Variation occurs as a result of a random mutation and this provides bacteria an increased level of resistance to antibiotics. When antibiotics is added it acts as the selection pressure. The conveys an advantage to the bacterium with the new allele, enabling it to survive. The surviving bacteria reproduces asexually by binary fission and the allele is passed on. Any bacteria that do not have this allele are selected against and will be killed by the antibiotics. The surviving bacteria continue to reproduce, passing the allele on each time until all of the population have resistance.
