The sympathetic and parasympathetic fibers serve the same organs. how can their opposing effects be explained?

Sympathetic and parasympathetic fibers serve the same organs, but they have opposite effects. The sympathetic system prepares the body for intense activity and fight or flight. The parasympathetic system relaxes the body, controls homeostasis and resting and digesting. It is the feed and breed system. 

It depends on the fish (I do not see any pictures) Any fish is an example of natural selection.

Explanation:

A good example would be that polar bears grew and evolved to have white fur to blend in the snow to sneak upon prey. However, their brown bear counterparts failed to do that. So due to better camouflage and hunting technique, the polar bears survived more up north.