If you are referring to selection pressure, when the selection pressure decreases, there will be weaker forces of natural selection. The angler fishes without the favourable traits would not be that strongly selected against and vice versa. In some cases such as predation selection pressure, the population of angler fishes in the habitat may increase
I believe this would be the cerebellum. This part of the brain is used for motor control like moving the fingers say and cognitive functions (to do with thinking) and also can affect the feelings of pain and pleasure.
