The evolution of the eye has fascinated evolutionist as far back as Darwin himself. Regarding the evolution of the eye he wrote in the first edition of his On the Origen of Species (1859),Organs of extreme perfection and complication.
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 possible degree.
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an 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, can hardly be considered real.
In the sixth edition (1872) of the same work he wrote,Organs of extreme perfection and complication.
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. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. 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.
In both of these editions, Darwin proceeds to lay out his understanding of the mechanism by which the evolution of the eye took place. The mechanism is given in more detail in the sixth edition than it is in the first edition.
150 years of study of the evolution of the eye have proved that Darwin was correct, we do see in nature animals with everything from the simple ability to perceive light to animals with fully formed eyes and the ability to very accurately perceive colors of light and to very finely focus that light. Furthermore, the mechanism by which the evolution of the eye could have taken place is now much better understood.
We very seldom find in the fossil record fossilized soft tissues such as eyes because they decay much more rapidly than fossilization can normally take place. Instead, we typically find fossilized bones and teeth. Therefore, we cannot expect to find in the fossil recorded a full spectrum of the evolution of the eye. We do find, however, this full spectrum in living animals. Indeed, we have today animals with everything from the simple ability to perceive light to animals with fully formed eyes and the ability to very accurately perceive colors of light and to very finely focus that light. Even having the simple ability to perceive light is a huge advantage over no sight at all, and all that is necessary for an animal to perceive light is cells in their body that are sensitive to light. With every improvement in vision come further advantages.
According to the theory of evolution, evolution is taking place throughout the plant and animal kingdoms, but much faster in some plants and animal than others. For example, in Horsetail plant (Equisetum telmateia), still looks very much like it did 100 million years ago. The same is true of the the American cockroach Periplaneta americana (even though its resistence to the poisons that we use to control it is evolving rapidly). On the other hand, some of the species of plants in the genus Clarkia are rapidly evolving, as is the Maliaria Mosquito (Anopheles gambiae). The causes of these different rates of evolution are complex, but they are mostly due the need, or the absence of the need, to adapt to environmental changes.
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