The above figure is from a paper in Proceedings B which shows a Dutch data set from right after World War 2. Controlling for several variables taller men and average height women have maximal fertility. The authors contrast the results from the United States, where it seems that shorter women and average height men have maximal fertility. This is kind of a big deal. The reasons for why the Dutch, who were the shortest Europeans two centuries ago, are the tallest nation in the world today, have been a matter of public discussion for over ten years (see this article in The New Yorker).
In the 19th century American whites were far taller than Europeans. European elites who toured the United States were reputedly shocked by the fact that American yeoman farmers were no shorter than them, as was the norm among the peasant classes in their lands of origin. This lack of size differential due to surplus of land in the early American republic was often compared with the relative social egalitarianism of the United States, along with its broader democratic ethos.
Obviously things have changed since the 19th century. The Malthusian conditions which ground down Dutch peasants in the 18th century no longer applied in the 20th century, and definitely not in the 21st century. Modern agricultural techniques mean that Northern Europeans are no longer nutritionally constrained. Not only that, but one could argue that today Northern European societies are more egalitarian than the the United States. Naturally there has been a focus on the environmental factors which might have shaped this difference in the distribution of heights between Northern Europeans and American whites of Northern European heritage.
But there are some biological issues which are likely relevant. Average human size, including height, actually peaked in the wake of the Last Glacial Maximum, ~20,000 years ago. Some of this is likely due to the nutritional changes enforced by the Neolithic Revolution, but the decrease in sizes predate that. Likely standard dynamics common to mammals, such as Bergmann’s rule, have also affected humans. The recent increases in height across the developed world have still not produced a population as imposing as that of late Pleistocene humans. Second, over the past few years plenty of genomic work has now argued for selection on height in Europe, explaining why there are small but persistent differences between populations in the north and south. Ancient DNA analysis has now confirmed this broadly result, as populations diverged in size due to local ecological pressures.
These new results suggest that selection is driving change in allele frequencies which control for height even today among the Dutch. The methods were pre-genomic. Basically they tracked fertility of individuals along with a bunch of variables, including height. There was no need to go into genomic details because there is a wide body of research which indicates that 80-90% of the variation in height in developed societies is controlled by variation in genes. In other words, height is a highly heritable trait. As per the breeder’s equation all you need to change a trait value for a highly heritable trait is selection:
Selection × Heritability = Response to selection
If there is selection but no heritability, then there is no response. If there is heritability but not selection, then there is no response. In this case the heritability is well known, and now they have shown selection in the Dutch population as an implication of differential fertility that tracks this heritable variation.
This framework is true for quantitative traits more generally. I wouldn’t be surprised to see that there is a fair amount of evolution going on in modern human populations, which large and robust data sets might be able to capture in the near future.
Citation: Does natural selection favour taller stature among the tallest people on earth? Gert Stulp, Louise Barrett, Felix C. Tropf, Melinda Mills, Proc. R. Soc. B: 2015 282 20150211; DOI: 10.1098/rspb.2015.0211. Published 8 April 2015
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