The Seeds Article that I mentioned in my last post, about the basics of evolution, talks at length about the idea that natural selection is an ongoing process in modern human populations. Their information primarily comes from a publication in PNAS (Proceedings of the National Academy of Science) written by John Hawks, Eric T. Wang, Gregory M. Cochran, Henry C. Harpending, and Robert K. Moyzis. Their article, “Recent acceleration of human adaptive evolution” argues that the rate of evolution has accelerated in modern humans because a larger population is capable of producing a greater number of random gene mutations. Hawks and colleagues suggest that between 7 and 10% of the human genome is currently evolving at the maximum rate. They argue that natural selection is still an active factor in human evolution because trends of change visible in HapMap (a massive survey of the genetic differences between selected populations from around the world) seems too consistent from person to person to be the result of genetic drift. You might say, “Yeah, that’s just common sense.” However, the idea that human evolution is still active is contradictory to the popular scientific thought of the past several decades that argued that the development of culture got rid of the need for evolution to adapt to our environments. Many scientists today are still arguing for this approach to human evolution. So where can we find evidence to support the newer theory that evolution is still active in modern humans? I can think of two relatively recent examples off-hand: the development of the ability for adult humans to digest milk and the development of the CCR5 gene variant that protects against HIV infection.
The first is probably the most well known to the general public just because of it’s prevalence in our daily lives. How often do you reach for a glass of milk with dinner, or pour milk over your cereal in the morning without thinking twice about it? The ability to drink milk without feeling sick as an adult doesn’t apply to everyone in the world. There’s an interesting table originally published in an article by Robert McCracken that shows the percentages lactose intolerant adults in different ethnic groups. According to McCracken, Sweden, at 4%, has the overall lowest percentage of lactose intolerant adults while Native Americans and Asian Americans have some of the highest percentages of lactose intolerant adults with 95% and 95-100% respectively. The National Digestive Disease Information Clearinghouse reports similar numbers with up to 80% or African Americans, 80-100% of Native Americans, and 90-100% of Asian Americans being lactose intolerant. The general trend overall is that European adults can drink milk while most adults in the rest of the world can’t. Why is this? Looking at the question from an evolutionary perspective we would probably immediately make a hypothesis that the ability to drink milk is the result of a genetic mutation in the past for certain human groups.
Scientists believe that before the domestication of cattle and other milk-producing herds all human adults were incapable of drinking milk because there was no need to drink milk. Children stopped drinking their mother’s milk after their first few years of life and in hunter-gatherer or foraging societies before herd domestication people didn’t have access to milk as a food source. Once nutrient-rich non-human milk became available to humans in pastoralist societies, adults with a genetic mutation that allowed them to consume milk had an additional food source, and thus a survival advantage over adults without the mutation in pastoralist societies. This mutation would then have been selected for because it provided a fitness advantage. Over time, probably the past 3,000-6,000 years, the percentages of adults with the ability to drink milk became what it is today. Two other, more technical hypotheses for why the ability to drink milk as an adult spread so successfully throughout many European societies are discussed in an article by Clare Holden and Ruth Mace.
A second interesting example is the CCR5 gene variant which might protect carriers against HIV. It is probably a much more recent mutation. Scientists believe today that this genetic mutation initially developed and was selected for in Europe during the time of the Black Death or bubonic plague. In their article, “Evaluating plague and smallpox as historical selective pressures for the CCR5-Δ32 HIV-resistance allele,” by Alison P. Galvani and Montgomery Slatkin provide a great discussion about this gene mutation and what scientists are discovering about its development and implication. Both of these mutations have developed and been selected for in recent human history, well after the development of culture. This suggests that using culture to adapt to our environments, technology, and modern medicine alone are not enough to stop evolution in its tracks. Perhaps Hawks and his colleagues are even correct in arguing that evolution is speeding up in humans today because our population size has gotten so big. We can look eagerly to seeing what scientists have to say about this hot topic in the coming future!
Here are some additional resources that I found helpful! -
Robert McCracken’s Article that I mentioned can be found here:
Robert D. McCracken, "Lactase Deficiency: An Example of Dietary Evolution,"
Current Anthropology 12 (Oct.-Dec. 1971, pp. 479-517) and Norman Kretchner, "Lactose
and Lactase," Scientific American 277 (Oct. 1972, pp. 71-78).
More information on Nutritional Adaptation:
http://anthro.palomar.edu/adapt/adapt_5.htm
Clare Holden and Ruth Mace’s article on the Lactase Gene:
http://findarticles.com/p/articles/mi_qa3659/is_199710/ai_n8778998/pg_1?tag=artBody;col1
Evaluating plague and smallpox as historical selective pressures for the CCR5-Δ32 HIV-resistance allele:
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=299980
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