There are a pair of articles in this week's edition of the journal Science that are almost certainly going to cause some excitement and controversy in the field of human evolution. Controversies in this area are nothing new, of course, but these articles seem to have all of the necessary ingredients for a spirited debate. They also seem to be almost certainly destined to be miscited by any number of unsavory individuals.
Although the two articles have slightly different sets of authors, both come from the same laboratory, and both focus on the same topic: natural selection acting on genes involved in the development of the human brain. Two different genes were examined, and in both cases specific versions of the genes - alleles - were found to be present in frequencies that indicate that they have recently been (or still are) the subject of strong selective pressure. In both cases, the alleles appear to be very new - younger than the appearance in modern humans. Finally, and here is the bit that's going be the most controversial part of this, the selectively favored alleles are less likely to be present in people from certain geographic locales.
The first paper discusses the evolution of a the gene known as Microcephalin. This gene got its name because a lack of function in both copies of this gene causes a genetic disorder called microcephaly. Individuals suffering from microcephaly have brains that are extremely small, but which have normal structure.
The team of researchers had previously investigated the evolution of this gene within primates, and had noted that this gene appeared to have been the subject of strong positive selection in the lineage leading up to apes and humans. They report, in the Science article, that during the course of this study they noticed that one version (or allele) of this gene appeared to be very common in the humans that they had looked at. They also found that this version of the gene caused an amino acid change in the protein that it produces, due to a single mutation changing amino acid 314 from aspartate to histidine. This kind of thing is very exciting to people who do evolutionary genetics, so they decided to investigate further.
They looked at sequences for this gene from 89 different humans, with a wide range of geographic origins. They found that the region of the gene that they were looking at contained 220 different places where there were variations in the specific nucleotide ("DNA letter") at that position. They also found that there were 86 different alleles present in the population that they had examined. One of these alleles was extremely common compared to all the others. Out of the 178 copies of the gene (each person has 2 copies) that they looked at, 59 of them - 33% - were that one allele. 18 individuals were homozygous for that allele (meaning that both of their copies were that allele). The next most common allele was present in 11% of the copies.
That's the type of data that screams "selective pressure" the second you look at it. But it is possible to be more scientific about investigating it. One of the things that you can do is to use computer simulation to figure out what the probability is that you would see the same results if the allele frequency was the result of chance. The investigators used a set of very conservative estimates about human population dynamics, and ran the simulation 5,000,000 times. None of the simulations yielded the observed results. This is a very strong indication that the common copy of the allele is selectively favored.
The investigators also looked at the age of the allele and the geographic distribution. They found that the allele appears to be quite young (~37,000 years). They also found, and this is the part that is likely to stir up controversy, that the allele is much less common in sub-Saharan Africa than in the rest of the world. (It seems to be most common in populations from the Americas.)
The second article looks at the evolution of ASPM, which, like Microcephalin, is involved in the development of brain size. As in the first study, an allele was found to be present with a frequency far greater than one would expect if the allele was neutral. This allele turns out to be even younger than the Microcephalin allele reported earlier, with an estimated age of only about 5,000 years. It also shows an uneven geographic distribution. As you might expect, the younger allele is less widespread than the Microcephalin allele. It is most common in Eurasian and North African populations, and it is quite uncommon in Native American, sub-Saharan African, and some East Asian populations.
Two studies, both of genes involved in brain development, both demonstrating extremely strong indications that one allele has experienced very strong positive selection, and both indicating that the positively selected alleles are more common in Europeans than in most Africans. I suspect that these results are going to bring joy to what passes for the hearts of a number of slimebags. They shouldn't, but they will. They will because the pussbuckets in question are always willing to grab onto anything they think supports their narrow-minded beliefs with both hands. They shouldn't get excited because these results actually don't say anything about intelligence.
We don't actually know why those alleles have been selected. The genes are involved in the development of the brain, and in brain size, but the alleles could be favored for entirely unrelated reasons. It's possible, for example, that people with that version of the gene are less likely to suffer from some illness or another. Even if the alleles in question have been positively selected due to their effect on brain size, these are only two of a large number of genes that are involved in the development of intelligence. These two genes have favored alleles that are present in certain populations. Other genes may have favored alleles that are present in other populations.
These two studies should serve as an excellent reminder that evolution, even in humans, is a continuous and complex process. They should not serve as fodder for various fools and assorted idiots.
Follow up: In response to accusations from one of the people commenting on this post, I have put up, in a new post, a slightly more detailed explanation of why we need to be careful drawing conclusions from these studies.
Evans, Patrick D et al. 2005. Microcephalin, a Gene Regulating Brain Size, Continues to Evolve Adaptively in Humans. Science. Vol 309, Issue 5741, 1717-1720.
Mekel-Bobrov, Nitzan et al. 2005. Ongoing Adaptive Evolution of ASPM, a Brain Size Determinant in Homo sapiens. Science. Vol 309, Issue 5741, 1720-1722.