Nature can be thought of as a blind knob-twiddler, an unconscious sculptor that chisels the gene pool, without any goal or purpose in mind. The process of natural selection acts, a posteriori, on preexisitng traits that have a heritable genetic component. A random variation of a heritable basis means that beneficial traits will emerge, which ultimately leads to such a process of non-random selection of randomly varying genetic equipment. The effects of selection, thus, produce a sort of illusion of design and false teleological implications as a consequence.
Not all variation is adaptive variation. Variation is a function of differential environmental heterogeneity. Variation itself is, also, non-directed and indeed, not all variation is adaptive. The environment only mediates a certain selection mechanism that tends in an adaptive fashion. It is the environment that provides the only source of directedness onto which evolutionary change proceeds.
Arguments against evolution tend to always single out a particular organ or biological system and use complexity to make the case for deliberate design by some sort of an intelligent agency. Aside from the fact that such an argument presupposes the existence of an even more remarkably complex entity and therefore creates more questions than it answers, it is flawed for many other reasons. It is an evasion of the responsibility to explain such complexity by thrusting forwards a mere presupposition that cannot be tested and does not have any observable consequences. And, secondly, it presupposes that complexity must mean design, shortsightedly basing such a presupposition on analogies of design by humans.
Intelligent design advocates always like to misinterpret Charles Darwin, when he wrote in The Origin of Species, that “To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.” Little, however, do they always go on with the rest of the quote when Darwin, thereafter, remarks: “Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory”.
Natural selection is more of a tinkerer than a designer (Monod, 1971). It works with what is available and while it does not lead to optimum results, it still gets the job done. The claim that an organ like the eye is perfectly designed is very flawed. The eye is, in fact, very poorly designed. Photosensitive cells point inwards away from the source of light such that light has to pass through the nerves and blood vessels and while it does, it degrades in the process. Further, an opening in the retina exists (called the blind spot) where nerve fibres pass through to the brain and where no rods and cones exist and therefore no image is focused. An engineer designing a television where a hole has to be constructed in the middle such that the wires pass through is a very bad engineer. In the vertebrate eye, the wires lie all over the surface of the retina and the photosensitive cells. And, the photocells are very loosely bound to the retina such that they can be detached easily after a trauma.
By contrast, other animals have very well-constructed eyes. The giant squid, for instance, has an eye where the photoreceptors point towards the light and where the nerves lie behind the retina and thus, where the optic nerve does not have to get in the way of the retina. Indeed, cephalopod eyes originate as a skin derivative rather than derive from the brain as they do in vertebrates. The same is true for pelagic fish. Such adaptations allow cephalopods and pelagic fish to target their prey in darkness. In addition, hawks can see much further than humans because they have a much greater density of light-sensitive cells. And, molluscs have mirrors instead of lenses.
Insects and crustaceans have compound eyes which are much more efficient at capturing light than vertebrate eyes. Compound eyes have hundreds or thousands of mini eyetubes (called ommatidia) radiating out from a central point over a convex surface, each receiving light at a different angle and focusing it onto a group of 9 photoreceptors and each seeing a different patch of the world around them. The number of tubes can vary from 1 tube/eye in fleas to as much as 30,000 tubes/eye in dragonflies. In compound eyes, the image that is constructed is not an inverted image.
One may be tempted to ask why has selection not led towards a better design for the vertebrate eye. The answer may lie in the notion that evolution proceeds in a successive stepwise fashion, wherein the steps are randomly produced but nonrandomly selected. An ancestor to vertebrates that would have had photosensitive cells and nerves (albeit pointing in the wrong direction) would have been still better than not possessing a non-functioning photosensitive cell altogether, at which point natural selection would have gradually start working onwards blindly and gradually. Whatever successive refinements that would have ensued would have still been based off the flawed start, which was still better than nothing nonetheless.
If one starts with photosensitive cells and each step increases the quality of the projected image and confers differential reproductive success, then a step in the opposite direction would be unfavourable and would thus be selected against. The evolution of the eye goes back hundreds of millions of years ago in unicellular protoctists possessing a simple eyespot (a light-sensitive body and photo-pigments to guide the organism in a directed fashion). As organisms became more complex, the eyespots also became more complex which gave rise to photoreceptors. Accordingly, as time went on, visual systems gradually became more and more adapted. For instance, curvature detects light signals more effectively and would have therefore conferred better directional selectivity for light. Such a trait would have therefore evolved, followed by the aperture for shape sensing and the lens for image formation and so on and so forth. All photoreceptor systems of all animals are related by common ancestry.
If an organ like the eye was truly “irreducibly complex” as intelligent design advocates often argue, it should not follow that flies should be able to develop compound eyes by expressing vertebrate genes. DNA sequencing of eye-forming genes in mice, humans, and Drosophila, indeed, shows that they are immensely homologous. These homologous genes even exist in invertebrates like sea urchins and nematodes that don’t even have eyes! The genetic equipment for eye formation developed in invertebrates long before vertebrate eyes developed. The Pax6 protein, for instance, is conserved throughout both vertebrate and invertebrate phyla. In fact, expressing the mouse version of Pax6 in fruit flies leads to the development of complex compound eyes in the flies which the mice do not possess. If those mice genes were designed by some designer specifically for mice, then how would it be possible that fruit flies would use them to develop compound eyes? This goes in direct contradiction with the claim that the eye is the result of irreducibly complex and independently formed parts. The parts are connected by signalling mechanisms that are intricately interconnected. But, it isn’t that they fit together like a jigsaw puzzle in order to function. Retinas can still function without lenses.
Jacques Monod, Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology (New York: Vintage, 1971).