This blog is intended to go along with Population: An Introduction to Concepts and Issues, by John R. Weeks, published by Cengage Learning. The latest edition is the 12th (it came out in 2015), but this blog is meant to complement any edition of the book by showing the way in which demographic issues are regularly in the news.

If you are a user of my textbook and would like to suggest a blog post idea, please email me at: john.weeks@sdsu.edu

Friday, February 14, 2014

Measuring Human Migration From DNA

Tracing the historical migration of people in eras prior to written records is not easy, and historians tend to rely on similarities between people (such as language) that seem as though they could not have happened just by chance, and thus must be evidence of population movements. A paper published today in Science and reported in the NYTimes takes this to a new level by studying human DNA. 
Though all humans have the same set of genes, their genomes are studded with mutations, which are differences in the sequence of DNA units in the genome. These mutations occur in patterns because whole sets of mutations are passed down from parent to child and hence will be common in a particular population. Based on these patterns, geneticists can scan a person’s genome and assign the ancestry of each segment to a particular race or population.
Now, geneticists applying new statistical approaches have taken a first shot at both identifying and dating the major population mixture events of the last 4,000 years, with the goal of providing a new source of information for historians.
Some of the hundred or so major mixing events they describe have plausible historical explanations, while many others remain to be accounted for. For instance, many populations of the southern Mediterranean and Middle East have segments of African origin in their genomes that were inserted at times between A.D. 650 and 1900, according to the geneticists’ calculations. This could reflect the activity of the Arab slave trade, which originated in the seventh century, and the absorption of slaves into their host populations. The lowest amount of African admixture occurs in the Druse, a religious group of the Middle East that prohibited slavery and has been closed to converts since A.D. 1043.
The Myers group [corresponding authors at Oxford University] has posted its results on a web page that records the degree of admixture in each population.
This is, in many ways, yet another approach to "family reconstitution" that demographers--especially at Cambridge University--have used to uncover the otherwise lost past. As the sample sizes increase, the ability to be more statistically precise will increase, and we will almost certainly to have a whole new perspective on human mobility.

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