Subject: How does natural selection apply to loss of organs or faculties?

Some creatures, such as fish, which live underground, are 
eyeless. I can understand how, for instance, snakes would evolve 
from burrowing lizards, leglessness being advantageous. But how 
about those fish? I would have thought eyes would become 
irrelevant as the fish adapted to life in darkness. Better touch 
or smell would help an individual to survive and breed, but what 
difference would it make if that individual had normal eyes, bad 
eyes, or even none. OK, eyeless individuals would be at no 
disadvantage and would survive, but why would the whole 
population become eyeless?

Re: How does natural selection apply to loss of organs or faculties?