Evolution question regarding something I saw on Planet Earth™?

Eyes are very vulnerable to damage and infection so have a high cost both in embryogenesis where the grow but also later in repair. Sensory interpretation and coordination takes up neural pathways in the brain. This is their negative selection.

Vision and the neural coordination with reduced reaction times in response to environmental changes is the positive selection.

In the caves there is no selection advantage to having eyes but there is a selection advantage to not having vulnerable eyes prone to injury. The negative selective pressure is far stronger than the positive selection since vision offers no benefit in the dark. Further the neural connections may be repurposed for use in coordinating other senses with survival behaviors useful in the dark if the mutation arise. New selection pressures can add genetic controls to alter previous developmental paths.

Natural selection is always the sum of all the many selective pressures for a phenotype.

The loss or degradation of a trait through time is known as regressive evolution. The first hypothesis assumes that the loss of eyes somehow enhances the efficiency of neural processing or reshapes the fish's morphology and physiology to better suit a life of total darkness. As a consequence, natural selection drives the regressive evolution.

The second hypothesis is that genes controlling the development of unnecessary structures become effectively neutral. Once the genes neither enhance nor hinder the organism's survival, the forces of natural selection that once maintained those genes in good working order no longer operate. The genes accumulate mutations that impair their function, and so the unnecessary structures governed by the genes degenerate. That view is summed up in the phrase "neutral-mutation theory." There is evidence that blind people develop above-average abilities in specific tasks related to hearing or to touching. There is also anecdotal evidence that sighted people have keener nonvisual senses when they cannot see - and that is why these weirdo creatures of the dark lose their eyes.

The advantage could be a simple as the fact that the salamanders without eyes no longer had to put-in the metabolic energy during development to grow the unnecessary eyes (and related structures) any more.

This is a *slight* advantage, but it is still an advantage.

I'm unconvinced by the "sensory input" argument, because in total darkness, there wouldn't be any sensory input through the eyes.

i think i understand...

"Is it simply one less sensory input system that the body has to deal with, therefore it doesn't waste time producing it?"

this is one reason...

another reason is that evolution doesn't SOLELY work on the basis of benifitial/deletrious... random evolution can and does occasionally happen, where there is no inherent benefit, but no malady either, in such cases the traits are passed on randomly

another reason is sexual selection, but this wouldn't have much to do with the cave salamanders

and then of course there's a possibility that it DOES serve some purpose (better refracts the dim light, covers up eye from scrapes, etc) that we just don't know about yet

"Is it simply one less sensory input system that the body has to deal with, therefore it doesn't waste time producing it?"

There's your answer.