Investigating the case of human nose shape and climate adaptation

<div><p>The evolutionary reasons for variation in nose shape across human populations have been subject to continuing debate. An import function of the nose and nasal cavity is to condition inspired air before it reaches the lower respiratory tract. For this reason, it is thought the observed differences in nose shape among populations are not simply the result of genetic drift, but may be adaptations to climate. To address the question of whether local adaptation to climate is responsible for nose shape divergence across populations, we use Qst–Fst comparisons to show that nares width and alar base width are more differentiated across populations than expected under genetic drift alone. To test whether this differentiation is due to climate adaptation, we compared the spatial distribution of these variables with the global distribution of temperature, absolute humidity, and relative humidity. We find that width of the nares is correlated with temperature and absolute humidity, but not with relative humidity. We conclude that some aspects of nose shape may indeed have been driven by local adaptation to climate. However, we think that this is a simplified explanation of a very complex evolutionary history, which possibly also involved other non-neutral forces such as sexual selection.</p></div

Sample size, mean and standard deviation of phenotypes by sex and population.

<p>Sample size, mean and standard deviation of phenotypes by sex and population.</p

ADMIXTURE Ancestry proportions (k = 6) of genotyped individuals used in Qst–Fst analyses.

<p>Their ADMIXTURE proportions are compared with samples from the HapMap dataset. Each vertical bar in the panels is an individual and the colors represent the proportion of ancestry derived from each of 6 clusters (k = 6). In each panel, the HapMap samples are arranged on the left with a three-letter acronym for the population they are from (e.g. GIH refers to the Gujarati Indians from Houston) listed under them. The samples from our dataset are arranged to the right of the HapMap samples with their population designation (e.g. South Asian) under them.</p

ADMIXTURE ancestry proportions (k = 6) of 1,825 individuals with primarily European ancestry, who were used to estimate <i>h_g²</i>.

<p>Each bar represents an individual and the different colors represent the ADMIXTURE proportions from six clusters (k = 6). The ADMIXTURE proportions for HapMap European populations (CEU and TSI) are shown on the left for comparison.</p

Geographic distribution of parents’ birthplaces.

<p>Individual points represent the birth locations of the parents with a line connecting two parents. A single point indicates that the two parents were born in the same location. Climate values at these locations were used to test for signals of climate adaptation.</p

Sensitivity plots showing critical values of <i>c/h</i>^<i>2</i>.

<p>Change in median Qst of height, skin pigmentation, and nose shape measures as a function of <i>c/h</i>^<i>2</i> is shown as a solid red line. Median Fst is shown as a solid blue line. The lower and upper 95% bounds for Qst and Fst are shown as dashed red and blue lines, respectively. The critical value at which the lower bound of Qst meets the upper bound of Fst is shown on the upper left corner of each plot. Lower critical values indicate the Qst–Fst is more robust to <i>c/h</i>^<i>2</i> assumptions.</p

Results of test for climate adaptation.

<p>Results of test for climate adaptation.</p

ADMIXTURE ancestry proportions (k = 6) of 409 Cape Verdeans, who were used to estimate <i>c</i>.

<p>Each bar represents an individual and the different colors represent the ADMIXTURE proportions from six clusters (k = 6).</p

Qst–Fst results across all populations.

<p>The bootstrapped distribution of Qst–Fst for each phenotype (shown by a violin plot) is compared against the expected value of zero under neutrality (horizontal dashed line). Phenotypes, which exhibit accelerated divergence (using a Bonferronni corrected p-value threshold of 0.0071), are shown in red.</p

Boxplots of phenotypes by population and sex overlaid with the individual data points.

<p>Height is measured in centimeters and melanin index is measured in percentage reflectance (Methods). Linear distances are measured in millimeters (mm) and area are measured in mm². Points are individual observations and the color of the boxplots and points represents sex with blue indicating males and red indicating females.</p