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.
The two fish are of the same species because they can mate with each other. Also, the characteristics of the blind cave fish enable them to live in caves. Eyes need energy. So, developing bodies with no eyes allowed these fish to function more efficiently.
