Xavier was conducting a long term study of two populations of butterflies. Both populations had the same number of individuals. Both populations lived in forests that were the same size. Population A lived in a forest that was full of milkweed, the favorite food of this butterfly species. Population B lived in a forest that had limited amounts of milkweed and other foods. Over many years, Xavier found that one of the populations was showing signs of evolutionary change. Their mouth parts were changing and they could get food from new kinds of plants. Which statement explains why one population would be more likely to evolve in response to natural selection?