In the paper The Role of Geography in Human Adaptation, Graham Coop (et al.) moves beyond expressed phenotypes and investigates the traces of human adaptation found in the genomes of extant and geographically diverse populations. By analyzing nonsynonymous SNPs, the study found that signals expressed in the genome can be used to predict the patterns found within specific loci. Essentially, the patterns they identified demonstrated that closely related populations possessed fewer genetic differences than would be expected having been subject to differing ecological pressures over the past 100,000 years. These apparently resilient genetic patterns - they suggest - are indicative of a selection pressure that “has been strongly constrained by the historical relationships and gene flow between populations.”
From the Introduction:
“…Humans have spread out of Africa to colonize almost all of the world's land mass, and in the process have experienced a vast range of new climates, diets and ecosystems. Humans have also encountered new pathogens as they moved around the globe and moved into close proximity with domesticated animals, and as human population densities increased.
These changes in human ecology suggest that there has been ample scope for the action of natural selection in recent human evolution. Moreover, most species, including humans, probably face various additional selection pressures on a persistent basis: e.g., due to sexual competition, viability selection and resistance to evolving pathogens. Hence, it seems reasonable that our genomes would show evidence for recent selection…”
Evidence for natural selection is exactly what they find, however they also conclude that the power of selection on humans can be very weak and therefore easily influenced by historic patterns of migration and genetic drift.
The paper is well worth a look…
Coop, G., Pickrell, J., Novembre, J., Kudaravalli, S., Li, J., Absher, D., Myers, R., Cavalli-Sforza, L., Feldman, M., & Pritchard, J. (2009). The Role of Geography in Human Adaptation PLoS Genetics, 5 (6) DOI: 10.1371/journal.pgen.1000500