Human migrations and population structure: what we know and why it matters

The increasingly obvious medical relevance of human genetic variation is fueling the development of a rich interface between medical genetics and the study of human genetic history. A key feature of this interface is a step increase in the size and diversity of genetic data sets, permitting a range of new questions to be addressed concerning our evolutionary history. Similarly, methodologies first developed to study genetic history are being tailored to address medical challenges, including mapping genes that influence diseases and variable drug reactions. In this paper we do not attempt a comprehensive review of human genetic history. Rather we briefly outline some of the complications and challenges in the study of human genetic history, drawing particular attention to new opportunities created by the explosive growth in genetic information and technologies. First we discuss the complexity of human migration and demographic history, taking both a genetic and archaeological perspective. Then we show how these apparently academic issues are becoming increasingly important in medical genetics, focusing on association studies, the common disease/common variant hypothesis, the evaluation of variable drug response, and inferences about gene function from patterns of genetic variation. Finally we describe some of the inferential approaches available for interpreting human genetic variation, focusing both on current limitations and future developments.

Hierarchical patterns of global human Y-chromosome diversity

We examined 43 biallelic polymorphisms on the nonrecombining portion of the Y chromosome (NRY) in 50 human populations encompassing a total of 2,858 males to study the geographic structure of Y-chromosome variation. Patterns of NRY diversity varied according to geographic region and method/level of comparison. For example, populations from Central Asia had the highest levels of heterozygosity, while African populations exhibited a higher level of mean pairwise differences among haplotypes. At the global level, 36% of the total variance of NRY haplotypes was attributable to differences among populations (i.e., Phi(ST) = 0.36). When a series of AMOVA analyses was performed on different groupings of the 50 populations, high levels of among-groups variance (Phi(CT)) were found between Africans, Native Americans, and a single group containing all 36 remaining populations. The same three population groupings formed distinct clusters in multidimensional scaling plots. A nested cladistic analysis (NCA) demonstrated that both population structure processes (recurrent gene flow restricted by isolation by distance and long-distance dispersals) and population history events (contiguous range expansions and long-distance colonizations) were instrumental in explaining this tripartite division of global NRY diversity. As in our previous analyses of smaller NRY data sets, the NCA detected a global contiguous range expansion out of Africa at the level of the total cladogram. Our new results support a general scenario in which, after an early out-of-Africa range expansion, global-scale patterns of NRY variation were mainly influenced by migrations out of Asia. Two other notable findings of the NCA were (1) Europe as a "receiver" of intercontinental signals primarily from Asia, and (2) the large number of intracontinental signals within Africa. Our AMOVA analyses also supported the hypothesis that patrilocality effects are evident at local and regional scales, rather than at intercontinental and global levels. Finally, our results underscore the importance of subdivision of the human paternal gene pool and imply that caution should be exercised when using models and experimental strategies based on the assumption of panmixia.