So What did we learn???

The yellow circles are camouflaged in their habitat  and therefore are better able to 'survive'.They are adapted to their habitat as they have characteristics which help them to survive i.e. camouflage.

 Animals that are better adapted to their habitat are more likely to survive than those animals not adapted. Therefore, if you start with a population of animals with a variety of characteristics, over time that population will change, so that the animals that are adapted to the habitat become more common than those that are not. 

Hence, we get natural selection of those animals most suited to their habitat causing the population to change over time i.e. evolve.

This links with the story of the peppered moth.