In the forest, there are different types of butterflies which vary from light to dark.
Changes in Gene Pool
No more food??!!
The butterflies are having a hard time to survive because the spider and bird, predators, are eating them. The darkest butterfly survives longer than the others because it camouflages the best with the environment.
The darkest butterfly is the only one that will be able to survive in this environment, therefore it will pass its genes to future generations.
Because they blend in with the environment, they survived and only produced dark colored butterflies.