The story of how cavefish lost their eyes runs against the common tale of evolution, in which adaptive traits are gradually acquired until they come to define the species.

"It's a very cool story in terms of the speed of evolution," says Nicolas Rohner, one of the paper's authors.

In the case of the cavefish, they evolved from psychological distress created by an unexpected change in habitat, laboratoryequipment.com reported. Thousands of years ago, a group of Astyanax mexicanus, as they are less commonly known, were carried against their will from the rivers where they used to swim into underwater caves. They didn't have time to slowly adjust to nearly pitch black waters. Distress enabled them to channel favorable genetic mutations like loss of pigment, a stronger sensory system, and no eyes.

Cavefish underwent a process called "standing genetic variation," according to study researcher Suan Lindquist, who has extensive experience in the subject. During less stressful times, certain species' pool of genetic mutations are kept in check by a protein called HPS90. Dramatic circumstances, however, can deplete the protein and leave the organism open to all of its genetic possibilities. In the case of the cave fish, as Lingquist and fellow researchers proved in this study, stress triggered the right genetic expression -- or at least triggered the right genetic expression in enough fish, Laboratory Equipment reported. 

"The delicate balance of protein folding - especially that controlled by HSP90 - holds the key," said  Susan Lindquist, a professor of biology at MIT and an investigator at the Howard Hughes Medical Institute. "Moderate changes in the environment create stresses on protein folding, causing minor changes in the genome to have much larger effects. Because HSP90 governs the folding of the key regulators of growth and development it produces a fulcrum point for evolutionary change."

Lindquist had demonstrated the effects of HSP90 in other creatures like fruit flies, but never before in cavefish, according to Laboratory Equipment. Her study used fish of the same species living near the surface, exposed them to a drug that blocked HSP90, and triggered latent genetic mutations relating to eye-size (though not to the same degree as the cavefish). To prove the influence of HSP90 in a more natural environment, they raised the river fish in waters of a similar quality to the cave water (in terms of salinity), and found similiar results. 

"This is the first time that we can see in a natural setting where the stress came from and observe the variation that results," said Clifford Tabin, genetics professor at Harvard Medical School whose lab was used in the course of the study.

 "This is the first study showing that this HSP90-mediated mechanism can be applied to vertebrates for real morphological adaptive traits," said Robner.