Research: Investigations

Genes and Geography

One of the main topics of research in the Rosenberg lab is the study of human genetic variation. Patterns of genetic variation across populations from around the world reflect past migrations, changes in population size, and actions of natural selection, and can be used to make inferences about human evolutionary history. Additionally, understanding the geographic distribution of genetic variation helps in designing studies to scan the human genome for variants that underlie susceptibility to genetic diseases.

We have recently been studying the relationship between geography and levels of genetic variability across populations. We found in computer simulations that if a population develops by a sequence of expansions that begin at a certain location of origin, the amount of genetic variability observed in the population decreases with the distance from the starting point. This suggests that the source of an expansion can be localized by finding the putative origin for which the clearest pattern of decline in genetic variation is observed as one moves away.

When we applied this idea to human populations, we found that data on human variation matched a scenario with an African origin far more closely than they fit any other scenarios - the red points in Figure 1 are the ones that produce the best agreement with the data. A picture in which people sequentially expanded outward from Africa, with each next expansion involving a subset of the population at the frontier, explains most of the difference in levels of genetic variation among populations around the world.

Figure 1

What explains the rest of the difference? Geographic barriers such as the Himalayas and the Sahara Desert. Directly on opposite sides of a major barrier, people are about as genetically different from each other as are populations that lie about 3000 km apart but on the same side of the barrier (Figure 2).

Figure 2

This work is helping us to further understand the genetic history of the human species. One of our next phases of research will be to examine how patterns of genetic variability such as those we have been studying will affect the ability of studies that search for disease-susceptibility genes to replicate their findings across multiple human populations.

— Noah A. Rosenberg, PhD, November 2005