Response
Sir,Dr. Bossak alleges that ‘it is unclear who is supposed to have made these assumptions’—that the Black Death provoked a genetic shift that bestowed greater resistance to HIV for Europeans, and in particular those from northern Europe—we think our references leave no doubt as to who supports this thesis (references 1–5, 7–10 from the original article).
Dr Bossak asserts: ‘Cohn and Weaver argue that if the Black Death were responsible for the spread of the CCR5-
32 allele, then present-day frequencies of the allele should correlate positively with recorded mortality during the Black Death’. We do not make any such positive claim; rather, we have questioned those who have assumed that Black Death mortalities followed a north–south cline matching that of the geographical distribution of CCR5-32 in present-day descendants. By turning to historical research (references 15, 17, 27, 28, 29, 31, 32, 34 and particularly 16 from the original article, which relies on >40 000 death records across Europe) we show that the Black Death mortalities as well as plague mortalities through the early modern period more likely reflect the opposite cline, with the highest mortalities most often recorded in the Mediterranean and little or no evidence of plague in the highlands of Scotland or the northern parts of Scandinavia and especially Finland.
Dr Bossak also raises questions about using genotype evidence from present-day descendants to study characteristics of historical and pre-historical populations, as though we were the pioneers in such research and methodology. Unfortunately, we cannot make any such grandiose claims. With regard to the question of CCR5 distributions and the Black Death, we have pointed out the limitations of this particular genotype evidence (original article, referenced above, p. 501). Those who developed these methods, however, are aware of the problems of subsequent migration. Indeed, several decades ago, scientists such as Luigi Cavalli-Sforza used such evidence and methodr to establish new findings about global migration and long-term transcontinental integration and communication of races and societies in pre-historical times.2 More recently, his students and others have devised genetic methods for distinguishing the effects of recent immigration among populations.3
University of Glasgow
Glasgow
email: s.cohn{at}history.arts.gla.ac.uk
References
1. Cavalli-Sforza L and Bodmer W. (1971) The genetics of human populationsSan Francisco WH Freeman.
2. Cavalli-Sforza L. (1993) The history and geography of human genesPrinceton, NJ Princeton University Press.
3. Rannala B and Mountain J. (1997) Detecting immigration by using multilocus genotypes. Proc Natl Acad Sci USA 94 9197–201.
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