Principle 1: Genetic Variation In beetles there are two types. There are black and brown beetles. The beetles with the brown mutation can hide from birds that try to eat them and they can be inherited by a brown beetle parent.
Principle 2: Over Production & Struggle for Survival For the beetles that are brown survive more than the black beetles and so they can pass down that trait. The black beetles die and are easily seen by birds and the brown make dozens or hundreds of eggs. That will most likely make it to adult hood. And the black beetles will just be eaten by birds.
Principle 3: Differential Survival & Reproduction The brown beetles will survive longer by camouflage. They will grow and be able to reproduce to pass down that trait. And every offspring that comes will have that trait and keep passing it down.
Principle 4: Changes in the Gene Pool The brown beetles will live long and even longer throughout generations. But as they are living longer the black beetles are dying out and cannot be alive later on in the future.