Principle #1 Genetic Variation: Genetic Variation is differences is the same animal at the same time, most likely due to a mutation over the span of a few lifetimes.
Principle #2 Overproduction & The Struggle for Existence: When an animal overpopulates and most survive that's where it goes wrong, there isn't enough food for them all, so they'd have to compete against on another for that food. Add in predators of their species, and a whole lot will drop like flies.
Principle #3 Differential Survival & Reproduction: With the differences in the way they survive, one of the different variants will be able to survive better than the other, and the other will be reduced because of all the prey going after them. The one who survive will be able to repopulate.
Principle #4 Changes in The Gene Pool: As the dominant variant survives, the other will die out more quickly, and not be able to reproduce, letting the other variant to repopulate the gene pool and cover it with their entire genetics.