Principle #1 Genetic Variation: The genetic variation in the population are small bats and little bats.
Principle #2 Overproduction and the Struggle for Existence: The big and small bats have babies. The big bats are doing fine but the little bats are having trouble hunting because due to their size they can't hunt animals that will feed them both.
Principle #3 Differential Survival and Reproduction The bigger bats have a higher chance of surviving and reproducing because they can hunt bigger prey.
Principle #4 Changes in the Gene Pool There are almost no more little bats and bigger bats have become more common.