Question

Drug-resistant populations of microbes arise when

Answer 1

There is variation within the population, due to a mutation. The selection pressure is the drug. The microbes that have the advantageous gene which makes them resistant to the drug are selected for, and reproduce passing on the gene, increasing its allele frequency within the population over time. However, the microbes that do not have the advantageous gene, cannot survive and their number decreases within the popullation.