Adaptations - Genetic basis of Adaptation

Aim of the Experiment

To demonstrate that bacterial adaptations result from the natural selection of pre-existing genetic mutations rather than being induced by the environment.

Basic Information

• Topic: Genetic basis of adaptation.
• Scientists: Joshua Lederberg and Esther M. Lederberg.

Experimental Procedure

1. Master Plate: Bacteria were grown on a standard agar plate until discrete colonies formed.
2. Replica Creation: A sterile velvet disc was gently pressed onto the master plate to pick up bacteria, then pressed onto new plates to create exact spatial copies (replicas).
3. Antibiotic Exposure: Replicas were made on agar plates containing the antibiotic penicillin.

Observations

• On the penicillin plates, most bacterial colonies failed to grow.
• Only a few penicillin-resistant colonies survived and multiplied.
• The physical pattern and position of these surviving colonies were identical across all replica plates.

Scientific Interpretations

• Lamarckian View (Rejected): Penicillin somehow induced a change in the bacteria, causing them to adapt and survive.
• Darwinian View (Supported): Penicillin-resistant mutations already existed by chance in a few bacteria before they were ever exposed to the antibiotic.

Final Conclusion

• The experiment provides concrete proof for Darwin's theory of Natural Selection.
• It proves that adaptation results from the selection of pre-existing mutant forms.
• Because the colony patterns were identical, it confirms that the resistant mutants were present in the original master plate prior to antibiotic exposure. The environment (penicillin) merely selected the fit microbes; it did not create them.