A piece by Carl Zimmer in Thursday’s New York Times called my attention to a new paper in the Proceedings of the Royal Society Series B (reference and link to download below) by Gert Stulp et al. on the remarkable height of Dutch people and some evidence that natural selection (probably via sexual selection) is acting to promote their vertical ascent. (Stulp himself is 6′ 7″.)
When I visited Amsterdam and Groningen a few years ago, I was immediately struck at how tall the Dutch were; I was altitudinally challenged when talking to them. The new paper by Stulp et al. not only shows that they’re the tallest people in the world, but that they’re getting taller. And some of that increase may be due to genetic evolution. For a multi-generation study shows that, at least among men, the tallest Dutch people have the most children.
Here’s the present situation as described the paper (I’ve translated the cm into inches):
When it comes to height, the Dutch have a remarkable history. In the mid- eighteenth century, the average height of Dutch (military) men was approximately 165 cm [5 feet 5 inches]. This was well below the average for other European populations, and very much shorter than the average height of men in the United States, who towered over the Dutch by 5–8 cm [2-3 inches]. Dutch men are now the tallest in the world, having grown by approximately 20 cm [8 inches!] over the last 150 years. By contrast, male height in the United States has increased by only 6cm [2.3 inches] across the same time span. Equivalent differences in height are also observed between The Netherlands and other European countries. Indeed, it is notable that, while the secular trend in height has slowed or stopped in most North-European countries, it has continued for much longer among the Dutch, with the available evidence suggesting it has begun to slow only very recently.
Or, as Carl Zimmer notes:
Since 1860, average heights have increased in many parts of the world, but no people have shot up like the Dutch. The average Dutchman now stands over six feet tall. And while the growth spurt in the United States has stopped in recent years, the Dutch continue to get taller.
That increase of 8 inches in only three or four generations is remarkable, and is simply too rapid to be explained by natural selection alone. As the authors note, it’s plausible that most of this increase in height is due to improvements in health and diet, including the advent of universal healthcare and a greater equality of income than seen in most other countries, including the height-challenged U.S. The authors mention consumption of dairy products; I’d add to that herring and french fries with mayonnaise! Of course, one could in principle test how much of that height increase was due to genes by simply rearing Dutch people in a controlled, equalized environment along with people of other nations over the past 150 years, but of course that’s not practical. But one could at least compare the present height of Dutch reared at home versus those brought up in other cultures where they don’t have the supposed height-increasing factors of healthcare and cheese. We have no data on that, either.
The authors, though, tried to parse out the action of genetic evolution by looking at the offspring of Dutch people of different heights in a three-generation health study lasting from 1935 to 1967, and involving over 90,000 subjects. Lots of demographic data were collected, including fertility, age of puberty and menopause, whether or not individuals were in a long-term relationship, health, education, and income.
It’s a complicated study, so I’ll just give the most notable result: the corrected correlation between male and female height (expressed as standard deviations above and below the mean height) and number of children, a good measure of evolutionary fitness. Here are the graphs from Figure 1. I had to cut and paste in the scale for the X axis, and can’t line therm up well, but the five ticks on the X-axis (not including the origin) go from -2 to 2 standard deviations (0 is the average, and is the middle tick on the scale). About 16% of Dutch people exceed one standard deviation above the mean; about 2.5% exceed two standard deviations.
As you see, for males, on the left graph, the number of children produced is generally higher for males above the mean ( mean = 0, middle tick); males about 1 standard deviation above the average have the most children, and the number falls off beyond that. That implies that there is natural selection for taller males: the population should, if there is genetic variation for height, be increasing in height. (Ignore the “no. of children with current partner” line for the time being; you can see a discussion of that in the paper.)
Now the authors claim that they can’t say this is natural selection, for their definition of “natural selection” is “”differential reproduction of individuals with different genetic constitutions“, and we don’t know if those taller males who leave more kids have at least part of their height advantage based on genes. (It’s likely to be true, though, for there is substantial genetically based variation in height among other populations that have been examined). But if you construe natural selection, as some do, as “differential reproduction of individuals with different traits,” then this is indeed natural selection for taller males. But whether that selection causes evolution depends on the genetic basis of height variation. In either case we lack the genetic evidence to say that the Dutch male population is evolving to be taller. But if the data are suggestive.
For females, however, women of average height have the most children, and so there would be no direct selection on women to be taller. The graph on the right, which shows “fitness” related to deviation from the mean, is a classic example of stabilizing selection, in which individuals with the average trait value have the most offspring, and those on either extreme have fewer. That is a form of “pruning-away” selection that keeps the trait at a constant value.
I suspect, and this is suggested by the authors, that if there is selection, it’s sexual selection: taller males are more attractive as mates. That would itself lead women to become taller over time as well, for the genes that make males tall would also tend to make female offspring taller as a byproduct. (The authors give no data on whether female height has also increased in recent years.)
There is much more in the paper, but I’ll just add two points. First, this relationship is independent of other variables like education and income, so the correlation is unlikely to be spurious, say if taller males had more kids simply because they were better nourished, or provided more resources for their kids.
Second, why is this happening in the Dutch but not in other populations? (Well, it could, since there aren’t many similar studies, but work in the U.S. shows that men of average height and women of below-average height have more children.) The authors speculate on the reasons for the difference between the Netherlands and U.S., but it’s still not clear.
What is clear is that there is phenotypic selection for bigger males in the Dutch population, and that may well be responsible for part of the the striking change in height of Dutch males over the past 150 years. But surely most of that increase is due to cultural rather than genetic evolution: something in the Dutch culture that makes people taller. My theory: raw herring!
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Stulp, G., L. Barrett, F. C. Tropf, and M. Mills. 2015. Does natural selection favour taller stature among the tallest people on earth? Proc Roy Soc B. 282, http://dx.doi.org/10.1098/rspb.2015.0211
