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Genetic variability was "built right into" the bacteria. Did the nonresistant varieties get resistant by mutation? No. Resistant forms were already present.
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I believe a better description would be that the ability to have genetic variability has been built right into the bacteria. Whether there are instances of resistant forms already being present and becoming more prevalent or whether it is new mutations being generated, these are very good examples of what is also known as evolution. Variation within kind is once again a meaningless concept.
Regards
Pim