Author: Arslan A. Zaidi, Pennsylvania State University, USA
Competing Interests: Arslan A. Zaidi is an author of the article discussed in this blog.
Image Caption: An artistic representation of the findings from Zaidi et al. showing the evolutionary process underlying population-differentiation in nose shape. See the March issue image for a full description.
Image Credit: Arslan A. Zaidi and colleagues.
The nose is one of the most easily recognizable parts of the face and varies quite a lot, both within and between human populations. This variability—and the fact that the nose serves as a vital air-conditioning apparatus—has led to speculation that climate might have influenced the evolution of nose shape in some capacity. Studies looking at patterns of variation in the shape of the human skull have supported this hypothesis. On average, the nasal aperture tends to be wider in warm-humid climates and narrower in cold-dry climates. Similar trends are observed in the shape of the inner nasal cavity as well. In our latest study we extend these analyses to the shape of the external nose and find support for the climate-adaptation hypothesis.
Invoking divergent selection as an explanation
Genetic variation and, as a consequence, phenotypic variation, accumulates among reproductively isolated populations due to (random) genetic drift. Thus, genetic drift is the default (‘the null’) explanation for the observed differences in nose shape between human populations. To be able to say that divergent selection is responsible for these differences, one has to show that they are greater than those expected under genetic drift alone. This is a tall order for traits such as nose shape, for which the underlying genetic architecture is not well understood. However, it is safe to assume that nose shape variation among humans, both within- and between-populations, is heritable. This assumption allows one to compare the level of phenotypic differentiation among populations, measured by a statistic called QST, to the level of genetic differentiation, measured by FST, of neutrally evolving loci. If the QST of a trait between populations is much greater than FST, we can say that genetic drift alone is not an adequate explanation for the differentiation in the trait; selection must have played a role.
In the featured article from PLOS Genetics’ March issue, we measured seven aspects of nose shape (nares width, alar base width, nasal height, nose ridge length, nose tip protrusion, external surface area, and nostril area) in people from four different ancestries (South Asian, East Asian, West African, and Northern European). Next, we looked at whether the variation in any of these features among populations was greater than that predicted by the genome-wide FST distribution. Turns out, only nares width and alar base width show significant deviations from neutrality. The remaining traits, while they do differ among populations, are not different enough to warrant non-neutral explanations.
Is climate the culprit?
Once we determined that the divergence in nares width and alar base width among human populations exceeds neutral expectations, we asked: could local adaptation to climate have driven these differences? To answer this, we tested whether the geospatial patterning of these traits corresponds to that of temperature and humidity. Of the two traits, nares width is positively correlated with temperature and absolute humidity. In other words, nostrils tend to be wider, on average, in populations adapted to warm-humid climates, and tend to be narrower in populations adapted to cold-dry climates. This pattern is similar to the distribution of nose width reported previously and suggests that the differentiation in nares width among human populations may have been driven, at least in part, by local adaptation to climate. Why alar base width does not show a similar pattern with climate, despite the signal of population differentiation, is unclear.
Our study shows two things: i) while most features of nose shape seem to be evolving neutrally across human populations, dimensions related to the width of the nose appear to have differentiated more than would be expected under genetic drift, and ii) the width of the nostrils is correlated with temperature and absolute humidity. Taken together, these findings suggest that at least some of the diversification in nose shape may indeed have been influenced by climate adaptation.
How and when did the nose adapt to climate? Did selection influence nose shape in both cold-dry and hot-humid climates? Is selection still influencing nose shape? Can the observed diversification in nose shape be explained by some complex demographic history? Can it be explained by introgression from archaic hominins? What role did sexual selection play? Currently, answers to these questions remain elusive but will likely be disentangled by a careful study of the genetic variants underlying nose shape.
Why should we care about nose shape evolution? We should care about human evolution and adaptation in general as it informs much of what we know about prevalence and prevention of many diseases. One great example is that of skin pigmentation, which has darkened in regions of high ultraviolet radiation (to protect us from skin cancer and folate deficiency), and lightened in regions of low UV radiation (to help us absorb more UV necessary for vitamin D synthesis). This understanding informs us about when we should wear sunscreen or, as in my case—a dark person of Pakistani origin living in the US—supplement our diet with vitamin D. There are important social implications to this as well: understanding human variation in an evolutionary context helps dispel notions of there being fixed, fundamental differences among human groups that make some individuals universally better or worse than others.
Zaidi AA, Mattern BC, Claes P, McEcoy B, Hughes C, Shriver MD (2017) Investigating the case of human nose shape and climate adaptation. PLoS Genet 13(3): e1006616. https://doi.org/10.1371/journal.pgen.1006616
*Arslan A. Zaidi and Mark D. Shriver were the featured hosts of the PLOS Science Wednesday AMA of May 3rd, 2017, which you can read in full here: http://plos.io/2sNZAQc