Is the backward positioning of photoreceptors bad design?

Question

(Several questions have been asked about this topic but most are quite old and there has been at least one study since then that has attempted to answer this in a new way)

Since photoreceptors are placed at the back of nerve cells in vertebrates, these nerves have to then bundle up before passing to the brain, leading to a blind spot. This video (from a usually reliable channel) from 2:30-4:00 says that its bad design and is just something the initial eyes in vertebrates fixed on, and was too difficult to correct later.

But this report of a study from 2015 says that this arrangement helps vertebrates see colours more easily during daytime, and is not a bad design.

Even if this design does help in seeing colours better, the same effect could have been achieved through other means without having to create a blind spot, no?

Can this be called a bad design or not?

Answer 1

Properly speaking, you should not use the word "design" when talking about evolution. All you can describe are more or less fit variations. A variation that is advantageous in one context can be detrimental in another.

That being said, I assume what you are asking is whether the eyes could be better, as if an engineer would design them from scratch. The answer is "probably yes". The backward wiring of the photoreceptors is an issue that the brain has mostly been able to work around, but it is obvious that forward-facing photoreceptors would be easier to work with. There are multiple issues, one is the blindspot as you identified. Another one is the diffraction of the light by the ganglion cells fibers. Part the light is adsorbed too, so because of the non-homogeneous thickness of the macula there are color distortions near the center of the visual field. But again, it is not impossible that having a forward-facing retina would be more detrimental in some environment, although I fail to think of a case where a backward retina would be better.

As for the sources you cite, I watched only part of the video but it was very spot on. The science daily article however is very misleading. Frankly, the quoted scientist doesn't seem to have a very good grasp of evolution. Though the actual paper is more nuanced. Basically it is possible, as suggested by the authors, that some mechanism mitigates the scattering of light by the retina. That does not mean that the retina is backward because of that. Claiming as one of the authors did that they have "explained why the retina is built backwards" is very, very, bad logic. With forward-facing photoreceptors you wouldn't even have to have such a mechanism. And as pointed out by the video you linked, cephalopod's retinae are not backward.