Many creatures that live in caves are blind. Yet their ancestors were sighted. This is a great example of evolution.
I did my master’s research on the cave beetle, Ptomaphagus hirtus. This beetle’s ancestors scavenged for food underneath the leaf litter on forest floors. Since it’s dark under the leaf litter, eyes were of little use even to these immediate ancestors. But much earlier ancestors had eyes and used them.
Though the circumstances differed for the ancestors of other cave animals, like fish or crayfish, the natural selection process was similar. In the cave, except at the cave mouth, there is absolutely no light. So eyes are of no use at all. It takes energy to build eyes, and for living organisms, energy is like money. The less energy an organism has to expend, the better off that organism is in terms of evolution.
Here’s why: Let’s take my Ptomaphagus beetle. Like all living things, the Ptomaphagus beetle has to expend energy to mate and lay eggs. The less energy the beetle expends building and maintaining useless eyes, the more energy it has for building and laying high quality eggs, which lead to successful new beetles.
“Survival of the fittest” means the most fit beetles will pass on the copies of their genes to more or larger new generations of offspring. So in a cave, the most fit beetles will be those that lose genes necessary for building and maintaining eyes, for this loss will save energy that otherwise would be wasted. The very fit blind beetles will pass on all their genes to their offspring, including the mutated eye genes that no longer code for eyes. Eventually, all the surviving Ptomaphagus beetles will be blind.
Of course all these blind creatures still must escape predators and must find food, so some of the energy they save in not having vision, must be spent on other senses. I’ll have more to say about this in my next post. Stay tuned.