Question

Give an account of Lederberg’s replica plating experiment to show the genetic basis of adaptation.

Answer 1

1. Lederberg’s replica plating experiment demonstrated the genetic basis of adaptation in bacteria. They grew bacteria on a “master plate” containing many colonies, each derived from a single bacterium. Using a sterile velvet cloth, they transferred bacteria from the master plate to several replica plates. Some of these replica plates contained an antibiotic like penicillin.
2. They found that most colonies grew on the master plate but failed to grow on the antibiotic plates, except for a few colonies that were resistant. Importantly, the pattern of resistant colonies was the same across all replica plates, indicating these resistant bacteria were already present in the original population before exposure to the antibiotic.
3. This experiment supported Darwin’s view that adaptation arises from pre-existing random mutations (mutant genes), not from the environment inducing new mutations (contrary to Lamarck’s idea). The antibiotic did not induce resistance but selected the resistant mutants that were already there. Thus, natural selection acts on genetic variation present in the population, leading to adaptation over time.