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The Lederberg Experiment
In 1952, Esther and Joshua Lederberg performed an experiment that helped to show that many mutations are random, not directed.
Here is the experimental set-up for the Lederberg experiment. All you really need to know in terms of background information is that bacteria grow into isolated colonies on plates, and that you can reproduce the colonies from an original plate to new plates by stamping the original plate with a cloth and then stamping empty plates with the same cloth. Bacteria from each colony are picked up on the cloth and then deposited on the new plates by the cloth.
The hypothesis for the experiment is that antibiotic resistant strains of bacteria surviving an application of antibiotics had the resistance before their exposure to the antibiotics, not as a result of the exposure.
So the penicillin-resistant bacteria were there in the population before they encountered penicillin. They did not evolve resistance in response to exposure to the antibiotic.
Causes of Mutations
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Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology.