Expert: John Shea - 6/21/2006

Question
Hi, my question is regarding human adaptation to climate, for instance adaptation to sun exposure; as in the case of white people who move closer to the equator.  More specifically the Slavs of Eastern Europe.  Who originally all looked the same; before today’s southern Slavs migrated south from the north sometime in the 6th or 7th century AD.

I have always noticed a very big difference in appearance between the northern Slavs, and the southern Slavs.  I see that the northern Slavs (Poles, Czechs, Russians etc…) are fair skinned, and have varying degrees of blonds in their populations, on the other hand the southern Slavs (Bulgarians, Serbs, Croats, etc…) are a lot darker skinned and never have blond hair naturally.  You could say that the northern Slavs look like your average “white” person, while the southern Slavs look Middle Eastern.  My questions on the subject are as follows:


1. How long does it take for a group of people to evolve/adapt to their new environment?

2. If a large population of white people moved closer to the equator, how long would it take for them to evolve into darker skinned people?

3. Are the southern Slavs darker, because they evolved in the past 1500 years and adapted to life closer to the equator, adapted to more exposure to the sun?


4. Are the southern Slavs darker simply because they mixed with the native dark peoples of the region?


Thank you in advance.  

Answer
Dear Nick,
This is a little off the mark for my area, but I think I can help you.

1. How long does it take for a group of people to evolve/adapt to their new environment?
This depends on group size. In a small group evolutionary change can happen very rapidly.  Briefly, this is because in a small group, a genetic mutation has a better chance of becoming "fixed" (possessed by all members), -a phenomenon known as the "founder effect".  Large populations evolve slowly.

2. If a large population of white people moved closer to the equator, how long would it take for them to evolve into darker skinned people?
For reasons outlined above, it should take a very long time.  FWIW: there has been a running "experiment" in this among Northeast Europeans who settled in South Africa and Australia.  Not much evolutionary change towards darker skin (other than that caused by interbreeding with darker skinned Africans and Australian Aborigines), but given that these European -derived populations have some of the highest rates of skin cancer known, there is certainly evolutionary pressure there for darker skin.

3. Are the southern Slavs darker, because they evolved in the past 1500 years and adapted to life closer to the equator, adapted to more exposure to the sun?
If there is a difference, it probably has to do with latitude variation in exposure to sunlight.  Be aware, though, that although the term "Slav" may be 1500 years old, the ancestors of the people thus named were probably living in Eastern Europe for tens of thousands of years.

4. Are the southern Slavs darker simply because they mixed with the native dark peoples of the region?
This is possible.  Geneticists refer to such a one-movement of genes, in this case genes influencing skin color, as "gene flow".  Tropical human populations tend to be larger than temperate zone ones, so one might reasonably expect more movements of individuals and their genes into this region from the south rather than the other way around.

Just a note, though, the terms we use for people, like "Slavs" or "Celts" or "Bantu" are in most cases artificial groupings that mix together physical appearance, language, culture, and geography. They don't usually correspond to clear differences in genes (other than those related to particular morphological characteristics).  In fact, if you think about it, physical appearance, language, culture, and geography can all all vary independently of one another.  (This last point is one of the major insights of Franz Boas, a founding father of American Anthropology.)
FWIW: I am not sure there is all that big a difference in skin color between what you call Northern and Southern Slavs, but if there is, I would bet that much of it has to do with recent (Medieval-Era) population dispersals there from Scandinavia and from either the Black Sea region or the Mediterranean Basin.
Cheers,
John Shea