Dick Dawkins is well known in some circles for his story of the evolution of sight, or 'the eye'. It starts with a light sensitive spot on a worm, and goes from there.
His story is what I call a Victorian gross morphology fantasy. It considers only the macroscopic anatomy, has some vague inferences about underlying bio-chemical processes, and that's it.
Now let's think about the human eye and its orchestration of quasi independent sub-systems that all had to come together to give us what we have today.
Let's start with the eye itself.
Its shape, various membranes, muscles, nerves, blood vessels, chemical supplies (to maintain light sensitivity) and differentiated rods and cones all had to come together to work together. Any single element by itself would have been eliminated as biologically useless.
The eye-brain system would have to be functional. Nervous connections to the appropraite part of the brain would have to form in step with the formation of the eye as an anatomical component. The 'software' would have to keep step with the 'hardware'. All the eye does is create an image optically on the retina. This then gets decomposed into electric impulses that go to a part of the brain that then assembles these impulses into something that is mentally meaningful for the organism (us). Each step requires substantial 'evolution': optical accuracy and control, translation to electrical impulses, transmission to the brain, some form of re-coding, formation of the mental impression of an image, and then this impression's meaningfulness to us! The orchestrated systems then keep track of the images as they change, smoothing them into a stable continuous image of the scene around us. None of this complex processesing, no use for the eye. No eye.
Now this system has to coordinate information that comes from the eye muscles, that point the eye, and those that focus it. They also have to coordinate with the amount of light coming in as the pupil changes size due to movement of the iris.
This enables the eye-brain system to maintain its stability of orientation, focus and illumination as it assembles moving images.
The body also moves on its axes. The eye-brain-ocular feedback system needs to coordinate with the balance mechanism and its whole system: detection organs and a processing centre in the brain. This helps to maintain a stable image as we move, lean over, turn around...
While the eye as a sense organ is 'evolving' its controlling muscles, blood supply and ancilliary nervous network need to keep up with it.
The eye's housing and accessories need to 'evolve' in line with this: facial bone structure, eye socket, skin, and surrounding (non-occular) muscles and their control system have to keep in step. The external apparatus, and eye lubricant system needs to be there: tear ducts, detector and triggering mechanisms, eyebrows, eye lashes, the lids and blinking system have to happen.
This doesn't even touch on the complexity of the eye's internal anatomy and its fine visual control features.
Then there's hand-eye, foot-eye coordination where major systems of systems interact to enable us to write, play tennis, produce sculptures and do the washing.
I've not mentioned the integration of the eye's lubrication system with another meta-system: emotions. The connection between feelings of supreme joy or suffering, or pain (physical or emotional) and the production of tears!
Genetic information drives all this, and not a simple change in DNA 'letters' here and there, randomly. Because the genetic information system is multiplexed several 'letters' have to change in concord. And groups of multiplexed letter sets need to change in 'super-concord' to bring chemical, nervous, interpretive facility and musculo-skeletal anatomy to the party.
As Dawkins might assert: 'simple'!
