Irreducible complexity is an argument that was first put forward by Michael Behe, a biochemist and proponent of intelligent design. Behe himself defines an irreducibly complex system as acollection of well-matched and interacting parts that on the whole, contribute to the functioning of the system, but the removal of any one part could cause the system to cease functioning. Most proponents of intelligent design and creationism like to argue in favor of irreducible complexity by using the eye as an example. Even on the surface, the argument appears ridiculous and ignorant. Consider an individual suffering from a cataract. The crystalline lens of the person is affected to varying degrees (from slight to complete opacity). Equally, the vision too is affected to varying degrees. A slight cataract may prevent the individual from seeing as clearly as an individual with no cataract, but he or she can still see.
So what does evolution have to say? Well, having an eye is not a jackpot. It obviously isn't a case of black and white. There are shades of gray. Most evolutionary biologists argue that having something is better than having nothing. Having 10% of a complete eye is better than having no eye. or having 51% of an eye is better than having 49% of an eye. So how could the eye have evolved? Obviously, the earliest ancestors had no eyes (in the way that we know what an eye is), but they may have had a single layer of photo-sensitive cells (see Fig. 1a). Now the only thing such a layer of cells would have been able to do is to detect light and dark. Not too much, but could've still assisted the animal when it had to detect if there were any predators around in the neighborhood. Euglena, for example, has flat patches of photoreceptors.
Now the next stage in evolution would have led to an indentation being formed in the photoreceptor layer, which gradually deepened to give rise to a cup-shaped structure (see Fig. 1b). This was better. Now this animal could not only distinguish light from dark, but also the direction from which the light was coming, since there is a shadow formed. Cup- shaped light sensitive spots are seen in Planaria. As evolution proceeded, the cup got deeper and deeper and with even the slightest increase in depth, the animal was able to make out, with ever increasing accuracy, the direction of light. Later, the cup may have started to close at the other end, ultimately forming a roughly spherical structure with a hole at one end (see Fig. 1c). This structure would have been equivalent to a pinhole camera. One animal that does have a pinhole camera for an eye is Nautilus, a sea mollusk. Now the resolution of a pinhole camera depends on the size of the aperture. Smaller the aperture, sharper the image. But if the aperture is small, then the amount of light passing in would also be less and hence the image would be dim. Hence, in a pinhole camera, one is always compromised over the other.
In any case, Nautilus had a pinhole camera where the aperture was open. Sea water could constantly flow in and out. So as the process continued, the aperture would have been covered with a thin layer of transparent material (which wouldn't have contributed much at first except for protecting the eye), which may have been the earliest form of the cornea (see Fig. 1d). The cornea, as we know it today, has a refractive power of about 43 diopters and does contribute to focussing light (along with the lens). Gradually as evolution went on, the cornea from being just a sheet of transparent protective material may have gained some refractive properties, allowing sharper images to be formed (even when the size of the aperture was large). Although the cornea contributes the most to the focussing power of the eye, it's focus is fixed. What "tunes" the focus in response to objects at different distances is the curvature of the lens. And thus, was born the lens (see Fig 1e)!
We see how evolution beautifully explains the gradual formation of a complex structure and puts to rest two arguments, one being that of irreducible complexity and the other being that of the Watchmaker.