Dawkins demonstrates how the eye could have evolved in stages, each simple and a result of random variation, but connected by a natural selection of those organisms which become more successful. In the early stages even with an incomplete eye, the benefit of even partial vision discriminating only between light and dark would be more useful than no vision.The structure of the surface of the "new" (in 1987) optical storage disc is related to the structure of the eye of the moth. A computer program produces generations of biomorphs using incremental random variation.
See http://www.rennard.org/alife/english/biomintrgb.html for an applet Engineers apply the principles of evolutionary variation, for example in a windmill that uses stationary vanes of suitable design to concentration even gentle wind onto a smaller central rotor that turns more powerfully as a result.