Evidence for gradients of human genetic diversity within and among continents

Abstract

Genetic variation in humans is sometimes described as being discontinuous among continents or among groups of individuals, and by some this has been interpreted as genetic support for "races." A recent study in which >350 microsatellites were studied in a global sample of humans showed that they could be grouped according to their continental origin, and this was widely interpreted as evidence for a discrete distribution of human genetic diversity. Here, we investigate how study design can influence such conclusions. Our results show that when individuals are sampled homogeneously from around the globe, the pattern seen is one of gradients of allele frequencies that extend over the entire world, rather than discrete clusters. Therefore, there is no reason to assume that major genetic discontinuities exist between different continents or "races."

Figure 1

Geographic distribution of the samples used in the (A) population-based and (B) geography-based data sets. In A, each circle represents five to eight individuals and in B a single individual. The assignment of individuals into two inferred populations from the population-based (C) and the geography-based (D) samples. See text for details.

Figure 2

Assignment of 261 individuals from the CEPH diversity panel (Cann et al. 2002) according to their coefficients of ancestry of four inferred populations (A, B, C, and D). Green circles represent African individuals, blue circles European individuals, pink circles Asians individuals, yellow circles Oceanian individuals, and red circles Native American individuals. The appropriate representation would be a pyramid. The two faces of the pyramid represented here included all individuals at least once.

Figure 3

Geographic representation of the proportion of ancestry in inferred population B obtained by Structure. Red dots represent individuals defined by their longitude (x-axis), latitude (y-axis), and coefficient of ancestry (z-axis) in the inferred population B. The surface of the world was fitted to the xyz coordinates using a distance weighted least-squares smoothing method.

Figure 4

Assignment of Native American individuals using (A) correlated allele frequencies as used in Rosenberg et al. (2002) and (B) uncorrelated allele frequencies. In B, the assignment of the individuals has been determined using all individuals from the first subsample and four inferred populations as in Figure 2. In A, each bar represents a single individual, and the colors correspond to the coefficients of ancestry. In B, the coefficients of ancestry were averaged over the five individuals for each population. Blue and red segments represent the proportion of ancestry in inferred population B and D, respectively.