Friday, December 13, 2013
How the Blind Cave Fish Lost Its Eyes
by Joseph Bennington-Castro
Pictured above is a very peculiar fish that lives in a few North American caves. It looks very much like other fish, except in one respect: it has no eyes. The story of its adaptation to life in pitch blackness is one of the weirdest stories in evolutionary history.
Mutations vs. Cryptic Variation:
When people think about how evolution occurs, the classical modelgenerally comes to mind. According to this view, species experience random genetic mutations that confer novel traits when they move to a new environment. The most beneficial traits — those that help individuals better adapt to their new habitat — get passed along to subsequent generations and eventually spread throughout the population.
It’s a relatively simple, easy-to-digest model, but it’s not able to explain all cases of evolution. “Imagine if you had a quick change in the environment,” said Nicolas Rohner, a geneticist at Harvard Medical School in Boston, Massachusetts. “This evolutionary process would take too long.” To rapidly adapt to a sudden shift in environment, a population would have to have some kind of standing genetic variation already available, which nature then selects for.
But how can this “cryptic variation” be maintained and accumulate in a species without actually affecting individuals before the environmental shift occurs? Scientists proposed that something must keep the genetic variation silent under normal conditions; then, when a species relocates to a new, life-threatening environment, it becomes physiologically stressed and that silencing mechanism stops or breaks down somehow.The Mexican tetra, Astyanax mexicanus, is a freshwater fish found mainly in the rivers throughout central and eastern Mexico, as well as in a few rivers in the United States. At some point in the fish's distant past, populations of A. mexicanus got trapped in pitch-black, underwater caves. In their new — and vastly different — habitats, the fish lost their pigmentation and their eyes, and became better able to store energy and detect changes in water pressure (to find prey and each other).