In the forest, lives a population of dark rabbits. These rabbits all have the same allele which makes their fur color brown(BB). Each reproduce and survive, since they have the beneficial trait which helps them survive and reproduce making their population grow and pass down their genes.
Suddenly, a mutation occurred, causing variation of traits within the rabbit population. The gene of an offspring changed by a mutation in the genotype, causing a different protein, making a different trait that changes the phenotype. The offspring inherited a light brown fur color(bb). This lead to some rabbits having the ability to camouflage better than others.
A fox appears and easily spots the light brown rabbit because it does not have the best advantage in the environment, while the dark brown rabbits are harder to spot because of the inherited trait making them camouflage with the environment. The dark brown rabbits are able to survive and reproduce having fitness, passing their genes to their offspring. The survival of a rabbit depends on the obstacles that the environment has.
Over time, the environment changed, causing the light brown rabbits have the beneficial trait, making them harder to spot, while the dark brown rabbit was easier to spot in the environment
Since the light brown rabbits have the favorable trait in the environment, it is able to survive and reproduce, passing their genes to their offspring. One the other hand, the dark brown rabbits are easily spotted by predators, reducing the amount of the dark brown rabbit population. The population favored the mutated trait over the original trait.
Over several generations, the rabbits with the best variation are left. Since the light brown rabbits were able to survive and reproduce, the mutated trait keep growing among the rabbit population, while the dark brown rabbit allele started to fade away. Now, the mutated trait is the surviving trait among the rabbit population.