Admixture in North America

The history of North America has been marked by the encounter of populations from different continents. The discovery of the New World began a period defined by human migrations at a much larger scale than in previous history. This movement of people, voluntary or forced, changed profoundly the human landscape. As populations came into contact, admixture followed in varying degrees depending on the circumstances. Today, many people living in the US, Mexico, Canada and the Caribbean can trace their ancestry to more than one continent. The most important genetic contributions came from the indigenous Native American groups, Western Europeans and West Africans, although there have also been influences from other regions, such as East Asia and South Asia. We can reconstruct and interpret this history of migration and admixture using genetic markers. A very complete perspective can be obtained when analyzing autosomal markers (located on any of the chromosomes other than the sex-determining chromosomes, which are inherited from both parents), maternally transmitted mtDNA markers and Y-chromosome markers, which are transmitted from fathers to sons. In many cases, the maternal and paternal admixture histories are remarkably different, so including mtDNA and Y-chromosome markers can provide a much better picture than that offered by the autosomal markers. Thus, using genetic markers we can reconstruct history at the individual and population level, even in the absence of a historical record. In this chapter, I provide an overview of admixture in North America, with a particular emphasis on the two major admixed groups: African Americans (and African Caribbeans) and Hispanics. I also discuss the implications of the history of admixture for the distribution of the genetic variation involved in drug metabolism and drug response and the potential consequences of population stratification in candidate gene association studies in recently admixed populations. Brief History of the Main North American Admixed Populations African Americans and African Caribbeans The history of African Americans can be traced back to 1619, when the first Africans arrived at the British colonies (Jamestown), although as early as 1526 the presence of African slaves was reported in Spanish expeditions to what would become the United States (South Carolina, Georgia, Florida and New Mexico). Institutional slavery began very soon after. Nevertheless, it was not until the beginning of the 18th century that the arrival of enslaved Africans reached increased rates, in parallel with the demand for workers to cultivate the tobacco, indigo, and rice plantations in the southern colonies. The highest numbers arrived in the decade from 1790-1800 and the early years of the 19th century. In 1808 slave trade became illegal but it continued at a low rate for several more decades. Estimates of the total number of enslaved Suarez(Parra) 10/19/06, 9:00 AM 1 © 20 07 C op yr ig ht L an de s B io sc ie nc e. N ot fo r D is tr ib ut io n. Pharmacogenomics in Admixed Populations 2 Africans brought into the United States differ but generally accepted numbers range between 380,000 and 570,000. Although it is very difficult to precisely determine the ethnic origin of the enslaved Africans, information from shipping lists has provided an approximate picture of their geographic provenance. The slave trade affected a very wide area of western and western-central Africa, mainly the coastline between the present day countries of Senegal in the north and Angola in the south. The most important regions were Senegambia (Gambia and Senegal), Sierra Leone (Guinea and Sierra Leone), Windward Coast (Ivory Coast and Liberia), Gold Coast (Ghana), Bight of Benin (from the Volta river to the Benin river), Bight of Biafra (east of Benin river to Gabon), and Angola (Southwest Africa, including part of Gabon, Congo and Angola). Curtin1 has offered, based on data on the English trade of the 18th century (the peak of the Atlantic slave trade), estimates of the proportional contribution by areas. His analysis shows that Angola and Bight of Biafra were the regions contributing the highest numbers of slaves imported into the North American mainland (around 25% each). It is important to point out, however, that there were significant differences in ethnic origin depending on the port of entry in the US, and the figures for the Colonies of Virginia and South Carolina differed considerably. The history of African Americans has been marked not only by their forced migration from Africa, but also by their admixture with the other ethnic groups that they met when they arrived in North America, namely Europeans and Native Americans. However, few historical records address the issue of admixture. Additionally, there have been important factors that, in the time since the abolition of slavery until the present, have configured the present African-American population. Of special interest is the pattern of migrations of African Americans within the US over the past 150 years. In this sense, the redistribution of African Americans in the southern states during the 19th century and the Great Migration from the rural south to the urban areas in the north beginning after World War I are of particular relevance, and have had an enormous impact in defining the present distribution of the African-American population in the US. According to the 2004 American Community Survey, there are approximately 34.8 million people who identify themselves as Black or African American in the US (12.2% of the total population). The majority of the African American population lives in the eastern states of the US and the largest percentages of African Americans with respect to the total population are found in the District of Columbia, Maryland, South Carolina, Georgia, Louisiana and Mississippi (Fig. 1). Finally, during colonial times the situation in the islands of the Caribbean was quite different from the US mainland. In addition to the Spanish, who concentrated their colonization effort in Cuba, Hispaniola and Puerto Rico, other European powers (mainly England, France, and Holland) established colonies in the Caribbean during the 17th century.3 The indigenous populations had been decimated by warfare, disease and forced labor, and millions of Africans were forcefully brought to the Caribbean during the slave trade in order to work in the sugarcane plantations, which were the mainstay of the economy of the islands. It has been estimated that the British and French islands received around three million enslaved Africans between 1601 and 1870. In terms of absolute numbers, many more enslaved Africans were brought to the Caribbean than to the US during the slave trade. Additionally, the enslaved Africans in the islands of the Caribbean far outnumbered the white settlers. For example, in 1775 the number of enslaved Africans in Jamaica was approximately 200,000 and the number of white settlers around 12,000. The conditions of slavery were particularly harsh in this region and the death rates were extremely high, surpassing the birth rates.4 The abolition of the slave trade in 1807 and slavery in 1833 created a shortage of labor in the British West Indies, and tens of thousands of indentured servants (laborers under contract for a specified time in return for payment of travel expenses and maintenance) were brought to the region (mainly to Trinidad, British Guyana and Jamaica). Most of the indentured servants came from South Asia (India), China and the Portuguese islands of Madeira and the Azores, further defining the demographic picture of the islands of the Caribbean.6 Suarez(Parra) 10/19/06, 9:00 AM 2 © 20 07 C op yr ig ht L an de s B io sc ie nc e. N ot fo r D is tr ib ut io n. 3 Admixture in North America