Over at Eye on DNA Hsien-Hsien Lei has given over her blog over to Jon Entine to promote his new book Abraham's Children [Books About DNA: Abraham’s Children by Jon Entine].
Entine points out that Ashkenazi Jews (he is one) have higher frequencies of certain diseases like breast cancer, Tay-Sachs disease, Bloom's Syndrome etc. He notes, correctly, that Ashkenazi Jews are a relatively homogeneous population in spite of the fact that they are spread out over all of Europe and have since emigrated to North America and back to Israel.
There's nothing special about this group in terms of susceptibility to disease. Other genetically isolated populations, such as French Canadians, also have elevated incidences of some genetic diseases, and lower incidences of others. We usually assume that this altered frequency of alleles is due to random genetic drift: in this case the founder effect. Since the populations were founded by small numbers of people, there were certain alleles that just by chance happened to be over- or under-represented in the ancestors.
All reports suggest that Ashkenazi Jews descend from a small number of people who left the Middle East less than 2000 years ago. Possibly only four females contributed to most of the mitochondrial DNA in today's descendants (Behar et al. 2006). Most of these early descendants settled eventually in the Rhine Valley and from there they spread eastward. The main population expansion probably occurred after 1000 AD. Today their descendants number about 8,000,000 worldwide.
There is evidence for distinct subpopulations, suggesting a number of bottlenecks in the Middle ages (Feder et al. 2007).
Jon Entine is intrigued by stories that the Ashkenazi Jews have higher IQ's than many other ethnic groups. He offers the following speculation,
The book includes a chapter explaining the possible link between so-called Jewish diseases — certain neurological and LSD disorders, as well as DNA repair problems — and the high measured IQ of Ashkenazi (Eastern European origin) Jewry. Although this theory (most recently advanced by Gregory Cochran and Henry Harpending—two non-Jews—is best classified as informed speculation (as I acknowledge in the book) at the moment, it argues persuasively, I believe, that Ashkenazi Jews are a rather distinct population group in which positive selection pressures, balanced against the killer consequences of neurological disease mutations, have led to higher IQs. The “it’s environment and culture” argument are far less persuasive. This is only one chapter in the book–most of AC is a look at our shared Israelite ancestry and history–but it is the most provocative chapter. Wonder where others come down on this issue? Interestingly, “liberal” Jews (of which I’m one) are the one’s most uncomfortable about discussing or (even if they believe it) acknowledging this point.Leaving aside the truth of the premise, it seems very unlikely that there was selection for higher IQ. We're dealing with a polygenic trait (intelligence) in a population of about 100,000 (average) over a maximum of 70 generations and possibly less than 25 generations. It's very unlikely that the adaptive benefit of a 10% increase in IQ would have an effect in that time frame.
Assuming there really is a genetic difference in intelligence then it is far more likely that it's due to the same factors that are responsible for differences in the allele frequencies of other alleles. It looks like the people in group that left the Middle East were smarter than the ones who stayed!
[Figure Credit: The image is Figure 2 in Behar et al. (2006)]
Behar, D.M. et al. (2006) The Matrilineal Ancestry of Ashkenazi Jewry: Portrait of a Recent Founder Event. Am. J. Hum. Genet. 78:487-497. [PubMed]
Feder, J., Ovadia, O., Glaser, B. and Mishmar, D. (2007) Ashkenazi Jewish mtDNA haplogroup distribution varies among distinct subpopulations: lessons of population substructure in a closed group. European Journal of Human Genetics 15:498–500. [PubMed]