Will evolutionary pressures of dealing with the strong sunlight (cancers etc) eventually make the skin tone of descendants of todays white Australians go black in a few thousand years or so?
What evolutionary pressures? That term is only meaningful for conditions that prevent the genes from being passed on. Skin cancers don’t affect proportional reproductive abilities; i.e. people don’t die so young from skin cancer that they don’t pass their genes on.
And that’s ignoring the fact that skin cancer is curable; that modern humans don’t spend all day outdoors naked; that skin color is a complex of genes that factor in a great many variables; that intermarriage is already a much larger factor than in aboriginal days; and that we don’t know anything about what life in the future will be like.
Really, “evolutionary presures” is meaningless no matter the context. Selective pressures, on the other hand… And, of course, selective pressures do not operate solely to cull; they also dictate which traits serve as most (or “more”) beneficial at a given time in a given environment.
So what specifically, if not “evolutionary pressures”, makes the skin of people in sunny climes (say Africa) substantially darker on average, than the skin of people in less sunny climates (say Iceland)?
Evoluntionary pressure only occurs if the enviromental effect has an impact on the birth rate of the population. Skin cancer is not the main concern when it comes evolutionary fitness in a high-sunlight region; skin cancer tend to kill people only after they have reproduced, though having grandparents around does boost the survial rate of children in primitive societies. The main concern with high sunlight is that the UV destroys folic acid in the body, which causes all sort of bad effects, especially to unborn children.
Now, in modern Australia, folic acid supplements are readily available, as is sunscreen. These tools allow even pale people to reproduce quite readily in the bright Australian sun, much like the tools of clothing & fire allowed groups humans to in Artic regions for generations, without having to become really hairy to stand the cold. As for skin cancer, as I said before, it tends to kill people after they have kids, so it isn’t much of an effect. And with current welfare systems, extended families are not as important, and even single moms can have a gaggle of kids that will most likely survive to adulthood if they want to.
Of course, if there is a wide-scale technological collapse, I expect the average population of Australia would get much darker over time, though I it would probably be due to the Aboriginal population (who is, of course, pretty dark already, and well adapted to survial in Australia’s climate with low-tech- after all, they have been there 40,000 years.) suddenly gaining a significant survival advantage. The white population would have to wait for dark skin mutations to crop up before they start getting darker.
Fine “selective pressures” it is. Now put down that bio pedant stick you’re beating me with.
Given that the human race originated in Africa, the proper question might be “Why aren’t we all black?”. And the answer is that light skin makes it easier for the human body to synthesize vitamin D.
As Darwin’s Finch said, it’s selective pressure not evolutionary pressure.
The strongest selective pressure on human skin color has probably been on the basis of vitamin D production and protection against loss of folate, rather than protection against skin cancer. Skin cancer usually occurs too late in life to have much of a selective effect.
These selective effects are relatively slight however, and probably have taken tens of thousands of years to produce the variation in human skin color seen today. Given the availability of vitamin supplements today, natural selection on the basis of skin color is probably negligible. (However, selection could still be taking place due to social factors.)
Evolutionary pressures – specifically, selection for particular genes – take place in a natural environment. Birds have water-resistant feathers because birds which shed water survived where birds whose feathers absorbed water were less able to.
In humans, the range of skin colors results from the combination of two pressures – the human ability to produce Vitamin D from its precursors in the dermis when hit by sunlight, and the tendency of the skin to “burn” from ultraviolet radiation, also from sunlight. The latter has been implicated in skin cancers, etc.
So – people in hotter climates where skin was more exposed to light for a far greater proportion of the time tended to select for dark skin, in which the melanin helped to protect against burning. People in cooler climates, where the skin was much less exposed to sunlight over the course of the year, selected for paler skins, maximizing the amount of Vitamin D their rarer exposure would produce. (There are, of course, a number of other factors – no single socio-biological trait can be taken in isolation, but that’s a rough cut at a principal cause.)
Two Australians from Queensland, one of northern European extraction and the other an aborigine, living the same lifestyle, are not going to be subject to any of the evolutionary pressures mentioned.
If civilization were suddenly to be wiped out, and the survivors to eke a living from what’s left where they are, odds are that in several thousand years, selection would have selected for darker-skinned Euro-Australians, and they would resemble sterotypical Mediterranean folks in skin tone.
But with civilization as we know it, there’s no such selection going on – other than that the attractive deeply-tanned dirty-blond young woman or man on the beach is probably going to be considered desirable by the typical Australian, selecting for “good tanning ability.”
Consider the “evolutionary pressure” seal duly clubbed. Move on
(Thanks for info)
Sorry, I wrote my response before seeing your reply some posts up. But there’s no need to be defensive, or accuse people of pedantry when they are just trying to answer your question.
I greatly appreciate the infomation from both you and the Finch, and I was trying to be amusing. Didn’t mean to offend.
On top of the relevant medical advances, in today’s world, it’s not like generation after generation after generation will be likely to stay in Australia and intermarry with other families that have also remained there for generations. People move around a lot these days.
That effect is likely to influence the average Austraian skin colour far more than any selectve pressure. Most Australian immigration these days comes from Asia, so that in a couple of hundred years the average Australian skin colour may be close to the average skin colour in South East Asia.
Unless there’s some major collapse of human society, it is unlikely that any further evolution will ever occur that is defined by location. As a species we are simply far too mobile for it to be a factor.
Very well, then. 
(It sounded like you were offended.)
Once they do, will they ever go back? 
If you mean that natural selection is no longer a factor in human evolution, you are probably correct, in that humans can now act upon and change their environments to suite their own needs, rather than the other way around.
We don’t mean to harp on you and astro, but in this case you really have to use the right terms.