3 Questions about skin color

Factual Questions
1

OK, from what I understand, it is currently believed that the first people came from somewhere in Africa, and so were black.
Also, historically, the closer to the equator people lived, the darker their skin color was likely to be (such as shown on the map shown on this page when you scroll down Skin Color Adaptation)

OK, now for my questions.

  1. OK, so a group of people moves from Africa and settles down somewhere else further from the equator. Now not as much melanin is needed, so children who are born with less, do better, and eventually, though, I guess, natural selection, the generations gradually get lighter until they reach a level of melanin that is best suited for their environment, right? If this is incorrect, then how did skin color variation come about?

  2. How long does the process take?

  3. Does this mean that, for example, eventually, even if there’s no interracial marriages, the decedents of the white European settlers in Africa will eventually start having future generations with darker skin, until eventually it will be the same shade as the native Africans?
    Also, will blacks living in America or northern Europe eventually have lighter skinned children?

Thanks.

2

I think you answered your own #1 question. Not everyone moved, just groups of people. Some stayed in Africa while others moved to the north and then stayed there.

As for your third question, I was under the impression races have very slowly blended through interacial breeding (marriage) and not because of any latitudinal disparities.

3

Well no kidding. I never said that they all moved. Anyway, I’ll take it that you’re saying that my understanding for question 1 is correct.

I think that you’re misunderstanding me. What I’m asking is, is the mechanism that caused different groups of people to wind up with different skin colors still in effect today, and, given time, would it effect groups of people who’s skin color isn’t optimally suited for their environment. For example, if the European settlers in Africa were to be totally isolated from the native peoples, would their decedents eventually become darker anyway, because those born with darker skin would be better suited to their environment?

4

Okay, so assuming the Inuit came across the last landbridge, does fairly recent migration from (likely) Asia explain how they have relatively dark skin? 'Cause using the theory of “further North, lighter skin”, they ought to be very pale indeed.

I’ve actually always wondered about that.

5

According to this Wikipedia article, those with light skin in sunny areas are more likely to get skin cancer due to damage from UV radiation and to have folic acid deficiencies, as folate is apparently destroyed by UV-A radiation. Skin cancer can, well, kill you, and folic acid deficiencies in pregnant women can cause several unpleasant and very serious birth defects. Those with dark skin in less sunny areas are prone to deficiencies of vitamin D, and are more likely to get rickets.

So, assuming that cultural preferences for certain skin colors do not exist or have no effect on who gets to reproduce, the same selective pressures do still exist, though we now have ways to mitigate their effects. If white South Africans don’t wear sunblock and don’t cover their skin while outside, those with slightly darker skin will be less likely to get skin cancer - though that usually gets you after reproductive age, so it may not affect the baby production much. If white South African women don’t take or don’t have access to folic acid supplements during pregnancy, those with slightly darker skin will be more likely to give birth to children without neural tube defects. If black Brits don’t get foods with lots of vitamin D in them, those with slightly lighter skin will be less likely to be affected by the skeletal problems that come with rickets.

And on preview, zoogirl, the traditional Inuit diet is apparently pretty high in Vitamin D, (fish fish fish fish fish…) so if they’re getting it from their diet, they wouldn’t necessarily need light skin.

6

Good point Elfbabe, but another point I kind of forgot is that Inuit tend not to expose much skin at all, so melinin seems quite redundant. They must have had to cover up as soon as they hit the colder climate.

7

This is based on a common misapprehension about how evolution works. Let’s assume for a moment that darker skin is better in an equatorial region. Fair skinned people colonize such a region. Assuming no other factors are involved, over the course of numerous generations, some children will be born with darker skin. And some will be born with lighter skin. The changes occur at random.

The second stage is natural selection. Individuals are born with a random set of genetic characteristics; if they’re lucky, these characteristics help them survive and reproduce, if they’re unlucky, they don’t.

8

I haven’t seen any mention yet that skin color is also not unchangeable during a person’s lifetime. Sun tans are defense mechanisms for the skin in sunny climates. the ease with which a person tans may have no connection to how dark their skin is, but may still have a significant effect on their skin’s durability.

9

Well, yeah, I do understand that, I just must have worded my question poorly. But thinking about it, I guess that with modern medicine and advances and improvements in modern living, if you had a group of white settlers who kept to themselves, and didn’t intermarry, the survival rate between the darker born children, vs the lighter born children would be so reduced, or eliminated, that over time, the group probably wouldn’t become more and more darker, as opposed to ancient people who were much more at the mercy of nature and, as elfbabe mentioned, would be more prone to skin cancer thus, giving a greater chance for survival to those born with darker skin.

