Scientists may have finally determined why animals that live in caves often become blind.

An evolutionary biologist from Arizona State University has proposed a novel theory, which rejects an original Charles Darwin theory that eyes could be lost by disuse over time.

“We think that blindness in cavefish is indeed Darwinian, but ultimately this disproves Darwin’s original hypothesis of ‘disuse’,” Reed Cartwright, an ASU evolutionary biologist in the School of Life Sciences and a researcher at the Biodesign Institute, said in a statement.

According to Cartwright, eyes are not lost by disuse but rather demonstrate Darwin’s fundamental theory of natural selection, with blindness selected as favorable trait for living in a cave.

Cartwright’s research team chose to model a well-studied blind cavefish—the Mexican tetra—a small, docile, pink-hued fish just a few centimeters long that could easily make its home in an aquarium.

The species has inhabited caves for two-to-three million years, giving it five million generations worth of time to evolve blindness. The Mexican tetra was selected because there is also a surface-dwelling form that has retained its sight.

The researchers used computational power to investigate how multiple evolutionary mechanisms interact to shape the fish that live in caves.

“The problem we have in these caves is that they are connected to the surface, and fish that can see immigrate into the cave and bring genes for sight with them,” Cartwright said. “Under these conditions, we don’t typically expect to find such a difference in traits between surface and cave populations. Unless selection was really, really, strong.”

In their model, the selection for blindness would need to be about 48 times stronger than the immigration rate for Mexican tetras to evolve blindness in caves and Cartwright’s group estimates that a measure of fitness for blindness—called the selection coefficient—in the tetra is between 0.5 percent and 50 percent.

In the lab experiments the coefficients were high enough that a difference between surface and cave forms of the fish should have but wasn’t detected.

Cartwright’s team then referred back to a 1925 hypothesis that stated that the reason you have blindness in caves is because the fish that can see simply leave the cave.

“If sighted fish swim towards the light, the only fish that stay in the cave are blind fish,” Cartwright said. “They aren’t trying to get to the light anymore because they can’t see it.

“Which actually is a form of selection, and thus, Darwinian evolution in action.”

Cartwright explained that a fitness difference as big as 10 percent between sighted and blind fish may be difficult.

However, if over time enough of the seeing eye fish are systematically being removed, they will also be removed from the gene pool and that could be enough to drive the evolutionary process.