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Red-green colorblindness seems to make it harder for a hunter-gatherer to see whether a fruit is ripe and thus worth picking.

Is there a reason why selection hasn't completely removed red-green color blindness? Are there circumstances where this trait provides an evolutionary benefit?

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23  
Just a general note: evolution isn't a guided process. For a trait to disappear, there must be a strong pressure against that trait - usually either cost (e.g. cave fish eyes) or a more direct danger (e.g. black rabbit on snow). Dichromancy simply may not be disadvantageous enough (just like tetrachromancy, exhibited by some humans, isn't advantageous enough to spread). These kinds of variations are very important for the survival of the species - when the environment changes, that's the pool of adaptations evolution "picks" from :) – Luaan Mar 10 at 12:38
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Note that the presence of a trait does not mean it's adaptive. Selection is not the only mechanism of evolution and others mechanisms can spread and maintain maladaptive alleles in a population. – rg255 Mar 10 at 12:48
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Just to to Luaan 's point - and I doubt this is applicable for color-blindness - but sometimes a neutral or slightly disadvantageous trait (like color-blindness) may be bound together with a trait that is clearly advantageous. So when evolution selects for the advantageous trait, it "accidentally" also gets the neutral/slightly disadvantageous one as part of the "package". – Baard Kopperud Mar 10 at 15:09
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@BaardKopperud yes, a well-known example is Sickle-cell disease protecting against Malaria. – Stéphane Gourichon Mar 10 at 15:14
    
@StéphaneGourichon baard is describing linkage and genetic hitchhiking - sickle cell is an example of heterozygote advantage – rg255 Mar 10 at 15:49
up vote 55 down vote accepted

Short answer
Color-blind subjects are better at detecting color-camouflaged objects. This may give color blinds an advantage in terms of spotting hidden dangers (predators) or finding camouflaged foods.

Background
There are two types of red-green blindness: protanopia (red-blind) and deuteranopia (green-blind), i.e., these people miss one type of cone, namely the (red L cone or the green M cone).

These conditions should be set apart from the condition where there are mutations in the L cones shifting their sensitivity to the green cone spectrum (deuteranomaly) or vice versa (protanomaly).

Since you are talking color-"blindness", as opposed to reduced sensitivity to red or green, I reckon you are asking about true dichromats, i.e., protanopes and deuteranopes. It's an excellent question as to why 2% of the men have either one condition, given that:

Protanopes are more likely to confuse:-

  1. Black with many shades of red
  2. Dark brown with dark green, dark orange and dark red
  3. Some blues with some reds, purples and dark pinks
  4. Mid-greens with some oranges

Deuteranopes are more likely to confuse:-

  1. Mid-reds with mid-greens
  2. Blue-greens with grey and mid-pinks
  3. Bright greens with yellows
  4. Pale pinks with light grey
  5. Mid-reds with mid-brown
  6. Light blues with lilac

There are reports on the benefits of being red-green color blind under certain specific conditions. For example, Morgan et al. (1992) report that the identification of a target area with a different texture or orientation pattern was performed better by dichromats when the surfaces were painted with irrelevant colors. In other words, when color is simply a distractor and confuses the subject to focus on the task (i.e., texture or orientation discrimination), the lack of red-green color vision can actually be beneficial. This in turn could be interpreted as dichromatic vision being beneficial over trichromatic vision to detect color-camouflaged objects.

Reports on improved foraging of dichromats under low-lighting are debated, but cannot be excluded. The better camouflage-breaking performance of dichromats is, however, an established phenomenon (Cain et al., 2010).

During the Second World War it was suggested that color-deficient observers could often penetrate camouflage that deceived the normal observer. The idea has been a recurrent one, both with respect to military camouflage and with respect to the camouflage of the natural world (reviewed in Morgan et al. (1992)

Outlines, rather than colors, are responsible for pattern recognition. In the military, colorblind snipers and spotters are highly valued for these reasons (source: De Paul University). If you sit back far from your screen, look at the normal full-color picture on the left and compare it to the dichromatic picture on the right; the picture on the right appears at higher contrast in trichromats, but dichromats may not see any difference between the two:

dichromatic camouflage
Left: full-color image, right: dichromatic image. source: De Paul University

However, I think the dichromat trait is simply not selected against strongly and this would explain its existence more easily than finding reasons it would be selected for (Morgan et al., 1992).

References
- Cain et al., Biol Lett (2010); 6, 3–38
- Morgan et al., Proc R Soc B (1992); 248: 291-5

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4  
Second this. I'm a mild deuteranope and it took me a long time to understand what "camouflage" meant. Nature programs would always talk about things like the tiger hiding in the grass and it was obvious to me. – Loren Pechtel Mar 11 at 1:19
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I'm red-green colorblind (not sure which flavor) and I can't tell the difference between the full-color image and the dichromatic image :( that mannequin is wearing some really ineffective camouflage though. – jayhendren Mar 11 at 1:49
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@jayhendren et al. Just to clear the confusion, the full-color image is a mannequin wearing green-white military suits, while it looks orange on the dichromatic image. – Andrew T. Mar 11 at 7:22
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I'm giving +1 for the last sentence. I doubt any selective advantage for colour blindness outweighs the selective benefit for colour vision so it's more likely to be a failure to be selected out of the population than anything that's actively selected in. – Jack Aidley Mar 11 at 13:15
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I have full color vision, and still find the mannequin's camouflage to be wholly ineffective. If the fatigues were green and black, it might have been better than this green and white scheme. – Dan Henderson Mar 11 at 19:22

There seems to be some evolutionary advantages to red-green colorblindness. The paper in reference 1 (a summary can be found in reference 2) shows that people with red-green color blindness can differentiate between much more shades of khaki than unaffected people. This might help detecting camouflaged food in a green environment.

Reference 2 quotes an expert about this:

For example, it may have helped them spot potential food items in complicated environments such as grass or foliage, he suggests.

This fits with the observation that in a number of new world monkeys dichromatic and trichromatic animals are present in the populations. They found that the dichromatic monkeys have advantages in low light conditions.

References:

  1. Multidimensional scaling reveals a color dimension unique to ‘color- deficient’ observers
  2. Colour blindness may have hidden advantages
  3. A foraging advantage for dichromatic marmosets (Callithrix geoffroyi) at low light intensity
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1  
Anecdotal evidence: both my brother and I have mild red-green colourblindness and our colourvision in the dark, is much greater than both our sisters. We also, in general, have much better nighttime vision than everybody we know who isn't red-green colourblind, to the point where , when most people can't see anything, we can still navigate comfortably (but obviously not nearly as well as if there's any real light). I can't say if this is due to colourblindness or not. – Clearer Mar 10 at 11:22
    
FYI: Your ref #1 is on deuteranomaly, which is anomalous trichromatic vision. – Christiaan Mar 10 at 11:40

protected by Christiaan Mar 14 at 14:05

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