FINDING FOOD IN THE DARK
12 01 2007The current issue of Seed has a very interesting discussion on the evolution of eyeless cave fish and the idea of “use it or lose it�. The evolution of flightless birds and eyeless fish was a problem that confused even Charles Darwin. He believed that disuse would lead to degeneration of organs over time.
The articles author, PZ Myers, says that one possible explanation for why animals living in total darkness should lose their eyes may be an economical adaptation. It takes a lot of energy to form something as intricate and delicate and an eye. Energy that was being spent in this pathway during embryo development could be diverted to other growing organs.
Now lets through a wrench into that explanation. Mexican blind cavefish embryos make eyes. They form an eye cup and beginnings of neural circuitry, but then they stop. That’s a lot of work and energy spent for nothing. Another explanation is random chance, like there is a mutation that knocked out the genes needed for making eyes.That theory doesn’t fly either because the cavefish have all the genes required for making an eye; something is just switching off lens formation.
W.R. Jeffrey and his colleagues suggest the evolution of eyeless cavefish is based on pleiotropy. Pleiotropy means that a single gene may play several roles or have different effects on an organism. A master gene called pax6 controls eye development, but a signaling molecule called hedgehog plays an important role in setting up the midline of the animal. Hedgehog is also expressed in teeth, taste buds and jaw. So by diverting the hedgehog molecule from eye formation, more gets sent to the mouth region making this area more sensitive and easier to detect food in the dark.
PZ Myers plans on discussing the importance of developmental biology in explaining evolutionary phenomena in a regular column.
Fascinating stuff, as always.
I am continually amazed at the fact that you use your brain rather than just letting it atrophy, the way mine is doing. Thank goodness that Use It Or Lose It thing might not be airtight. I don’t want to lose my brain.
But is it possible that the mexican cavefish are still in the process of devolving from eyes? That eyes became useless, and just as it probably took a million years to develop eyes, it will take a million years for the fish to phase them out? Or is that exactly what you said and I just got confused?
Why is the phenomenon called Hedgehog? I think you’re making it all up!
Tiffany you’re cute. the phenomenon is not called hedgehog. The signaling molecule that helps establish a midline is called hedgehog. I don’t know why, but if you discover a gene or molecule you get to name it whatever you want. The stuff required to finish making eyes just got diverted to making other areas more useful. The fish can’t see any way, so why waste the time and energy. Now as time goes on, who knows? You could be right. We might see more and more cavefish in the population without even start-up eyes. But I think that the genes won’t go to waste; they’ll be converted into something more useful.