Cecil M. Lewis

Mitochondrial DNA and the peopling of South America

ABSTRACT The initial peopling of South America is largely unresolved, in part because of the unique distribution of genetic diversity in native South Americans. On average, genetic diversity estimated within Andean populations is higher than that estimated within Amazonian populations. Yet there is less genetic differentiation estimated among Andean populations than estimated among Amazonian populations. One hypothesis is that this pattern is a product of independent migrations of genetically differentiated people into South America. A competing hypothesis is that there was a single migration followed by regional isolation. In this study we address these hypotheses using mtDNA hypervariable region 1 sequences representing 21 South American groups and include new data sets for four native Peruvian communities from Tupe, Yungay, and Puno. An analysis of variance that compared the combined data from western South America to the combined data from eastern South America determined that these two regional data sets are not significantly different. As a result, a migration from a single source population into South America serves as the simplest explanation of the data.

Ancient DNA and genetic continuity in the south central Andes

Alternative models of residential mobility have been proposed to explain the development and spread of Tiwanaku influ ence across the south central Andes. Within the Osmore drainage, the rich Moquegua Valley has been hypothesized as the site of a significant colonization event (or events) whereby both the natural and human landscape was transformed and integrated into the expansive Tiwanaku state. In this research, the impact of altiplano colonization is inferred from tempo ral and spatial patterns of genetic variation within and among native groups. Mitochondrial DNA haplogroup frequencies are used as the measure of genetic variation. The haplogroup data are determined for Moquegua Valley archaeological sam ples (Chen Chen site; A.D. 785-1000) and are compared to published data from 58 other ancient and contemporary native groups. The results support temporal and spatial genetic continuity in the south central Andes for the last 1,000 years. Con temporary Aymara speaking groups are exceptions to this pattern, perhaps because of recent population decline. While the altiplano colonization hypothesis is not rejected, moderate gene flow and relatively large population sizes likely charac terized much of south central Andean prehistory regardless of the contribution from Tiwanaku colonization events.

Native South American Genetic Structure and Prehistory Inferred from Hierarchical Modeling of mtDNA

Genetic diversity in Native South Americans forms a complex pattern at both the continental and local levels. In comparing the West to the East, there is more variation within groups and smaller genetic distances between groups. From this pattern, researchers have proposed that there is more variation in the West and that a larger, more genetically diverse, founding population entered the West than the East. Here, we question this characterization of South American genetic variation and its interpretation. Our concern arises because others have inferred regional variation from the mean variation within local populations without taking into account the variation among local populations within the same region. This failure produces a biased view of the actual variation in the East. In this study, we analyze the mitochondrial DNA sequence between positions 16040 and 16322 of the Cambridge reference sequence. Our sample represents a total of 886 people from 27 indigenous populations from South (22), Central (3), and North America (2). The basic unit of our analyses is nucleotide identity by descent, which is easily modeled and proportional to nucleotide diversity. We use a forward modeling strategy to fit a series of nested models to identity by descent within and between all pairs of local populations. This method provides estimates of identity by descent at different levels of population hierarchy without assuming homogeneity within populations, regions, or continents. Our main discovery is that Eastern South America harbors more genetic variation than has been recognized. We find no evidence that there is increased identity by descent in the East relative to the total for South America. By contrast, we discovered that populations in the Western region, as a group, harbor more identity by descent than has been previously recognized, despite the fact that average identity by descent within groups is lower. In this light, there is no need to postulate separate founding populations for the East and the West because the variability in the East could serve as a source for the Western gene pools.

Difficulties in rejecting a local ancestry with mtDNA haplogroup data in the South-Central Andes

This study assesses whether local genetic drift within populations can be rejected as a sufficient explanation for mitochondrial DNA haplo group frequency changes between contemporary and prehistoric population samples in the South-Central Andes. Differences in the frequencies of haplo group s between populations are a popular line of evidence for assessing population history. The null hypothesis of haplo group frequency change is a stochastic force inherent to finite populations called genetic drift. Genetic drift is particularly influential in small populations. Innumerable historical events can result in low population sizes, and the simplest scenarios for these events are those occurring locally. In this study, simulations are used to provide a baseline for the amount of haplogroup-frequency difference expected from local genetic drift over time. The results from the simulations are compared to observed data from 23 population samples, including six prehistoric population samples. The study concludes that local genetic drift cannot be rejected when comparing a prehistoric population to a contemporary population. For the South-Central Andes, these results have dire consequences when attempting to infer genetic exchange. This study demonstrates that more informative genetic data are required for such inferences.