Two pulses of white admixture in American slaves?

By Razib Khan | July 9, 2012 11:54 pm

I noticed today an interesting paper in Genetics by Simon Gravel, Population Genetics Models of Local Ancestry. As indicated by the title this is a general paper where the method is the main course. But, there was an interesting empirical result which I want to highlight:

Comparing the ancestry variance from the African-American data to those predicted by the demographic models, we find that the pulse model predicts a genealogy variance of 0.0005, whereas the variance in the model with two distinct pulses is 0.002. The total variance in the African-American sample is 0.0047, of which we infer that 0.0041 is due to genealogy variance (using the method described in Appendix 3). Thus the model with two pulses of migration is again more realistic than the single pulse model; the fact that it still underestimates the variance can be due to a combination of factors that have not been modeled: our demographic model may be underestimating low level, very recent migration because of the parameterization as two discrete pulses of migration, and both population structure and errors in ancestry assignment may be adding to the observed variance.

To the left is a screenshot which represents a slice of the technical meat of the paper. Most people aren’t going to be able to penetrate this. So how to evaluate? The author presents an empirical prediction. I’ve read a bit about American slavery, a few years back, and I don’t recall any mention of two pulses. This isn’t too surprising, as there wasn’t that much cliometrics. But if this is attested in the literature it would certainly increase my confidence in the utility and power of the method of the paper. By their fruits you shall know them!

More generally, this sort of analysis of phased data sets is obviously the future. A more detailed topography of genomic variation is going to open up a huge window onto the human past.

MORE ABOUT: Genomics
  • Hemo_jr

    So we know that the pulses came before 1865, within the slavery period?

  • Spike Gomes

    I have a notion of when the first pulse is (I’m figuring when the Melungeons, Redbones and Black Dutch and We-Sorts all sort of generally emerged from the historical murk), no clue as to the second pulse.

  • Walter Sobchak

    The history of slavery in the US is that it began in the 17th century, but had its real growth in the early 18th century. The importation of slaves was banned in 1808 and the first decade of the 19th century saw a spike in importation:

    Date Amount
    1620-1700 21,000
    1701-1760 189,000
    1761-1770 63,000
    1771-1790 56,000
    1791-1800 79,000
    1801-1810 124,000
    1810-1865 51,000
    Total 597,000

    The years after 1808 until 1861, saw a rapid expansion of cotton plantations across the frontier south. Most of the slaves who labored there came from the eastern seaboard, particularly places like Virginia that were less suitable for plantation agriculture.

  • ohwilleke


    (1) Africa to USA with admixture in USA plus (2) Africa to Caribbean with admixture there followed by Caribbean to USA.


    (1) Slavery era plus (2) post-Civil War era admixture enhanced by hypodescent rules and small size of African American population relative to white population in much of the USA.


    as illustrated by figures in comment #3,(1) a pre-Independence plus (2) a pre-1808 ban surge.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at


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