A population of Sherwood Moths live in the forest, they have quality traits that can help them to survive in this environment. However, they go through the mechanism on Natural Selection.
Principle #1 Genetic Variation Sherwood Moths have encoded traits in their DNA, which can determine their physical traits. Although, some traits could be caused by a mutation that occurred when the DNA was copied. That would be one of the reasons why some moths are lighter than others.
Principle #2 Overproduction & Struggle for Survival Some mutations in genes can be favorable, but not in this population of Sherwood Moths. Since the mutation has made the moths lighter in color they have the least chance of surviving. They find it harder to camouflage with it's surroundings, thus making them much visible to predators like bats.
Principle #3 Differential Survival & Reproduction Since some of the moths had the unfavorable trait of being lighter, they wouldn't have made it to reproduction stage. Which will end with their traits dying with them and the survivors will past down their traits to their offspring.
Principle #4 Change in Gene Pool If the survivors make it to reproduction their genes will be pasted on. However, since most of the survivors in the Sherwood population are dark than there would be more darker colored offsprings than light colored offsprings.
In the end only the fittest will be able to live on in their environment, and perhaps there could be more mutations that could further on help them adapt better.