I have little time to post this morning, but I call your attention to a really dreadful piece of science journalism at CNN.
It refers to a new paper in PLoS Genetics by Arslan Zaidi et al. (reference below, free access) describing how natural selection based on climate (temperature and humidity) may have molded the nose shape of populations of humans in different parts of the world (I’d call these groups “races” but I’d get my tuchas chewed for that). Here’s the paper’s abstract:
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.
We know of course that selection has been responsible for many local adaptations in humans (see here for a summary), so this is nothing new, though it’s an interesting piece of work. Sexual selection may also be responsible, as the authors say, though it’s not the kind of sexual selection that leads to sexual dimorphism (in this case, to any different nose shape between males and females).
Now look at the CNN headline reporting this result (clicl on screenshot to go to article):
The headline (which probably wasn’t written by author Susan Scutti) gets natural selection completely wrong, implying that it’s something that involves genetics and the selective pressure itself as different and separable entities. Of course we know that if climate-based natural selection caused evolutionary changes in nose shape, those changes would have to be genetic! Climate, after all, is not some Lamarckian force that molds an nose shape that gets passed on without the intervention of genes. Climate cannot evolutionarily mold nose shape, at least in a heritable way, without genes!
The authors of the PLoS paper discuss the differential replication of gene forms (alleles) based on their contribution to well being mediated through nose shape. That process involves both climate and genes interacting in a nonrandom way. The headline is grossly misleading, though Scutti herself seems to get it pretty much right in the article (but see below).
I was sent this headline by Richard Dawkins, who was just as appalled as I was. Here’s what he wrote (quoted with permission):
But if you read the CNN story it turns out, as you would expect, that the study shows natural selection, in different climates, has shaped the nose. In what possible sense is that NOT genetics?
Well, as I said, the story itself is okay, but the headline is horrible.
But there’s still a bizarre bit of Scutti’s story. Here’s what she reports further:
So it’s easy to understand why many people, past and present, “have this sense that human populations are very distinct and have been separated for a long time,” said Mark D. Shriver, lead author of the study and a professor of anthropology at Penn State University. Still, he noted, “human populations have always split and come back together, split and come back together, so there’s no separate origin.”
In fact, genetic differences between various population groups is not that great. Using noses as just one example, said Shriver, “the surface, the appearance of people in different populations is much greater than what the genetic differences show on average.”
There are three things wrong here. Yes, human populations have exchanged migrants for a while—ever since forms of transportation came about. And this process is accelerating. But the statement that “populations have always split and come back together” and that “there’s no separate origin” is flatly wrong. Populations don’t meld completely and then split again: they simply send individuals back and forth, and historically have maintained many of their genetic and phenotypic differences.
Further, there is a separate origin for many populations. Native Americans, including those in North and South America, came over the Bering Strait about 15,000-20,000 years ago. They did not repeatedly fuse back to their Eurasian ancestors and separate again. Ditto with Polynesians, the indigenous people of Australia, and so on. I have no idea what Shriver is talking about.
In the second paragraph, I am simply confused by Shriver’s statement that “the surface, the appearance of people in different populations is much greater than what the genetic differences show on average.” What genetic differences is he talking about? For surely there are substantial genetic differences involved in the nose shape differences, be they small differences in the frequency of alleles at many loci, big differences in the frequency of alleles at a few loci, or a mixture.
Perhaps he is saying that the allele frequency differences in nose shape (and other distinguishable traits among populations) are greater than those of the “average” gene, including “neutral” sites where different gene forms make no difference in appearance, phyisology, or so on. That would be a nod to the fact that wholesale genetic differentiation of our genome hasn’t had time to evolve over the 60,000-100,000 years since we spread out over the globe from Africa. But if Shriver meant that, why didn’t he say it more clearly, and why didn’t Scutti ask him to clarify it? After all, there’s no good way to compare the differences in the configuration of a character like the nose with the frequency differences of genes in the genome. They are apples and oranges.
This is the kind of dire science reporting, with the journalist not asking the right questions (not atypical for science journalists who haven’t had extensive training in science), and therefore the body of the article (and the headline!) remaining confusing. It was confusing for me, and I’m an evolutionary biologist.
Zaidi A. A. et al. 2017. Investigating the case of human nose shape and climate adaptation. PLoS Genetics http://dx.doi.org/10.1371/journal.pgen.1006616.