In the classic view of evolution, organisms undergo random genetic mutations, and nature selects for the most beneficial ones. A recent study in Science adds a twist to that theory: variability already present in a population's genome may remain hidden in times of plenty but come unmasked in stressful situations, ready to help with adaptation.

At the theory's core is a protein called HSP90. It binds to other proteins to keep them properly folded. Work over the past few decades by Susan Lindquist, a professor of biology at the Massachusetts Institute of Technology and a co-author of the Science paper, has established that when HSP90 is distracted from that task, as might occur in a stressful new environment, traits that were once uniform suddenly show lots of variation.

The thought is that when HSP90 holds proteins in a certain shape, it compensates for minute variations that have crept into them over generations. When stress diverts HSP90, the proteins' alternative forms are released, triggering the wider variety in traits. Natural selection can then act on those new traits, spurring adaptation.

The study's lead author, Harvard Medical School's Nicolas Rohner, tested the idea on the Mexican tetra, a river-dwelling fish. In the distant past, populations of Mexican tetra ended up in underwater caves, a new environment to which the fish adapted by losing their eyesight.

Rohner and his colleagues raised surface fish in water treated with an HSP90 blocker. Those fish, they found, had greater variations in eye and eye-socket sizes. Stressing surface fish with water chemically similar to cave water also yielded offspring with a greater than normal variety of eye sizes.

Although the findings do not prove that HSP90-masked variation helped the fish change their eyes, they lend the idea plausibility. Exactly how stress on HSP90 induces variation is still mysterious, but it is a topic of active research.