Measuring cranial variation using geography as a proxy for neutral genetic distances

Certain anatomical features of the human skeleton are known to vary with geography and climate. To what extent each variable contributes to our physical makeup is less clear. The problem is that populations with similar climate are geographically close to one another. Even if we find shared traits among populations from similar climates it may be just as a result of geographic proximity (and thus clinal gene flow), rather than shared common ancestry.

As I mentioned in my previous post, anthropologists often compared cranial data to matched microsatellite datasets. However, it is rarely possible to get an exact match between the cranial and microsatellite populations. The anthropologist will instead use populations that are genetically similar and which may or may not be representative of the target population. Another option is to substitute microsatellite data with geographic distances, since studies have found a strong correlation between genetic distance and geographic distance (Manica et al. 2005; Ramachandran et al. 2005; Romero et al. 2008). This allows us to get around the need to match phenotypic data with genetic datasets.

A recent paper by Betti et al. used geographic distance as a proxy for neutral genetic distance. They set out to test the extent to which cranial differences can be explained by geographic proximity, by comparing pairwise phenotypic distances among populations and pairwise geographic distances using isolation by distance (IBD) models, as well as comparing pairwise cranial distances with climatic variables after correcting for IBD. Geographic distances were calculated as the shortest distance over land between populations while avoiding areas greater than 2000 metres above sea level. Intercontinental land bridges were also factored into their model.

Their study found geographic distance (and by extension genetic distance) to be a strong predictor of cranial variation. Minimum and maximum temperatures were also a significant predictor of cranial differentiation but not as strong as geographic distance. It also appears that much of this climate-related variation is influenced by the populations from exceptionally cold climates. A previous study by Roseman also found that populations living in extremely cold climates showed greater selection. Betti et al. suggest that this may be due to culture acting as an environmental buffer, with the buffer breaking down at extremely cold climates, after which cranial plasticity takes over.

Since climate and geographic distance covary, not considering isolation by distance leads to an overestimation of the effect of climate on cranial differences between populations. Not surprisingly facial traits showed the strongest correlation with climate. In summary, this study suggests that cranial measurements are predominately influenced by neutral evolutionary processes, especially in populations that do not live in extremely cold climates.

References:
Betti et al. 2009. The relative role of drift and selection in shaping the human skull. Am. J. Phys. Anthropol. in press.

Manica A, Prugnolle F, Balloux F. 2005. Geography is a better determinant of human genetic differentiation than ethnicity. Hum Genet 118:366–371.

Ramachandran S, Deshpande O, Roseman CC, Rosenberg NA, Feldman MW, Cavalli-Sforza LL. 2005. Support from the relationship of genetic and geographic distance in human populations for a serial founder effect originating in Africa. Proc Natl Acad Sci USA 102:15942–15947.

Romero IG, Manica A, Goudet J, Handley LL, Balloux F. 2008. How accurate is the current picture of human genetic variation? Heredity 102:120–126.

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Above photo by jacorbett70 under creative commons license.

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