About 200 generations ago, there was a population of fish. There were many different traits whiten this population. Some fish were blue, some were orange, some were larger than others, some had larger eyes and mouths, and their fins were all different sizes. Being small was a good trait for the fish because they were harder to see when people came to the lake to fish, which wasn't very common at first.
Over time people started to build houses and live on the land around the lake. This caused fishing to be more common in the lake. When this started to happen, the bigger fish began to stand out more and they often were caught and eaten.
After many generations, fish that were smaller survived and bigger fish went extinct. This caused the variation to go down and the distribution to favor smaller fish.