A researcher is doing an experiment to test the effectiveness of antibiotics against Strep throat bacteria (Streptococcus). The researcher uses a petri dish filled with the bacteria and then adds penicillin, a type of antibiotic. Within two days, the pathogen’s population decreases by 99 percent. Over time, the population begins to increase and continues to survive, even with doses of penicillin in the petri dish. This is an example of