If I understand natural selection correctly - a mutation will eventually spread throughout a population if it gives member of that population a reproductive advantage.

A accept that more cave fish will lack eyes than other species, as it doesn't matter whether they can see or not. To the seeing cave fish - they are as good as blind in their environment, regardless of vision. However, why would being born without eyes lead to a reproductive advantage? Or is it just a matter of the seeing cave fish seeing light - swimming towards it, and thus leaving the environment? While the blind cave fish would have other reasons for staying. Food, calm water, consistent temperature, etc. - with no external stimuli such as light to encourage them to go anywhere else.

Never mind. I think I answered my own question.

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Spock Jenkins of the Vulcan Jenkins'.