Evolution is natural selection. The answer lies in part due to the old maxim: "If you don't use it, you lose it". In caves, no light. Thus the eyes of the animals involved tend to atrophy. And since food is at a premium, anything unused tends to be discarded. If a fish is born without the need for eyes, then it has an advantage that the ones born with eyes, in terms of growth. It's not growing body parts that it doesn't need.

In the caves, poor eyesight was not a disadvantage - whereas any fish/lizards able to use other senses better did have an advantage - so as they changed and evolved, eyes - being no longer necessary - were not selected for. Other things were, and those same lizards/fish have some highly evolved senses for surviving in a dark environment...

I think that random mutations which impaired vision were not selected against -- therefore they accumulated. So over time, the percentage of fish with working eyes would go down. I don't think there would be an active selection of blind fish over sighted fish -- just no selection, plus random mutation. It's just like any trait that doesn't affect survival one way or the other: red flowers survive just as well as yellow or white flowers, therefore we have a mixture, but white flowers are rare because you need multiple coincidences in the genes. If the principle must have a name, I'd call it the absence of selection pressure leading to randomization of a trait.

1) "During adaptation, some structures may lose their original function and become vestigial structures. Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes, the non-functional remains of eyes in blind cave-dwelling fish, wings in flightless birds, and the presence of hip bones in whales and snakes. Examples of vestigial structures in humans include wisdom teeth, the coccyx, and the vermiform appendix." Source and further information: http://en.wikipedia.org/wiki/Evolution 2) "Natural selection: Darwin cites moles as an example of mammals that have organs that have become vestigial and are being phased out by natural selection: The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind. One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse. (Charles Darwin, The Origin of Species) Research The blind forms of the Mexican tetra have proven popular subjects for scientists studying evolution: A recent study suggests that there are at least two distinct genetic lineages among the blind populations, arguing that these represent a case of convergent evolution. One theory is that because of its dark habitat, the fish embryo saves energy it would normally use to develop eyes to develop other body parts, and this developmental choice would eventually dominate the population. This is called economical adaptation. However, studies have shown that blind cave fish embryos begin to grow eyes during development but then something actively stops this process and flesh grows over the partially grown eyes. Another theory is that some Mexican tetra randomly don't develop eyes (which is represented by broken genes in the fish's genome), and this lack of eyes spreads to the rest of the population despite having no advantage or disadvantage. This is called the Unified neutral theory of biodiversity. In one experiment studying eye development, University of Maryland scientists transplanted lenses from the eyes of sighted surface-form embryos into blind cave-form embryos, and vice versa. In the cave form, lens development begins within the first 24 hours of embryonic development, but quickly aborts, the lens cells dying; most of the rest of the eye structures never develop. Researchers found that the lens seemed to control the development of the rest of the eye, as the surface-form tetras which received cave-form lenses failed to develop eyes, while cave-form tetras which received surface-form lenses grew eyes with pupils, corneas, and irises. (It is not clear whether they possessed sight, however.) The evolution of trichromatic color vision in primates occurred as the ancestors of modern monkeys, apes, and humans switched to diurnal (daytime) activity and began consuming fruits and leaves from flowering plants. (see-Evolution of color vision, Evolution of color vision in primates)" Source and further information: http://en.wikipedia.org/wiki/Blind_animals