A new paper in The American Journal of Physical Anthropology surveys the variation of genes across latitudes and longitudes. The authors found that both latitude and longitude were significant in the Americas, while only latitude was significant in Eurasia. They used microsatellites, which is fine by me. The main issue which they acknowledge is that their sampling of populations is rather sparse in certain areas. Let me jump to their conclusion:

Our results suggest a reduced speed for gene flow in the Americas since its initial peopling that, even after accounting for lower levels of genetic diversity in Native American populations, has led to more genetic differentiation in the Americas than that observed between Eurasian populations. Not only is the level of genetic differentiation greater between Native American populations, but it is greater per kilometer of latitudinal distance than genetic differentiation between Eurasian populations per kilometer of longitudinal distance. If a lack of gene flow between populations is an indication of little cultural interaction (i.e., assuming that technologies travel by demic diffusion), then a lower latitudinal rate of gene flow suggested for Native American populations may partly explain the relatively slower diffusion of crops and technologies through the Americas, when compared with the corresponding diffusion in Eurasia. Thus, our result that genetic differentiation increases more rapidly with latitudinal distance between Native American populations than with longitudinal distance between Eurasian populations supports the hypothesis of a primary influence for continental axes of orientation on the diffusion of technology in Eurasia and the Americas (Fig. 1).

In some ways this paper is extending and testing the argument in Guns, Germs, and Steel. Here is my primary concern: agriculture may have radically reshaped the genetic variation patterns in Eurasia over the last 10,000 years. This is especially obviously true in Southeast Asia. The variation that we see in the New World among indigenous people may then be a snapshot of a region where mass population replacements had just not gotten as advanced in the Old World. It seems that gene flow and cultural exchange would have a much more explosive connection than is implied in the model of demic diffusion used to generate many of the statistics in the above paper.