Principle #1 In the squirrel population, there are some born with brown fur and some with grayish fur. This trait is caused by a mutation and can only be inherited.
Principle #2 The squirrels with brown fur (mutation) pass it down as the grayish squirrels die because they can easily be spotted by predators. Overall, squirrels with the mutation (brown) survive and pass down their genes but grayish squirrels don't. Therefore, the selective pressure in this case is the predators/environment.
Principle #3 The squirrels with the mutation (brown fur) inherited their genes. As they grow up they pass down the mutation to future generations. Therefore, When the brown furred squirrels reproduce the pass down the mutation.
Principle #4 Every generation will increase the amount of brown furred squirrels and decrease the amount of grayish furred squirrels (Squirrels-that are brown-survive/reproduce, grayish-die and don't reproduce).