Final answer:
Yes, natural selection has occurred when an animal discovers a new food source and grows very large, resulting in increased reproductive success.
Step-by-step explanation:
Yes, natural selection has occurred in this situation. Natural selection is the process by which organisms with advantageous traits are more likely to survive, reproduce, and pass on their traits to the next generation.
In this case, the animal that discovered the new food source and grew very large has an advantage over others in the population, allowing her to produce more offspring.
Over time, the genes for large size and increased reproductive success will become more common in the population, leading to evolutionary change.