“I agree that a eye in a totally dark world is a liability. Yet an eye has mutated to be a little less sensitive/functional remains as large an liability as the fully functional eye.

The only selective advantage would be the sudden appearance of an absent eye--the hopeful monster in reverse.” ”

>>IF there sighted versus non-scighted phenotypes exhibited idnetical fitness we could expect that we’d see a population displaying a mix of sensitive and slighty-less-sensitive eyes, just as we see a mix of blond and brunette phenotypes in the human population. They’d be equivalently fit and natural selection would act neither to conserve or oppose them.

At a later time, of course, additional genetic changes could transform lsighty-less-sensitive eyes into eyes that were not only completely insensitive abut which also did confer a fitness advantage (reduced risk of infection, for example), at which point blindness would be selected for and the frequency of alleles resulting in blindness would increase over generations.

Additionally, if the mutation resulting in the loss of sensitivity was a function of inactivation of gene expression--if the salamanders no longer expressed a gene product that conferred no change in fitness of itself--there would be a sufficient energy savings to drive selection for the salamanders that failed to express the gene

Even if the mutation caused no change in fitness the blind could become uniform throughout the population by mechanisms such as founder’s effect or genetic drift.