By “eventually” I mean, through evolutionary processes.
First let me preface by saying I know very little about evolution. Whatever I was taught in highschool is just a haze now. I’m not a creationist either. I’m just curious about the subject.
Let’s pretend we have a colony of 500 white people suck in a small island right at the equator. The island has some trees but exposure to the sun is continuous and inevitable. The growth rate is kept at 0% so they’re basically replacing themselves.
If someone can explain the process in layman terms that’d be great as well.
My incredibly layman’s understanding is that people of darker skin would have a slightly better chance of surviving long enough to reproduce due to the hot climate favouring darker skin, and therefore over a long period of time (many generations) the skin of the population would get darker and darker as the darker skinned survivors pass on their dark skin genes.
If there was a lot of hot sun those with the lightest skin would be most likely to suffer from skin cancer.
So over many thousands (tens of thousands) of generations those who produced more melanin would have a better chance of survival than those who produced less. Any random mutations that caused an increase in melanin production would be favoured.
Of course, most ‘white’ people can actually go pretty dark if they are continuously exposed to a lot of sunlight so the effect might not be very pronounced.
A more interesting question, IMO, is what evolutionary advantage did having lighter skin have confer that caused people who migrated to cooler climes to lose their colour.
The population would evolve, assuming it could be kept going.
Whether it evolved into black people you don’t really know. Assuming that people are selected for darker skin, which in your scenario seems a reasonable guess, then yes, over enough time, assuming the population kept going, it’s possible that their skin would become darker. That doesn’t necessarily mean they would become “Black” in the sense you probably understand it, meaning looking like West Africans; curly hair, larger lips, that sort of thing. They might have darker skin the way Sri Lankans do, for instance. They might look like the Melungeons.
In this case the way evolution is working is by natural selection. We’re assuming that dark skin is a survival trait; that people with slightly darker skin have a better chance of passing on their genes than people with lighter skin (I would assume because lighter skin increases your chances of getting skin cancer.)
However, people being people, it seems equally likely your population would not so evolve simply because it’s unlikely that the likelihood of skin cancer will ever have an effect on breeding likelihood. People are pretty good these days about staying out of the sun, after all.
Along comes a Zebra stripe mutation, with its slightly darker average skin color. The ladies find it especially attractive, so within a few generations, everyone is stripey, despite the somewhat better protection from cancer offered by uniformely dark pigmentation.
If I remember from college anthropology (just a general ed requirement, I’m by no means an expert, so correct me if I’m wrong), lighter skin is more conducive to the production of Vitamin D than darker skin.
Vitamin D can be produced in the skin, when it’s exposed to ultraviolet light, and we need Vitamin D to live. At higher latitudes, with less UV light, the risk of skin cancer was much lower. Of course, vitamin D production was also lower, so people evolved lighter skin to make use of what UV light they could get.
Humans use sunlight to produce Vitamin D in the upper layers of their skin. In higner latitudes its thought that winters with little sunlight and humans spending as much time inside where its warm led to paler skin being an advantage, since it aids production of the vitamin.
Now adays lots of food is artificially fortified with vitamin D, so its not really an issue.
I thought it was an urban myth that Negros evolved because of sun exposure?
There are deserts all over the world. You have Asians in Mongolia with the Gobi desert. Indians like the Navajo in North America deserts. Arabs in Saudi Arabs. None of these people are black. They do have darker complexions, but aren’t black.
As a kid I met some older black men that my uncle had hired. They had spent a lifetime in the sun working in the fields. They literally were the color of ebony. Coal black. You don’t see that much any more because so many people work indoors.
There is a balance between the advantage that paler skin has in allowing adequate Vitamin D production and the disadvantages it has in not only increased rates of skin cancer but also, and maybe more so, in increased light driven destruction of folate, a B vitamin that is involved in blood cell production and protects against neural tube and other defects in embryonic and fetal growth. Probably the folate issue is the more critical one as it has the more direct and more immediate bearing on reproductive success.
Skin color actually has actually adapted to dark and to pale and back again in a variety of populations over evolutionary history.
Both points are discussed in detail in this article (pdf)
The speed of this process may be faster than you may think. (Same scientist interviewed a few years later on NPR.)
And it has a variety of consequences beyond bone health, including more kids that wheeze, and diabetes and cancer risk. Adults in Northern climes, such as Canada, but even Arizona have high rates of deficiency.
Vitamin D fortification has been insufficient. This Mayo Proceedings review makes the case for both taking supplemental Vitamin D and for "sensible suberythemal sun exposure ":
The thing is, this experiment has pretty much been done a number of times (although not with a limited population), and none that I know have turned “black”, even if we just refer to skin color. Could it happen? Of course. Would it happen? Maybe and maybe not.
All white people of European descent have a particular genetic defect that causes us not to properly produce melanin. Interestingly, lighter skinned people of Asian descent have a defect in a different gene, which means that lighter skin evolved at least twice independently of each other.
So, the first question is, are all of your people on Whitey Island of European descent or is it a mix of European and Asian whites? This will make a big difference.
In order for black skin to evolve, two things have to happen. First, you have to get your whites to develop a corrected (or at least functioning) version of the gene that regulates the production of melanin. This will be easier if your population is a mix of European and Asian whites, since each has a proper copy of the other’s defective gene. If you don’t have a mix of Europeans and Asians then the only way you’ll get a properly functioning gene is for the malfunctioning gene to mutate into a functioning one, which could take a very long time.
The second thing that has to happen is that there has to be some sort of environmental pressure to select for this gene. Just being on an island isn’t enough. Humans make shelter and clothing and can adapt so that the sunlight isn’t an issue. You’d have to make it impossible for your residents to make clothing and shelter so that the direct sunlight would become an issue. Basically, if extreme sunlight isn’t killing your whiteys, they aren’t going to evolve.
Define “black”. (And yes, the issue here is skin color alone.)
As dark as the majority of “Black” Americans? Well, many in Southern India, especially those who are of the Dalit group (formerly designated as “untouchables”), are darker than that. Depending on what theory of migration into Australia you accept, the Australians aboriginals probably began similar in tone to Asian populations and are darker than the average American Black. The Vedda of Sri Lanka, darker than that. Heck, many populations indigenous to more equitorial climes, many now gone, have/had skin darker than that. As Black as the darkest African Black? Probably not, at least not unless humans lose their proclivity for clothing and shaded shelter, as noted above…