these are two types of organisms , the first fish is a camouflage and the second fish is bright colored that most likely catches a predators attention unlike the camouflage that can blend in the environment and can hide.
the first organism is a camouflage fish that won't most likely catch a predators attention and can blend in it's environment from a predator which will most likely survive and reproduce.
the second organism is a bright color fish that will most likely not to survive because it will catch a predators attention because the predator will most likely thinks it's food , the fish can try to escape but will likely fail because there will be a organism faster than them so the bright colored organism will most likely dead and won't reproduce.
the changes in the gene pool is that there will be more of the camouflage organisms than the bright colored organisms. for example the camouflage organism can hide from there predators and will most likely succeed and the bright colored organism can try to escape but 90% will mostly likely fail.