variation. At the beginning their are two different colors which are light and dark.
struggle to survive. The struggle is trying not to get eaten by the lion. However the dark ones will be the ones who will not survive because of their color which doesn't camouflage in that environment.
survival. The light ones are the ones who will survive and not the dark ones. The light ones have an advantage to survive because they can camouflage in that environment.
change in gene pool. At the end their will only be light ones and no dark ones. After a while the light ones will reproduce and pass the genes. Which means he environment will be full of light ones.