The evolution of the eye has focused research interest ever since Darwin identified the eye with its ‘‘inimitable contrivances’’ as a vexing problem for evolutionary theory (1859). Gradual evolution seemed implausible because ‘‘intermediate’’ forms of the eye seemed unlikely to be adaptive and selectable. Since Darwin’s original challenge, however, a surprisingly large number of cases of independent evolution of image-forming eyes have been documented.
Translation: Contrary to evolutionary expectations, biology presents us with a wide variety of vision systems. They are too different to have evolved from a common ancestor. The evolutionary spin on this surprise is that vision must have independently evolved many times (after all, the fact that vision must have evolved, somehow, is not in question).
Furthermore, various living species with completely functional forms of eye organization are now known, which could be viewed as ‘‘intermediate’’ between a simple photoreceptive patch and the complex image-forming eye seen in cephalopods and most vertebrates.
On the other hand, they could not be viewed as intermediate. It all depends on whether we are following the evidence. In fact, the biochemistry of even simple, non image forming, eyes is profoundly complex.
Although the fact of repeated evolution of image-forming eyes, as well as the capacity for functional intermediates, is thus firmly established, the mechanism of the evolutionary process is still speculative.
Translation: We may have to contrive just-so stories to explain evolution, but we will continue repeating that it is a fact.