Color Blindness and How Easy Evolution Really Is
I love learning new things. I was especially happy when Neil Shubin’s “Your Inner Fish” was made into a three part series on PBS. (View here for a short time.)
I loved the book (even did a chapter-by-chapter review that was a lot more fun that Darwin’s Doubt). But it was really nice to see things that weren’t in the book. New information. New evidence for evolution and our common ancestry with… well… everything.
One new thing I learned was particular interesting given my interest in the creationist movements. One often hears about how difficult it is for complex things to evolve. Take vision for example. The eye is really complex and all the parts have to work together or there’s no vision. Well, that’s true… now. But it hasn’t been true throughout the history of life. The eye has evolved to be more complex as those animals with really good vision were simply better at surviving.
But a scientist named Jay Neitz (featured in the third episode of Your Inner Fish) and several other scientists reported on just how easy it is to change the vision systems in the eye. The paper was in 2009 in Nature.  PDF
First we need to talk about how color vision works. Humans have three types of cone cells in the eye. These are designated S, M, and L by the types of EM radiation that those cones respond to. Remember the phrase ROY G. BIV? That’s the visible light spectrum (a rainbow). Red is the longest wavelength, while violet is the shortest wavelength. S-cones is our eye respond best to short waevlengths, the violet and blues, and very weakly to the other colors. The M-cones respond strongly to yellows and greens, but weakly to blues and reds. While the L-cones respond strongly to reds and oranges and weakly to blues. The combination of how these cones react give us a full spectrum of color vision.
Well, most of us.
Some people have a mutation in one of the genes (there are several) that controls the light sensitive pigments in our eyes, called opsins. The genes for these opsins are on the X chromosome. This is why there are more males with color-blindness than females. Females have two X chromosomes, so they have a ‘backup’ copy of the gene if one is damaged. This also explains the concept of tetrachromacy, which MAY mean that some women have four different cones instead of the usual three. Perhaps, they can actually differentiate more colors than us plain old trichromacy people.
What I learned from Your Inner Fish was that the blue/green opsin in the L/M-cones seems to be a relatively recent innovation in evolution. They are really close together on the X_chromosome and they are really very similar. It’s almost as though the gene segemnt was duplicated, then one gene mutated and now responds to a different part of the spectrum than the other. That’s one hypothesis.
Dr. Neitz was wondering if it’s really that simple. Could such a single change, a few mutations in a gene, really allow an animal to see more colors? Or would the brain simply not be prepared for it.
In squirrel monkeys (Saimiri sciureus), some females have trichromatic vision, but all males are red-green color blind. The authors trained some male monkeys (I hate that term) to identify the part of a touch screen that had different colors. If they got the selection correct, they got a treat.
These tests are exactly like the color-blindness tests that you get at the optometrist’s office. Except the monkey’s got a reward for getting it right.
After testing for more than a year, two males were injected with a virus that had been modified for a gene therapy role. The recombinant virus contain a human L-opsin gene with an L/M opsin enhancer and promoter. And yes, the three injections of viral particles was just underneath the retina. Shudder.
If the opsins were all that were needed to see three colors, then the monkeys would be able to see colors that they hadn’t seen before. But if some rewiring of the brain was required, then the monkeys still wouldn’t see red/green even with the gene producing the L-opsin.
A brief aside here. This is also a test of creationist claims. If things like color vision are as complex as creationists think, then just a simple change like this shouldn’t change anything in the monkeys. But if we do see a change in the monkeys ability to see colors, then the creationist claim is wrong. Simple changes to genes can produce very dramatic changes.
Twenty weeks after the injections, the two males were no longer color blind. They could register all color differences. In fact, if I’m reading this paper correctly, it appears that the human L-opsin is superior to the squirrel monkey L-opsin because the males could actually see colors more effectively than the females.
So, we see that the monkey brain can handle the additional inputs in an entirely different color spectrum. And it can readily use that additional information in a direct (profitable) way.
And all that is required is a gene. This supports the claim that gene duplication followed by mutation is a method by which new abilities can be generated in populations of organisms.
The other exciting thing is that this procedure may provide a way to restore full color vision to humans with certain types of color-blindness.
 Mancuso, K. et al. Gene therapy for red-green colour blindness in adult primates. Nature 461, 784–7 (2009).
 There is also an opsin gene on the human #7 chromosome. That’s the S-type.