Q.8 Evolution of drug-resistant bacteria is an example of
1. genetic drift
2. natural selection
3. bottleneck effect
4. founder effect
Drug-Resistant Bacteria Evolution: Natural Selection Example
Antibiotic overuse drives bacterial populations toward resistance through heritable survival advantages. The correct answer is 2. natural selection, where antibiotics act as the selective pressure favoring pre-existing resistant mutants.
This real-world Darwinian process threatens global health, with resistant strains like MRSA evolving rapidly in hospitals and communities.
How Natural Selection Drives Drug Resistance
Bacterial populations contain rare mutants with resistance genes (via mutation or horizontal transfer). When antibiotics kill susceptible cells, resistant ones survive, reproduce, and dominate—shifting population allele frequencies directionally.
Classic example: penicillin-resistant Staphylococcus aureus emerged post-1940s as survivors proliferated under treatment pressure. Overuse in agriculture accelerates this, creating “superbugs.”
Explanation of All Evolution Options
Each mechanism explains genetic change differently—natural selection is directional and adaptive.
| Option | Mechanism | Characteristics | Applies to Drug Resistance? |
|---|---|---|---|
| 1 | Genetic drift | Random allele frequency changes in small populations | No—resistance is non-random, fitness-based selection |
| 2 | Natural selection | Survival/reproduction of fitter variants under pressure | Yes—antibiotics select resistant mutants |
| 3 | Bottleneck effect | Drift after population crash reduces diversity | No—not directional; random survivors |
| 4 | Founder effect | Drift in new small colonies from limited founders | No—occurs in colonization, not ongoing selection |
Why Other Mechanisms Don’t Fit
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Genetic drift: Chance events, irrelevant to consistent antibiotic pressure.
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Bottleneck: Temporary crashes (e.g., disasters) cause random loss, not targeted resistance gain.
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Founder effect: Subset migration creates drift, seen in island species—not hospital-acquired resistance.
Natural selection predicts resistance rises with antibiotic exposure, validated by lab evolution experiments showing stepwise MIC increases.
Correct Answer: 2. natural selection
