Blind, pale, and condemned to scraping a living deep underground, cave creatures have a repulsiveness akin to Tolkien's Gollum. Yet, they were among Darwin's favorites. And although his views on their evolution have fallen out of favor, new research suggests he wasn't entirely wrong.
Darwin imagined that bizarre cavern-dwelling insects and fish would evolve when surface dwellers strayed into a cave. Isolated from their brethren aboveground for many generations, "disuse will ... have more or less perfectly obliterated [their] eyes, and natural selection will often have effected other changes," Darwin wrote.
A more recent theory on cave evolution--called adaptive shift--doesn't require isolation. It holds that cave dwellers can adapt to the dark while still interbreeding with their aboveground kin. The classic example is the fish Astyanax fasciatus, which lives in Mexican rivers. Wherever a river flows into and out of a cave, eyed fish rub shoulders with their blind relatives, and in the laboratory the two types can be crossbred.
Now Ulrike Strecker of the University of Hamburg, Germany, and colleagues have shown that Darwin was right about isolation, at least for Astyanax. They netted 116 fishes from underground lakes and rivers in the Sierra de El Abra in Mexico. They also caught 62 fishes in streams just outside the caves. DNA profiling clearly distinguished the cavefish from the others, even when they were living close together. This means that the cavefish cannot have descended from the eyed fish nearby but must have evolved inside the caves a long time ago. Also, one of the four cave populations was a genetic oddball, meaning that it must have evolved independently from the rest. So Astyanax fasciatus may have been a red herring.
The new study, which appears in this month's issue of Molecular Ecology, receives acclaim from specialists in cave evolution. Stewart Peck of Carleton University in Ottawa, Canada, who was sympathetic toward the adaptive-shift model, calls it "a marvelous advancement." But he adds that there is still much work to do before we will know when Darwin's theory holds and when the adaptive-shift one does.