Final answer:
A population of bacteria can develop antibiotic resistance through natural selection, particularly due to the overuse and misuse of antibiotics in humans and livestock, which favors the survival and reproduction of resistant strains.
Step-by-step explanation:
The phenomenon of a strain of bacteria that was previously sensitive to an antibiotic becoming resistant to the same antibiotic is a result of natural selection. When antibiotics are used excessively or incorrectly, such as not completing a full course of prescribed antibiotics or using them for viral infections like colds or flu, it gives an advantage to the few bacteria that have mutations or characteristics enabling them to survive the antibiotic's effects. This selective pressure means that only those bacteria with resistance will survive and reproduce, hence increasing the proportion of resistant bacteria in the population over time.
Furthermore, the misuse of antibiotics in livestock contributes significantly to the development of antibiotic resistance. In the United States, a large percentage of antibiotics are used in animal agriculture, often at low doses, which promotes the development of resistance. These resistant bacteria can then be transmitted to humans, creating a major public health concern.