Variation: difference of characteristics in similar or in the same species.Overpopulation: a species produce too much offsprings, Survival of the fittest: the species that have "better" traits will survive.Inheritance/Reproduction: the offsprings have genes by both the mother and the father.
Survival of fittest
Two different types of bat, one white and the other one black in the same habitat.
There are much more offsprings, and competition takes place. Black bats prevail on the white ones because they have a better camouflage in the cave.
Traits: Black skinWhite skinGrey skin
Black bats are the fittest in a dark cave, because they can hide from predators easily.
The offspring of the two types of bats have a new color, due to the unexpected codominance of the two traits.