A population of bacteria is exposed to an antibiotic. Initially, most of the bacteria die, but a small fraction survives. Over several generations, the entire population is resistant to the antibiotic. Which mechanism best explains this phenomenon?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Antibiotic resistance in bacterial populations emerges through natural selection acting on pre-existing genetic variation. Within any bacterial population, random mutations in chromosomal DNA or horizontally acquired plasmids (such as R plasmids carrying the bla gene encoding β-lactamase) generate heritable differences in antibiotic susceptibility. The enzyme β-lactamase hydrolyzes the β-lactam ring of penicillin-class antibiotics, breaking the amide bond through nucleophilic attack mediated by a serine residue at the enzyme's active site, thereby inactivating the drug before it can bind transpeptidases (penicillin-binding proteins) required for peptidoglycan cross-linking. Alternative resistance mechanisms include efflux pumps like the AcrAB-TolC complex, which uses proton motive force to actively export tetracycline molecules against their concentration gradient, and target-site modifications where point mutations in the rpoB gene alter RNA polymerase's rifampicin-binding pocket through changes in amino acid side-chain stereochemistry and charge distribution.

Why Other Options Are Wrong

Natural selection operates when three conditions are met: variation exists in a heritable trait, that variation produces differential reproductive success, and the advantageous allele increases in frequency across generations. The selective agent—the antibiotic—creates an environmental filter. Bacteria possessing resistance alleles survive, reproduce through binary fission, and pass resistance genes to daughter cells. Over multiple generations, the population's allele frequencies shift directionally, demonstrating microevolution through descent with modification.

PILLAR 2 — STEP-BY-STEP LOGIC

The stimulus describes a classic natural selection scenario with three distinct phases. Phase one: initial exposure kills most bacteria because the wild-type alleles (conferring antibiotic sensitivity) predominate in the population. Phase two: a small fraction survives—these individuals already possessed resistance-conferring alleles before antibiotic exposure occurred. This pre-existence is critical; the antibiotic does not create or induce resistance mutations in individual bacteria. Rather, it eliminates susceptible phenotypes, leaving resistant genotypes as the sole survivors. Phase three: over several generations, these resistant bacteria reproduce, and the population becomes entirely resistant because the selective pressure continues to favor the resistance allele.

The correct answer (A) identifies natural selection as the mechanism because the scenario demonstrates all requirements: heritable variation (resistance alleles present before exposure), differential survival (resistant bacteria live, sensitive bacteria die), and directional change in allele frequency across generations (resistance becomes fixed in the population). The antibiotic functions as the selective pressure—an abiotic environmental factor that screens phenotypes rather than generating new mutations within individual lifespans.

PILLAR 3 — DISTRACTOR ANALYSIS

Option B (genetic drift) traps students who confuse random evolutionary forces with selection. Genetic drift involves stochastic changes in allele frequencies unrelated to fitness—such as bottleneck effects from random mortality. The stimulus specifies that survival depends on resistance to the antibiotic, indicating a deterministic, non-random filter on phenotypes, not chance elimination.

Option C (gene flow) might attract students who recognize the population changes over time. Gene flow requires migration between populations exchanging alleles. The stimulus describes a single population with no mention of external bacteria introducing resistance alleles. The resistance was already present as standing genetic variation within the original population.

Option D (acclimation or Lamarckian adaptation) ensnares students who misunderstand that individual bacteria can develop resistance through exposure. This reflects the incorrect Lamarckian view that use or disuse modifies traits within an organism's lifetime. Bacteria cannot physiologically acclimate to antibiotics at the individual level; a given cell either inherits functional resistance genes or it does not. The molecular machinery—enzymes, efflux pumps, modified targets—requires specific DNA sequences that arise through mutation before exposure, not as a physiological response to it.

Option E (mass mutation during exposure) confuses students who believe the antibiotic causes resistance mutations to arise. While spontaneous mutations do occur during DNA replication, the mutation rate per base pair (~10⁻¹⁰) is far too low for a population to generate sufficient resistance alleles simultaneously upon exposure. The surviving fraction described in the stimulus represents pre-existing variants, not newly generated mutants.