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For whatever reason, evolution decided those wavelengths should be overlapping. For example, M cones are most sensitive to 535 nm light, while L cones are most
by georgecmu 1y ago
For whatever reason, evolution decided those wavelengths should be overlapping. For example, M cones are most sensitive to 535 nm light, while L cones are most sensitive to 560 nm light. But M cones are still stimulated quite a lot by 560 nm light—around 80% of maximum.
The reason is simple: genes coding the long wave opsins (light-sensitive proteins) in these cones have diverged from copies of the same original gene. The evolution of this is very interesting.
Mammals in general have only two types of cones: presumably they lost full color vision in the age of dinosaurs since they were primarily small nocturnal animals or lived in habitats with very limited light (subterranean, piles of leaves, etc.) Primates are the notable exception, and have evolved the third type of cone, enabling trichromatic color vision, as a result of their fruitarian specialization and co-evolution with the tropical fruit trees (same as birds, actually).
So, what's interesting is that New World and Old World primates evolved this cone independently. In Old World primates the third cone resulted from a gene duplication event on the X chromosome, giving rise to two distinct (but pretty similar) opsin genes, with sensitivity peaks at very close wavelengths. As a note, because these genes sit on the X chromosome, colorblindness (defects in one or both of these genes) is much more likely to happen in males.
New World primates have a single polymorphic opsin gene on the X chromosome, with different alleles coding for different sensitivities. So, only some (heterozygous) females in these species typically have full trichromatic vision, while males and the unlucky homozygous females remain dichromatic.
Decent wikipedia article on the subject: https://en.wikipedia.org/wiki/Evolution_of_color_vision_in_primates https://en.wikipedia.org/wiki/Evolution_of_color_vision_in_p...
Types of opsins in vertebrates:
https://en.wikipedia.org/wiki/Vertebrate_visual_opsin https://en.wikipedia.org/wiki/Vertebrate_visual_opsin
- im3w1l 1y ago> So, only some (heterozygous) females in these species typically have full trichromatic vision Wow that's wild how heterozygousity can be that helpful. Makes you wonder if there are other genes like that.
- rozab 1y agoSome human females have functional tetrochromatic vision. https://jov.arvojournals.org/article.aspx?articleid=2191517 https://jov.arvojournals.org/article.aspx?articleid=2191517
- im3w1l 1y agoNo I meant like if there is some other gene where the two different variants are synergistic to each other.
- andyfilms1 1y agoThis is a good biological explanation. The physical explanation is, if the sensitivities didn't overlap, our spectral sensitivity would not be continuous. There would be valleys of zero sensitivity between the cones, and a continuous wavelength sweep would result in us seeing black bands between colors.
- aetherson 1y agoGray bands, or more realistically just desaturated bands. There'd still be sensitivity to light through rods (black and white), and even if the peaks of wavelength sensitivity were highly separated there would still be some cone response to wavelengths that didn't stimulate them strongly.
- __MatrixMan__ 1y agoThis is only tangentially related, but I have always wondered why chlorophyll absorbs blue and red, but reflects green--green being sunlight's brightest component. It's almost as if there was some evolutionary pressure towards being very visible in sunlight which is more important than evolving ways to collect as much sun energy as possible. When I guess at this I end up with something along the lines of reflected green being used as a signal to a neighboring plant: "I'm already here, grow in some other direction instead." There is some evidence that plants do this (https://en.wikipedia.org/wiki/Crown_shyness https://en.wikipedia.org/wiki/Crown_shyness, https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep11604091 https://onlinelibrary.wiley.com/doi/10.1111/1365-3040.ep1160...) but it's not clear that the need to do so is so strong that it would overshadow the drive to collect as much energy as possible. Or perhaps there's something to do with the physics of absorbing light to drive a chemical reaction that makes it better to absorb at red and blue while passing on green (450nm and 680nm are not harmonics--so if this is the case it's more complex than which sorts of standing waves would fit in some chemical gap or other).
- amelius 1y agoRed is nature's warning signal, and blue was already taken by the sky, so the only option left was green. Just kidding of course, it is an interesting question.
- amelius 1y agoMaybe it has something to do with keeping the albedo of the planet at a higher value.
- pinkmuffinere 1y agoThis is fascinating, I’d never realized there is this seeming-paradox! Thanks for mentioning it
- privatelypublic 1y agoHave you looked into band-gaps? Also remember that these are random processes with selection pressure keeping those who survive to reproduce. Assigning a will to such processes makes them and the results harder to understand- imho. Theres probably something more efficient at converting light into simple sugars.
- cyborgx7 1y agoI'm pretty sure that line of the article didn't mean to imply that we don't know, or aren't sure, only that it goes beyond the scope of the article and isn't directly relevant to the topic at hand.