Ripe fruit is rarely green, it is red or yellow in which case trichromat color vision is a selective advantage, or it’s blue or violet etc. In which case both trichomats and typical dichomats can pick it out with little problem. Like any good evolutionary theory we don’t know for sure but can test it, just have to avoid “Just So Stories”.
Color blind people can see a range of colors within the red-orange-yellow-green range but it leads to greater color confusion because a very bright red can be confused with a dimmer green for example.
Evolution doesn’t pick the best solution by the way. It selects against something that causes disadvantages in survival and/or reproduction. Some traits exist because there’s no reason to remove them, not because they’re good.
Black and white vision is rare, it either involves brain damage, or loss of two cones. Loss of blue and red, or blue and green cones is rare because they’re completely unrelated genetic factors (one somewhat uncommon and one very uncommon) and it requires both to happen together. Or they’re lacking the evolutionary related red and green cones, which due to the structure of the cone matrix means they are invariably legally blind and probably confers a severe disadvantage in survival, and a moderate to substantial one today (driving may not be advised for example)
Sure, generally speaking. I think green grapes are a mutation and limes were invented by humans. Kiwi are a bunch of colors, not sure about proto-avocados. But of course survival just needs you to eat some fruit, not every variety.
Achromatic (black and white) vision comes in three “flavors”:
Brain damage, where the brain no longer (or never did) process color information coming from the eyes. Oliver Sach’s “The Case of the Colorblind Painter” is about one such person This is cerebral achromatopsia. One way it differs from other forms of achromatopsia is that the person may retain normal visual acuity and normal tolerance for daylight, which is not a feature of the other two forms.
Achromatopsia caused by non-functioning cones. That is, none of the person’s cone cells are functional and they see only with the rod cells. In other words, they really do see only light intensity and not hues. Other features are photo-phobia and day-blindness - normal daylight is not tolerant and is literally blinding to them. They also have lower than normal visual acuity which is not correctable with lenses, and they have nystagmus, meaning basically twitchy eyeballs that jiggle back and forth even when focusing on something. This is a genetic form of colorblindness that is not sex-linked, it affects men and women equally.
Blue cone monochromacy. This person has only blue-sensing cones, the others are not functional. This one is sex-linked, with mostly men affected (for a woman to be affected her dad would have the disorder and her mother would have to be a carrier). This has similar symptoms to #2 but because the person does have functional cones their visual acuity is usually better than with rod-only vision. This one is also three times more rare than #2.
You are correct that driving is not advised, in fact, I’m not sure anyone with #2 or #3 is allowed drive anywhere in the world. You’re correct that there is a form that renders both red and green cones non-functional, but it’s actually the LESS common of the two genetic monochromatic vision types.
It’s interesting that evolution in primates have but the red and green cones in the most acute part of our visual field. It’s almost as if those two are more important than the blue cones, huh?
Distinguishing red and yellow fruit against a backdrop of green foliage might have been important to our primate ancestors, important enough to affect color vision and its inheritance. Sure, there’s also texture and smell, but while you can see a ripe red fruit from 30 feet away you can’t feel it and primates don’t have the greatest noses.
So… I was looking through newspaper archives for an entirely unrelated reason and came across this gem! From the Cleveland Plain Dealer, August 15th, 1943:
Ah yes, forgot about rod monochromacy. I’m not sure how the rest is contradicted though? How are you distinguishing blue cone monochromacy and “form that renders both red and green cones non-functional”? And with just blue, or without any cones they see better when not using their central vision.
I’ve met a blue cone monochromat who I’ve heard can legally drive, but I don’t know how often he actually does and can’t comment whether it’s advised, but sounded like it wasn’t the greatest idea.
Hence the “not quite” - I wasn’t contradicting all of your post.
Non-functional red and green cones vs. no functioning cones at all. Sorry if that wasn’t clear.
Blue monochromats have a visual acuity significantly better than those without functional cones. In US visual terms blue monochromats have a visual acuity between 20/60 (which might just barely qualify them to drive) and 20/200 (legally blind). Complete monochromats (rod only) have a visual acuity between 20/100 (under optimal conditions) and 20/200.
For people with complete colorblindness the lack of ability to perceive color is probably not the biggest issue they have with vision.
