Annual Review of Genomics and Human Genetics The Genetic Diversity of the Americas

The history of the Americas involved the encounter of millions of Native Americans, Europeans, and Africans. A variable admixture of these three continental groups has taken place throughout the continent, influenced by demography and a range of social factors. This variable admixture has had a major influence on the genetic makeup of populations across the continent. Here, we summarize the demographic history of the region, highlight some social factors that affected historical admixture, and review major patterns of ancestry across the Western Hemisphere based on genetic data. 277 Click here to view this article's online features: • Download figures as PPT slides • Navigate linked references • Download citations • Explore related articles • Search keywords ANNUAL REVIEWS Further A nn u. R ev . G en om . H um . G en et . 2 01 7. 18 :2 77 -2 96 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y C ol um bi a U ni ve rs ity o n 09 /0 1/ 17 . F or p er so na l u se o nl y. GG18CH13-Ruiz-Linares ARI 30 July 2017 11:55 INTRODUCTION The United Nations includes in the Americas a total of 35 countries and 18 dependencies, currently comprising approximately 1 billion inhabitants, or approximately 14% of the world population (99). In addition to a heterogeneous native population, whose immigration from Asia starting ∼15,000 years ago spanned millennia, from the late fifteenth century, the so-called New World has received immigrants from across the planet. The genetic diversity of the present-day Americas is thus, in a sense, the genetic diversity of the world. However, four factors had a prominent impact on the current genetic makeup of the Western Hemisphere: the variable native population density at the arrival of Europeans, the extent of European immigration to specific geographic areas, the degree to which certain parts of the New World were involved in the African slave trade, and the extent to which natives, Europeans, and Africans admixed on the continent. These three populations provided the genetic ancestry components that have become predominant across the Americas, with contributions from other parts of the world generally being smaller and geographically more restricted. Understanding the genetic diversity of the Americas has a range of evolutionary, anthropological, and biomedical implications. To provide a sharper focus, we circumscribe this review mainly to patterns of genetic diversity (particularly continental ancestry and admixture) and their historical correlates, and do not extend into the implications of these patterns for other research fields. We also largely limit ourselves to reviewing solely native, European, and African ancestry studies across the continent. In reviewing this literature it is noticeable that, other than studies of Native American populations, population genetic analyses have rarely attempted to examine genetic diversity across the whole of the Western Hemisphere. Rather, these studies have concentrated mostly on either the United States or Ibero-America (i.e., the former Spanish and Portuguese colonies) in isolation. This literature often also shows different research approaches in that population genetic studies of the United States have usually examined separately the genetic diversity of European Americans, African Americans, and Hispanics. By contrast, studies of Ibero-American countries usually examine the extent of Native American, European, and African ancestry without reference to perceived ancestry labels. As a contribution to a rapprochement between these somewhat divergent literatures, here we review genetic ancestry studies across the Western Hemisphere in the context of a common historical narrative. The basis for this narrative is the fact that similar demographic forces have been at play across the continent, but various social factors have influenced the extent of admixture in different regions. In particular, historical analyses have suggested that Iberian America has differed from non-Iberian (particularly British) America in a range of social features that could have affected the frequency of admixture. We therefore summarize some key historical and social factors that have helped shape the genetic diversity of human populations across the Americas (particularly rates of admixture across the continent). For simplicity, in what follows we at times refer to the Americas (comprising North, Central, and South America and the Caribbean islands—i.e., the Antilles) as “America” (and “American” as an adjective referring to the continent as a whole), while throughout the text we refer to the United States of America as the United States (or the US). THE DEMOGRAPHIC HISTORY OF AMERICA A range of studies date the initial settlement of the American continent towards the end of the last glaciation, some 15,000 years ago, by people who migrated from northeast Asia across Beringia (the land bridge that connected Asia and North America at the time) into Alaska (28, 35, 40, 62). From the northwest tip of North America, humans dispersed southward, reaching Tierra del Fuego at 278 Adhikari et al. A nn u. R ev . G en om . H um . G en et . 2 01 7. 18 :2 77 -2 96 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y C ol um bi a U ni ve rs ity o n 09 /0 1/ 17 . F or p er so na l u se o nl y. GG18CH13-Ruiz-Linares ARI 30 July 2017 11:55 17.17 Population size (millions) 9.28 4.49 1.85 0.60 0.13 0.03 0.02 Greenland (DK)