Hi Dr. Oakley,
I am writing a research paper and came across your paper entitled, Opening the “Black Box”: The Genetic and Biochemical Basis of Eye Evolution. I was hoping you could give me your perspective on a question that is part of my research interest.
Since a simpler mechanisms for phototransduction would theoretically work, why would evolution favor a more complicated phototransduction cascade with intermediates such as transducin and PDE? I would greatly appreciate any insight you could provide me.
The implication in the e-mail is that evolution is a force that produces sleek perfection. Expensive solutions to problems should not arise by evolution (or at least they should not be maintained), especially if the complexity is unnecessary. This is a modernist view of biology, a view that can be found in 20th Century biological research, and a view that is also common today among students, and the general public outside the field of evolutionary biology. It is a view that results from an often unstated assumption natural selection is a supremely powerful force that leads to perfection.
From this Modernist, Bauhaus perspective, it is indeed perplexing to learn that opsin initiates a complex, baroque, Rube Golddberg-like cascade to turn light energy into a nervous impulse. This cascade includes reactions from opsin->transducin->PDE->CNG; each protein signaling in one way or another to another protein down the line - and this description is even VERY simplified compared to the actual complexity!
So the question is, why would evolution "favor this complicated phototransduction cascade", when all that seems to matter is that opsin signal directly to the CNG ion channel protein to cause the nervous impulse.
The most direct answer is that evolution is not an Intelligent Designer, rather it is a bricoleur, a tinkerer. Evolution acts upon what is available, and things that are useful are kept. In the case of the phototransduction cascade, evolution co-opted existing components: an existing GPCR cascade gained light sensitivity. We know this because the components of phototransduction pre-date opsin (e.g. here). Phototransduction was not invented from scratch, in the most efficient way possible. Instead, it was cobbled together using available parts.
This can be conceived as an example of a phylogenetic or historical constraint. In other words, history matters. All living things and all components of living things share a common history. Because of this, and because of the interdependence of components of living things, it is usually not easy to completely re-invent something. The number of shared genes in all animals (for example) clearly illustrates that history matters. Components are used and re-used, not invented anew.
This answers the proximate question, of why phototransduction is so complex. But doesn't address the question of why all GPCR cascades are so complex. I don't know the answer to this, but perhaps the complexity allows for flexibility. In fact, GPCR cascades are supremely flexible, and underlie signaling from outside to inside cells for many processes in animals, including vision and other senses, hormone signaling, metabolism, development, reproduction, etc, etc.
Interestingly, this question showed me yet another new perspective on the flawed argument for Intelligent Design. ID proponents suggest that when we see something outlandishly complex, then it must have been designed by an intelligent agent. However, as this question points out, extravagant complexity is not a sign of intelligence. Why use 50 components when 2 will suffice? Elegant simplicity is far more intelligent than excessive complexity. Again, evolutionary biology provides a logical and plausible explanation for the biological processes that we are coming to understand.