-resistant tuberculosis, natural selection means?

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

Answer: (A)

Natural selection acts on heritable variation, so individuals with traits that help them survive under a specific pressure are more likely to reproduce. In drug-resistant TB, some bacteria acquire mutations that make them less affected by the antibiotic. When the drug is present, those mutant bacteria survive while others are killed, and they pass the resistance genes to their offspring. Over time, the resistant traits become more common in the population, producing TB that is harder to treat with that antibiotic.

The idea that the environment creates the survival of resistant bacteria is part of the concept, but the crucial point is that the helpful trait is genetic and heritable, enabling its spread. The notion that the antibiotic weakens due to host metabolism isn’t what drives the bacterial population’s change, and the belief that the population becomes resistant to all antibiotics isn’t accurate—the resistance is usually specific to particular drugs, and not every bacterium becomes universally resistant.