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A vast majority of humans get at least some grey hair as they age. As far as I know this applies to both genders and all races. Presumably this means that at least some grey haired humans have noticeable reproductive advantage, or maybe they had it in the recent past.
Theoretically, because this feature is so prevalent, there must be a strong evolutionary pressure to keep it. Am I right? If so, what is it?
Presumably this means that at least some grey haired humans have noticeable reproductive advantage, or maybe they had it in the recent past.
No it doesn't. Natural selection is not that strong, it doesn't optimize every single possible physical trait towards maximum reproducing.
And as others have mentioned, having lots of grey hair usually happens after reproduction is over. Historically, lots of women did a lot of reproducing before they had any grey hair.
Grey hair is one of many age related traits. Other traits showing a positive correlation with age include Parkinson's, cancer, and Alzheimer's. There are two key theories as to why age-related disease & decline occurs. But the key message is just because something evolves, it doesn't mean it's advantageous.
First of all is mutation accumulation (MA) theory. This theory basically suggests that as we age there is less likelihood that selection can remove the alleles that cause genetically determined aging. If we consider something like cancer, which is clearly deleterious it becomes clearer, though grey hair may be deleterious (reduces fitness) it is at least less obvious. If a cancer has early onset, affects people aged 10, then it is highly unlikely to spread through a population because those carrying the allele are unlikely to reproduce. If a cancer has late onset, causing cancer at 60 years old, then there is a much lesser chance it will affect reproductive success - most 60 year olds don't have more children in the future. In the case of MA the effect of selection is weak if a trait reduces survival post-reproduction. The same could be true of grey hair, even if it is costly it's arrival late in life is not that surprising.
The other theory is antagonistic pleiotropy (AP) which suggests that genes which improve fitness might reduce survival. For example, the genes that cause cancer at age 60 might also make you better at attracting mates, fertilising, and raising offspring. In this case it would be beneficial to carry the cancer gene (from an evolutionary perspective). I think there is some evidence (but can't find it right now) that grey hair is associated with higher fitness, in that case it could be that grey hair has evolved directly because of sexual selection (people might prefer silver haired partners) or because of AP - grey haired people are better at other fitness related things because of pleiotropy.
So to answer your question: direct selection is not necessarily the reason that grey hair has evolved, it could be because of mutation accumulation or pleiotropic gains associated with the genes affecting grey hair.
You can read more about these theories of aging in this answer and for references.
I do not think there is a reproductive advantage in gray hair - it's the other way around:
Normal colored hair has a reproductive advantage. But it also has a cost in terms of substances needed to build it.
I make the assumption here that grey hair - which is often also more sparse - has a lower cost in terms of material.
I think we are investing the cost for the part of the live that is exposed to direct evolutionary reproductive pressure.
Later, we do not longer invest the cost.
That makes sense if we assume that hair becoming gray is a process of degradation.
It is not supported by an investment, but by the absence of an investment, following from absence of evolutionary pressure.
I think grey hair does have an evolutionary advantage related to Vit D. Vit D is crucial to life and severe deficiency may lead to death. The main source of this vitamin is the production in skin cells through exposure to UV light.
As you might know, humans who live in colder climates with limited sun exposure lost the pigmentation in their skin. This is to allow more UV rays to reach the deeper layers of skin to produce Vit D. Humans living in sunny areas had the opposite need as their skin pigmentation is protective against skin cancer, and it still allows enough UV rays to produce Vit D in sunny areas. This is why people with pigmented skin living in cold climates are recommended to take Vit D oral supplements, and why people with less pigmentation in hot climates are more vulnerable to skin cancer.
Grey hair allows more UV rays to reach the skin due to the lack of pigmentation in the hair. Humans in general need more Vit D as they age and start becoming vulnerable to osteoporosis and fractures. This explains why grey hair commonly starts at age 40-50 when the bones start to become increasingly weaker.
My theroy is that gray hair allowed a predator to pick out the weak one in the group and focus on it as a possible food source allowing the younger ones to escape and survive.
Gray hair is also a signal to a potential mate that you might not be providing young healthy genes.
Both of these could have allowed our species of human to succeed over others.
