Stumbled upon this free reference for eye evolution that seems both concise and descriptive.
"Although the eyes that have resulted are radically different from each other, it turns out that the photoreceptors upon which they are based are remarkably similar to each other, and indeed are derived from a common ancestral type, so that one can now conclusively reject the claim of Salvini-Plawen & Mayr (17) that "Photoreceptors have originated independently in at least 40, but possibly up to 65 or more different phyletic lines". As was so aptly pointed out by Jacob (18), it is important to realize that evolution works by 'tinkering' with what is already available, and without any overall 'purpose' (such as to give rise to vision). In the case of the evolution of phototransduction, photoreceptors, and retina, numerous examples of such tinkering will become apparent in the Sections ahead; see also Goldsmith (19)."
Webvision: The Organization of the Retina and Visual System [Internet]
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