darkuser Wrote:
-------------------------------------------------------
> Sorry, it just gets more confusing for me...
>
> bernard Wrote:
> --------------------------------------------------
> -----
> > This is NOT a society permission question.
> Whiter
> > people will have more stillbirths because of
> the
> > amount of solar radiation and therefore will
> have
> > fewer descendants-- exactly what
> "reproductive
> > advantage" means. Indeed natural selection
> applies
> > to humans not just to other organisms.
>
> The layman's question would be whether this means
> that it is potentially dangerous for lighter
> skinned people to live in equatorial zones.

The question is phrased too strongly :-). However, just remember what I just wrote about the current situation in Australia. Whites have a much higher incidence of skin cancer in situations of higher solar incidence, but remember that we can adapt culturally--clothing, sunscreen, hats, air conditioning, etc. all of which will protect us from the ultraviolet

>
> >
> > You can see for yourself how much variation
> there
> > is in human skin color. Look at the range
> from
> > Swedish blondes to the darkest Nigerians and
> all
> > the variations in between. All this range
> comes
> > from different combinations of possibly 8
> genes.
> > Look at how dark even lighter colored people
> can
> > get if exposed to enough sunlight, i.e. sun
> > tanned.
>
> This idea of random skin pigmentation is what
> wireless and i are having problems understanding.
> Clearly, it isn't random. White parents will
> rarely give birth to a black child. It is
> difficult to believe that it is not somehow
> genetic and inherited. Are you saying if were to
> live in Africa, my descendents will be darker
> simply though the recombinations of the 8 genes?
> Can a Swedish blond become a dark Nigerian through
> this process over thousands of years?

We have to talk about populations not individuals. The two aspects are 1)variation-- in any population there will be a range of traits-- even among Swedes you have blonder/whiter persons and darker persons. If instead of focusing on one superficial trait, we look at a broader range of genes, we get the following results: of all the genetic variation of humans any ethnic group, tribe, or nationality will have 85% of the total variation, the amount of genetic variation between, say East and West Africans is 6%, the difference between "races" white/black, black/Asian, etc is only 10%.
We need to get some basic biology here-- it might be too much but perhaps it will be useful.

For a moment lets pretend that skin color is a Mendelian one gene trait (+ is Black and dominant; - is white and recessive. All traits have two alleles (i.e. genes) one coming from the mother and one coming from the father. Recessive means that in order for the gene to be expressed (i.e. seen) both alleles have to be (-). Lets have a pure white father (--) and a pure black mother (++) mate. Each child will get a RANDOM gene from the mother and the father (here is where the randomness of the process comes in). Each child will have the genotype (+-) and will look black. If then two of these first generation males and females mate what you will get the following (lets assume that we get all the possible types in a row-- actually since this is a statistical process you would have to have a large number to besure to get the statistical reult; i.e. a coin flip is a 50/50 chance of heads or tails, but you could get 5 heads in a row) you will get one child (++), 2 children (+-) and one (--)
one child will look white (--) and 3 will look black (+-) or (++).
So far so good

2) the second word is necessity lets suppose that (--) phenotypes have a high rate
of stillbirths due to folate deficiency in Equatorial areas. This is what is "selecting" and determining "fitness" in the particular environment. Let's also make the picture more complicated we now have a population of a 1000 "white" people and skin color is determined by 4 genes i.e. the genotype is now (++++) to (----). The more (+) in the genotype the darker the person. The variation in our population would be as follows-- a pretty "white" population

B genotype (----) 50%
C genotype (+---) 20%
D genotype (++--) 20%
E genotype (+++-) 10%

If we mate a B and C we will get a ratio of 3 (----) to 1 (+---) by random selection of genes from each parent. The one (+-) will have a little advantage over the (--) in being able to give birth.

If we mate a B and D we will get equal numbers of (+---) and (+---)

if we mate a C and D we will get 37.5% (----), 37.5% (++--), 12.5% (+---) and 12,5% (+++-)

etc. The basic thing to see is that the random combination of the genes from each parent will produce the whole range from (----) to (++++) you get some of this from mating 2 Ds , 2Es, one E and a C, or a D and E. BUT remember that the selction factor is weeding out Bs and possibly Cs from the population. Over time the resulting population will be heavily D and E with little C and B, i.e. the Swedish population is now "Black"


> > There are no taxonomic relationships--
> remember
> > there are no races-- I'm just pointing out in
> fact
> > how useless skin color is for classifying
> since it
> > is so subject to adaptation to degrees of
> > sunlight. The reason it takes a while is that
> it
> > is not a severe challenge (that is why I went
> to
> > DDT as an example because of its strong
> > selectivity). An example in humans, is
> malaria.
> > Malaria is such a severe selective factor
> (causes
> > so much mortality) that any adaptation that
> > provides some relief spreads through the
> > population much faster. For example,
> sickle-cell
> > gene in Africa reached some 30% of the
> population
> > of Cameroon in about 1000 years.
>
> Ok, so is it right to say that it is possible to
> discern from racial characteristics the
> approximate geographical or latitudinal range of
> one's ancestry? You see, most people would say
> that this info. equates to taxonomic
> relationships: presumably if your ancestors lived
> in an equatorial zone for 10'000 yrs, they shared
> the same gene pool with those who also lived
> there.

To some extent, but this is still not taxonomic, if taxonomic means being genetically related, which is the usual definition. Nigerians and Negritos may be similarly skin colored but they are NOT genetically related-- they are very very different.

From what you're saying, an extreme and
> unrealistic example may be the ancestors of a
> Nigerian who lived in Scandinavia for 10'000
> years, migrated to Africa and lived for just as
> long, and then migrated to Scandinavia and lived
> for another 10'000 yrs before returning to Africa.
> Clearly, racial characteristics will not show the
> first three or four migrations, and will only
> indicate that last 10'000 years of adaptations.

If the only thing you are loking at is skin color- which is susceptible to environmental effects. This is why scientists do not use this. If we look at a bunch of genes, specially those that are not adaptive, then we would not have this problem. You would be able to trace their history to their original migrations. Assuming your example, if you had a population in Africa, that looked black, but did not have a mtDNA (mitochondrial DNA) haplotype L2 or L1 (which are characteristic of Africans) but rather had a R haplotype (characterictic Swede) we would know that the origin of that population was Sweden and that it had migrated to Africa.
BTW your example has to assume that this group only had children with each other and did not have intercourse with any Africans while they were in Africa--very doubful-- but these are hypohetical :-).

> Therefore, racial characteristics do not have
> taxonomic value. However, this is a highly
> unrealistic example. Are genetic tests showing
> that this is indeed the problem, if not the
> potential problem?
>
>
>
> PS. the questions i ask in my posts are my
> attempts to understand what you're saying and none
> of them are debating your comments if you
> understand what i mean....
I do and I welcome the chance for reasoned dialogue
Bernard