Joanne Wrote:
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> What about the moths that adapted to pollution in
> the 19th century -- only a generation or so?


I don't know why there seems to be such a problem with skin color and the whole concept of mutation. perhaps there is a problem with the idea of mutation and adaptation. What you have to know is that at any given time there is a large variation for any trait in a population. This variation occurs through mutations, geteic drift and also through sexual selection. In any particular environment some aspects will be favored and will come to dominate. If the environment changes, then other aspects will become favored. This is what happened with the peppered moths there have always been BOTH darker and lighter varieties. In the early 19th century, most moths were light colored because their color blended in well with exiting tree bark and the mimicry protected them from bird predators. When the pollution of the industrial revolution turned the bark of trees dark light colored moths stood out and birds got them. After a while, most moths were the dark kind. After 1945, when the air in England was cleaned, the balance again shifted in favor of the lighter colored moths. The point is that there was no mutation and/or recessive gene expression involved.

The key point to always keep in mind is that organisms do NOT mutate after the fact to adapt to an environment. If the capacity is not already in the gene pool and the environment changes what happens is that the species goes extinct. A different example, insect resistance to insecticides (or bacterial resistance to antibiotics). When you first use DDT on a field you kill most of the insect pests; but there is a preexisting range in the ability of the insect to deal with toxic materials in general. Some don't do well and are killed in large percentages, others are better equipped and survive better and reproduce. In the next generation, a larger percentage is "detoxifier able" and so on. After a while, the farmewr has to use more and more DDT to control pests and eventually the insecticide is useless. As a matter of fact, there are spacies of bees in Brazil that eat DDT as food.

Genomes will have lots of genes for traits that may not be useful at the time. Mutations occur all the time, if they are neutral in effect they just acumulate. If they happen to be beneficial in the present environment-- great they will help the reproductive success of the individual. If they are deleterious, or lethal, they will kill the individual if expressed or remain dormant if recessive until mating produces a homozygous individual, who then dies. Everyone of us this minute has dozens of lethal recessive mutations in our gene pool. An example, the mutation for sickle cell anemia has probably occurred thousads of times. For most of history it then dissapeared as the carriers died out. However, about 2000 years ago when carriers of this mutation expanded their range and became subject to the ravages of malaria and the Anopheles gambia mosquito, it turned out that individuals who were heterozygous (one normal, one sickle cell allele) did not die of sickle cell AND were somewhat resistant to malaria. The selective pressure of malaria was so strong that eventually some 30% of some populations carried the sickle cell gene. Again, the mutation had occurred BEFORE the selection pressure of the environment came into play.

Now to skin color. (You should really read my posts) Chimpazees (and presumably the precursors of Australopithecus) have light colored skin protected by hair-- However, they also havethe capacity ot manufacture melanin-- this trait is probably as old as mammals (or before). When hominids lost their hair-- or moved out from forests to African savanna Those who were able to manufacture more melanin were selected for, and the populations grew darker... However, you would get a range of dark from the lighter brown to a dark black depending at what latitude in Africa they lived in, what kind of forest covering there was, etc. The capacity for lighter skin color was always there and when eventually they moved to higher latitudes (where dark skin was maladaptive and light skin adaptive) the population got lighter skin color. One of the reasons trhe Khoisan are lighter in color is that they did not live originally where we find them now. The Bantu expansion displaced them out of forested Central Africa.

Skin color is NOT a recessive/dominant trait. it does not require any recent mutation. I also don't see why you keep invoking a bottleneck. It closely resembles the peppered moth example-- but at a much slower rate. Reread my post on Livingston's computer simulation on developing skin color with 4 genes assumed. Blue eyes are recessive but they are NOT exclusively assocated with either white skin or blond hair. There are many Touareg Berbers with very blue eyes (all you need is one mutation in a founder population)

I see that no one has read (or at least commented on my "genetics" post)-- It would be useful in getting us to the same starting point.

Bernard