10

Well, in the website that I mentioned in the OP, it says this

So, to some extent, how well someone can, or can’t tan is determined by their skin color, or at least, it can be a factor.

11

I could be totally wrong here, but I’d always thought that white skin was a result of inbreeding. If you go with the most evolutionary theories, and believe that humans originated in Africa, you can see that the greatest variation in features occurs among those Africans. They way I’ve thought of it, a group of people migrated up North, and they happened to have lighter skin. Thus, of course, they started a whole race of people with lighter skin. This is also backed up by the fact that there is much less variation in other features among white people than among black people, thus pointing to the fact that white people might have come from a smaller pool of ancestors.

12

That isn’t what is currently believed. It is one hypothesis, and less supported by evidence than some of the alternatives, which are:

  1. Homo sapiens is a forest animal, although an undisputed descendant of H. erectus that evolve din the savannas. Chimapanzeees are also forect animals and tend to have pale or even white skin. So the first humans were white. An extension of this under multiregionalism is that the Europen populations of erectus and neaderthalensis were white and that these either evolved independatly to modern humans, or were partially replaced by H. sapiens but that their white skin was retained, as was the brown skin of the oriental erectus populations. Multiregionalism is currently in disfavour with the scientific community, but it remains a valid theory with its own champions.

  2. H. erectus and archaic H. sapiens were all brown skinned, like the Khoi san and similar people today. These people posess an odd pigmentation type where all the melanocytes are heavily clustered together. In black and white people the melanocytes are widely dispersed, suggesting a common recent lineage. Taken with other presumed primitive features of the San it seems like the most plausible human ancestral condition.

The same way as all other variation. To start with every phenotpype has disadvantages and advantages associated with it. The only question is whether the advantages outweigh disadvantage. Skin type is no different. Pale skin may favour a newborn child in a cold winter, but it may prove fatal to the same individual in a hot summer 10 years later. Some variation will always be present because some traits will be detrimental or beneficial to some individuals at some times, even in the same location. Sexual selection helps speed this process along.

Greater than 10, 000 years at least. Tasmanian Aborigines were isoltaed for that long at least, and showe no signs of developing skin as light as that for comparable latitudes elesewhere.

Theoretically yes, but it assumes that they are unable to alter their environment to cope, and it assumes that there is no benefit to having white skin that outweighs the penalties. Clearly this is not true at the moment, but with gradual physical changes over hundreds of generations or rapid social changes then that may become the case.

13

This is something I wondered.

Can evolutionary pressure have an effect over the course of one lifetime? Is the following correct:

Natural selection is the mechanism by which evolution works ie you pass your genes on by means of reproduction. Those more likely to reproduce are more likely to pass on their genes. Therefore you get more of that type in the next generation and the next and so on. Correct?

Does one of your father’s sperm contain 100% of his genes? And your mother’s egg contain 100% of her genes?

And then the two things mix to form one unit and half of the genes from each are switched off? How does it know which genes to keep and which to discard? Does your body, in effect, have 200% of genes (ie your father’s 100% and your mother’s 100%) it’s just that 100% of them are switched off - half your mother’s and half your father’s?

In deciding which genes to switch on and which to switch off, the newly-fertilised egg doesn’t really know since it’s never lived. The sperm and the egg however have formed part of a living being so they have experience. So are certain genes already switched on or off before the two ever meet (the sperm and the egg)? Pre-programmed rather than leaving it up to the newly fertilised egg to make the decision which genes it will need and which it won’t.

Each gene performs a certain function but can we trigger that gene to be an “on gene” or an “off gene” during our lifetime?

So if you are black and move to a cold country then (in several generations) your descendants skin will get whiter anyway because the “get whiter” genes will tend to be the “switched on” genes? And likewise if a white person moved to Africa, the switched on genes will tend to be the “get darker” ones?

Such that, in a coupla thousand years, black people would look like white people (in terms of skin colour) if they lived in a cold climate and white people would look like black people (in terms of skin colour), if they lived in a hot climate?

Or am I wrong?

14

You’re wrong JoJo. Sperm, and ova, are haploid cells whoich means that they have exactly 50% of the chromosomes of the individual that produced them. So a sperm contains only 50% of your father’s genes. Which 50% is pure dumb luck since the allocation is totally random. No room for selection pressure in random events like that.

15

What about the fact that they would be at grater risk for melanoma and other skin problems associated with having light skin in a tropical climate?

16

ok I was just wondering.

But if it’s pure dumb luck every time, doesn’t that mean that there is no room for evolutionary change? The changes must happen at some link in the chain? Since the newly-fertilised egg doesn’t know what to do surely the information must come from the experienced sperm and the experienced egg?

