Evolution of the eye
The initial stage has a transparent cell layer, a light-sensitive cell layer, and a dark pigmented bottom cell layer.
Each new generation is formed from the optically superior eyes in the previous generation; changes that made the optics worse were rejected, as selection would reject them in nature.
After about 400 steps the photoreceptor layer and pigment layer have buckled in to form a retinal pit.
This continues to deepen until about 1000 steps, when the structure approximately corresponds to a pin-hole camera eye.
The optical properties of the eye are further improved by the evolution of a lens. This type of eye is common among aquatic animals.
The lens shape then changes, and the iris flattens, to improve the focussing properties.
The complete evolution of an eye like that of a vertebrate or octopus takes about 2000 steps.