The classical view of evolution is that species experience spontaneous genetic mutations over time that produce traits that either benefit them, or don't. Now, though, it seems as if there may be some other mechanism that drives evolution--at least when it comes to blind cavefish.
At some point, many thousands of years ago, a population of Astyanax mexicanus was swept from its river home and into the unfriendly confines of underwater caves. Facing a dramatically different environment, the fish were forced to adapt as they lived in near total darkness. The fish lost their pigmentation and developed heightened sensory systems to detect changes in water pressure and the presence of prey. They also lost their eyes.
The eye loss in particular is considered to be something called "standing genetic variation." This argues that pools of genetic mutations, some potentially helpful, exist in a given population but are normally kept silent. The manifestation of these mutations doesn't emerge until the population encounters stressful conditions. But what keeps these mutations at bay in the meantime?
Past research has shown that the heat shock protein HSP90 silences genetic variation in a variety of organisms. In order to see whether HSP90 had been an evolutionary role-player in cavefish, the researchers devised a series of complex experiments. The scientists raised surface fish in the presence of a drug that blocks HSP90 activity, which mimicked a stressful environment. These fish showed a significant variation in eye size. Cavefish raised in the same conditions showed no increase in variation in the size of their eye orbits.
While these results were seen in the lab, though, the researchers wanted to be sure they were applicable to nature. That's why they measured the conditions in the cave water and surface waters that are home to these fish. In the end, they found considerable difference in conductivity, as measured by salinity, between the cave and the surface. Because low conductivity can trigger a heat shock response, the researchers raised surface fish in water whose conductivity equaled that of native caves.
So what happened? The results were essentially the same. The fish showed significant variation in eyes size, which hinted at the environmental mechanism for evolution.
"This is the first time that we can see a natural setting where the stress came from and observe the variation that results," said Clifford Tabin, one of the researchers, in a news release.