Answer: The environmental change put selection pressure on the population of the bacteria so that some trait variations had a competitive advantage for surviving over time.
Explanation: So, in Addie’s case, the environmental change was the antibiotics. The antibiotics led to the weaker bacteria to die off while the stronger bacteria reproduced and grew larger in population. This is called selection pressure. So to sum it all up, the bacteria population became more resistant because of the pressure selection out into them by the antibiotics. So, the weaker bacteria died off and the stronger one grew larger and larger, becoming antibiotic resistant. Sorry if I’m too late but I hope this clarifies your question!