Apparently not much:
In 1954, Syuichi Mori, a biologist at Kyoto University, put flies into a cave of their own. He took eggs from ordinary flies of the species Drosophila melanogaster put them in milk bottles, which he placed in pots and covered in dark cloth. There they lived in utter darkness. He tended to the flies, generation after generation, dividing them into three separate lines. Meanwhile, he reared three lines of flies in normal light for comparison.
Mori wondered what sort of changes would occur in his dark flies. Would they lose their internal clock, controlling the daily cycles of their bodies? Would they stop responding to light? Would they evolve in other, unexpected ways?
These flies did not:
…They still have normal eyes, for example, complete with pigments. Last year, Michio Imafuku and Takashi Haramura reported that the dark flies still had their body clock. If they exposed the flies to three and a half hours of light, the insects became active and sluggish in a 24-hour cycle.
For a Drosophila melanogaster, 57 is 1400 generations; for a human, 1400 generations is 30,000 years. In all of that time in the dark, the flies only had a small number of mutations. What could this mean? One, permanent darkness is not a dramatic enough change for this insect; two, mutations are too gradual to cause dramatic changes in the species. Huge transformations only happen by leaps, not by steps; and leaps can only occur under a great, environmentally induced stress on the species. If the stress is not overwhelming, you only have mutations (steps); and if you only have mutations, you can only have small or minor changes.