17

That is a penlaty to having white but it doesn’t outweigh the benefits of having white skin in southern Africa by a huge stretch. Look at the income levels and HIV infection rates for black and white South African’s for two of the more obvious starting points.

This is precisely what I was saying above. Every phenotpype has disadvantages and advantages associated with it. The only question is whether the advantages outweigh disadvantage. If being born with plaer skin results in a .0005% chance of dying of melanoima but being born with darker skin results in a 25% chance of dying of AIDS then you can guess which phenotype is being selected for.

No. There is no information involved in directing evolutionary change. Evolutionary change is the selction of information, not direction by it. In simple terms it works because you got your father’s good gene and lived your brother got the useles gene and died. His dying wasn’t luck, it was a direct result of the genes he received, but which genes he received was pure dumb luck. The gene is the information, it was selected for, it didn’t do the selecting.

18

This is pretty much confined to West Virginia nowadays.

19

Folks, ya’ll are focusing on skin cancer much too much. Skin cancer tends to kill you after prime reproduction age, meaning natural selection doesn’t do much to eliminate it. Sure, having grandparents does give a kid a slight advantage, but other adults in an extended family can provide similar benefit.

These days, I’m leaning towards the folate and vitamin D angle. A woman must have adequate folate levels to successfully reproduce. Lack of them greatly increases the risk of neural tube defects that, prior to the 20th century, killed the kid a birth or shortly thereafter - and that’s natural selection in full strength. Darker skins protect folate by reducing damage from the sun. And if you take a closer look at skin color and latitude you’ll find equitorial people who live in forested areas or those with frequent cloud cover have a tendency to be a somewhat lighter shade of dark brown than those living in, say, Sudan which is not only equitorial but arid, so there is seldom cloud cover and not a canopy of vegetation overhead. Likewise, people in high latitudes (that’s Inuit, as an example) have less atmosphere above them to filter out damaging sun, and during the summer the sun shines up to 24 hours a day - so for a couple months their sun exposure may be quite intense. And contrary to rumor, it does warm up in the summertime, sometimes to 80 or 90 degrees due to all that constant sunlight, which will prompt anyone to peel off that fur parka for a bit. So there may be some pressure to keep skin pigment. This may also account somewhat for the Tasmanians and Australians keeping their dark skins - those folks didn’t wear much clothing and thus got a lot of sun exposure, particularly in the arid regions of their territories. Likewise, frequent ocean voyages in primative boats with little shade might have exerted some pressure on Polynesians to keep their brown.

On the flip side, a dark skin interferes with vitamin D production - and that is a factor pushing for lighter skin. A pregnant woman needs adequate vitamin D not only for herself but also for that developing baby. If the child’s skeleton doesn’t get a good start in the womb there will be consequnces later on. In the sun-intense regions, you can produce sufficient vitamin D even with a very dark skin - but it’s almost universal for the women of a given ethnic group to be slightly lighter than the men. Possibly because they need the vitamin D. The less sun you get - such as by going north - the more pressure for lighter skin to allow for sufficient vitamin D. And since those less-sun-intense regions also result in less damage to folate, that lighter skin isn’t as much a drawback as it might be somewhere else.

A diet heavy in vitamin D food sources might take off the pressure towards lighter skin, resulting in people who retain dark pigmentation because it does no harm. This may be happening with the Inuit - but with the Inuit of today you also have to take into account that at least one of the early non-Inuit artic explorers was African-American and left his genes behind. Some of his descendants are aware of the relationship, some may not be, but there are a number of relatively dark, somewhat curly haired Inuit who are more representative of modern mixing than some “pure” ancestral state. People move around and they screw each other, which keeps us all one species.

So, really, this can get quite complicated - we’ve already uncovered cancer, folate, and vitamin D as factors in the natural selection of skin color, and we need to add in cultural factors for sexual/cultural selection on top of that. No wonder we come in so many shades!

20

I hope I’m not mangling my genetics here, but I think it would take much longer for a light-skinned population to evolve dark skin in response to their environment than it would for a dark-skinned population to do the reverse. The genes for dark coloring, be it skin, hair, or eye, are dominant, right? So darkly pigmented people can be (and often are) carriers for lighter genes, but lightly pigmented people cannot be carriers for darker genes.

Genes don’t just come out of nowhere, so a mutation would have to occur before our light-skinned population could begin to produce darker offspring. As mutations are, by their very nature, random, this could take a really long time. If the dark-skinned population did include carriers of lighter genes, no initial mutation would be necessary before natural selection could start doing its thing in response to new environmental pressures.