The current issue of Seed has a very interesting discussion on the evolution of eyeless cave fish and the idea of âuse it or lose itâ?. 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. Thatâs 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 doesnât 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.