In this image, there are two genetic variations to this species. One of the variations include a large snake that blends into the ground very well . The other is purple but very small.
Here, as you can see the more fit variation of the species is over producing. While, the purple snakes that cant blend in with its surroundings are being spotted more easily and are going extinct. While all animals here are struggling to survive.
The smaller snakes that can move faster, hide in small areas, and blend in the best are going to be the ones who reproduce more often. They will also evolve the best.
Eventually, the entire gene pool will become only small brown snakes, because they are the fittest in their environment. All the other snakes will most likely die out, because they are not adapted .