Replica plate experiment by Lederberg showing antibiotic resistance proves

Options

A. genetic drift
B. natural selection
C. chemical mutation
D. founder's effect

Answer

### Step-by-Step Solution:

1. **Understanding the Experiment**: The replica plate experiment was conducted by Joshua and Esther Lederberg to study the genetic basis of antibiotic resistance in bacteria.

2. **Setting Up the Experiment**: In this experiment, the Lederbergs cultured a large number of bacterial cells on a nutrient medium. They then transferred these cells to different plates containing antibiotics.

3. **Observing Resistance**: After applying the antibiotics, they observed that some bacterial colonies were able to survive while others were not. This indicated that certain bacteria had pre-existing mutations that conferred resistance to the antibiotics.

4. **Conclusion on Adaptation**: The Lederbergs concluded that the antibiotic resistance seen in the surviving bacteria was not a result of new mutations occurring in response to the antibiotic exposure, but rather the selection of pre-existing resistant mutants that were already present in the population.

5. **Linking to Natural Selection**: This process of survival and reproduction of the resistant bacteria is an example of natural selection. The environment (in this case, the presence of antibiotics) selected for those bacteria that had the advantageous trait (antibiotic resistance).

6. **Evaluating the Options**:
- **Genetic Drift**: This is a random process that affects allele frequencies in a population and is not what the Lederbergs demonstrated.
- **Natural Selection**: This is the correct answer, as the experiment showed how environmental pressures (antibiotics) select for certain traits (resistance).
- **Chemical Mutation**: This option is incorrect because the experiment did not focus on mutations caused by chemicals.
- **Founders Effect**: This is also incorrect as it refers to genetic variation that occurs when a small group from a larger population establishes a new population.

7. **Final Answer**: The correct answer is **Natural Selection**.