In the fish population there are blue fish and there are brown fish. the blue fish can only hide from the predator will the brown fish can camouflage with the sand at the bottom of the ocean.
When the fish sees the shark coming the brown fish start to hid in the sand and some of the blue fish get eaten by the sharks and there are some blue fish that survive. The brown fish start to reproduce and there are a few blue fish left.
The brown fish as a high percent chance that they will survive for more generations and keep reproducing than blue fish that will get eaten by other predators and have few of them.
When the fish started to reproduce from generation to generation the blue fish start to decease in each generation and the brown fish start to increase in each generation.