Just as ants and their subterranean fellow-travelers lose their wings underground, so numerous different kinds of animals that live in the depths of dark caves where there is no light have reduced or lost their eyes, and are, as Darwin himself noted, more or less completely blind. The word ‘troglobite’ has been coined for an animal that lives only in the darkest part of caves and is so specialized that it can live nowhere else. Troglobites include salamanders, fish, shrimps, crayfish, millipedes, spiders, crickets and many other animals. They are very often white, having lost all pigment, and blind. They usually, however, retain vestiges of eyes, and that is the point of mentioning them here. Vestigial eyes are evidence of evolution. Given that a cave salamander lives in perpetual darkness so has no use for eyes, why would a divine creator nevertheless furnish it with dummy eyes, clearly related to eyes but non-functional?

Evolutionists, on their side, need to come up with an explanation for the loss of eyes where they are no longer needed. Why not, it might be said, simply hang on to your eyes, even if you never use them? Might they not come in handy at some point in the future? Why ‘bother’ to get rid of them? Notice, by the way, how hard it is to resist the language of intention, purpose and personification. Strictly speaking, I should not have used the word `bother’; should I? I should have said something like, `How does losing its eyes benefit an individual cave salamander so that it is more likely to survive and reproduce than a rival salamander that keeps a perfect pair of eyes, even though it never uses them?’

Well, eyes are almost certainly not cost-free. Setting aside the arguably modest economic costs of making an eye, a moist eye socket, which has to be open to the world to accommodate the swiveling eyeball with its transparent surface, might be vulnerable to infection.

So a cave salamander that sealed up its eyes behind tough body skin might survive better than a rival individual that kept its eyes.

But there is another way to answer the question and, instructively, it doesn’t invoke the language of advantage at all, let alone purpose or personification. When we are talking about natural selection, we think in terms of rare beneficial mutations turning up and being positively favoured by selection. But most mutations are disadvantageous, if only because they are random and there are many more ways of getting worse than there are ways of getting better. Natural selection promptly penalizes the bad mutations. Individuals possessing them are more likely to die and less likely to reproduce, and this automatically removes the mutations from the gene pool. Every animal and plant genome is subject to a constant bombardment of deleterious mutations: a hailstorm of attrition. It is a bit like the moon’s surface, which becomes increasingly pitted with craters due to the steady bombardment of meteorites. With rare exceptions, every time a gene concerned with an eye, for example, is hit by a marauding mutation, the eye becomes a little less functional, a little less capable of seeing, a little less worthy of the name of eye. In an animal that lives in the light and uses the sense of sight, such deleterious mutations (the majority) are swiftly removed from the gene pool by natural selection.

But in total darkness the deleterious mutations that bombard the genes for making eyes are not penalized. Vision is impossible anyway. The eye of a cave salamander is like the moon, pitted with mutations: craters that are never removed. The eye of a daylight-dwelling salamander is like the Earth, hit by mutations at the same rate as cave-dwellers’ eyes, but with each deleterious mutation (crater) being cleaned off by natural selection (erosion). Of course, the story of the cave-dweller’s eye isn’t only a negative one: positive selection comes in too, to favor the growth of protective skin over the vulnerable sockets of the optically deteriorating eyes.

Excerpted from ‘The Greatest Show on Earth’ by Richard Dawkins