Principle #1: Genetic Variation: In the bat population, there are bats that are short faced, and there are those that have long narrow snouts. The bats with shorter faces have a better bite, which gives them a good variety (in terms of their diets).
Principle #2: Overproduction & the Struggle for Existence The bats that have long and narrow snouts are not able to eat figs that are hard, which means that they will starve. In order for the bat to survive, it needs to have a short face so that it can eat properly. The selective pressure in this case is the long narrow snout trait that is passed down.
Principle #3: Differential Survival & Reproduction Bats with shorter snouts have a better bite, which lets them survive. The key to survival is to have a good bite so that they can eat a variety of foods.
Principle #4: Changes in Gene Pool As future generations pass, the population of bats with shorter faces will go up since they can easily adapt to their environment, while the population of bats with long narrow snouts will go down since they cannot survive in their environment.