Frederick
Morgan – Off for the Honeymoon (Wikicommons) Over the past 2,000 years, the
British gene pool has shifted toward alleles that favor lighter hair, sunburn,
and educational attainment. Was this because high-status men tended to mate
with blonder, fairer women?
Have
we evolved over the past two thousand years? Until recently, the answer was thought
to be 'no.' Cultural evolution took over from genetic evolution around the time
farming took over from hunting and gathering, some ten thousand years ago, thus
putting our ancestors on a path to increasing social complexity: sedentary
living, growth of towns and villages, formation of states, trade and
specialization of labor, and so on. It was culture that changed during recorded
history, not genes.
Well,
things are not that simple. Genes and culture have coevolved with each other.
Yes, culture has been changing rapidly over the past ten thousand years. But so
have genes. During that time, our genetic evolution has been driven by adaptation
not only to natural environments but also to cultural environments. Increasingly
so. We live more and more in cultural environments of our making (Chen et al.,
2016; Cochran and Harpending 2009; Hawks et al. 2007).
In
what ways have we changed genetically during the past ten thousand years? In
the ways we digest food. With the shift to dairy farming, and the resulting
increase in milk consumption by adults, natural selection favored those who could
digest milk sugar, an ability previously confined to infants.
We
have also changed in the ways we think and behave. That kind of evolution is
not difficult. A few point mutations may alter a behavior by changing its
timing, its intensity, or its threshold of stimulation. Other alterations have
been much more polygenic. Cognitive ability, for instance, seems to have
increased through mutations at many genes, with each mutation causing only a tiny
fraction of the increase.
Because
recent evolutionary change has so often been polygenic, we need to examine it in
relation to many genetic variants spread over the entire genome, i.e., by means
of genome-wide association studies. Such studies can take many forms. A recent
one, proposed by Stern et al. (2020), may be better than earlier versions,
particularly in avoiding biases due to population structure and population
stratification.
I
nonetheless have a few reservation about this proposed method:
1.
Population stratification can be a factor in evolutionary change. Let's take the
work of Gregory Clark on the growth of the English middle class. He found it
grew steadily from the twelfth century onward, its descendants not only growing
in number but also replacing the lower classes through downward mobility. By
the 1800s its lineages accounted for most of the English population. Parallel
to that demographic growth, English society became more and more middle class
in its values. "Thrift, prudence, negotiation, and hard work were becoming
values for communities that previously had been spendthrift, impulsive,
violent, and leisure loving" (Clark 2007, p. 166). Isn't that evolutionary
change through population stratification? Or am I missing something?
2.
The new method can reveal only evidence of directional selection. It thus fails
to capture other interesting forms of selection, like diversifying selection.
How the British have
evolved over the past 2,000 years
Stern
et al. (2020) used their method to study how the British population has evolved
over the past two thousand years. They found increases in the prevalence of lighter
hair, in tanning and sunburn, in age at first birth, in bone mineral density,
and in the risk of type 2 diabetes. They also found decreases in the risk of neuroticism
and in the risk of high glycated hemoglobin levels.
Some
of these changes correlate with each other. In such cases, we should step back
and try to identify the common cause.
Lighter hair, more
sunburn ... and higher educational attainment
Over
the past 2,000 years, the British gene pool has shifted toward alleles that
favor lighter hair, sunburn, and educational attainment. These changes in
allele frequency correlate with each other, so what, exactly, was driving the overall
change?
There
is genetic linkage between light hair and pale skin, but it's weak. In fact, pale
skin often coexists with dark hair. Moreover, we still have to explain the link
to educational attainment. The common cause for all three changes may have been
sexual selection mediated by social class. In other words, high-status men
tended to mate with blonder, fairer women.
This
form of sexual selection was observed in a Japanese study on social class and
skin color. Upper-class men were shown to be fairer-skinned than lower-class
men, even when the latter were factory workers and not farmers and even though
the measurements were taken on unexposed skin. Wealthier men have a wider range
of prospective brides and can thus choose the fairest women, for "skin
color has long been regarded, by the Japanese, as one of the criteria for
evaluating physical attractiveness, especially in young females" (Hulse
1967). Similarly, in India "[w]ealthy landowning families often have a
tradition of seeking light-skinned brides among poorer members of their
subcaste. It is very common to find a high concentration of lighter-skinned
people among established land-owning families" (Béteille 1967).
Darwin
discussed this sexual selection with reference to English social classes:
Many persons are convinced, as it appears to me with justice, that our aristocracy, including under this term all wealthy families in which primogeniture has long prevailed, from having chosen during many generations from all classes the more beautiful women as their wives, have become handsomer, according to the European standard, than the middle classes; yet the middle classes are placed under equally favorable conditions of life for the perfect development of the body. (Darwin 1936[1888], p. 892)
Until
the 20th century, higher social status meant higher fertility (Clark 2007). Thus, the
physical and mental characteristics of the upper and middle classes tended to
displace those of the lower class.
Higher risk of Type
2 diabetes and glycated hemoglobin
Why
would natural selection favor type 2 diabetes? Isn't diabetes harmful? It is,
in a modern environment that lets you ingest calories almost without limit.
That wasn't the case in Britain for most of the past two thousand years. During
that time, food was scarce for most people, and natural selection favored the
ability to get as many calories as possible out of our food.
Less neuroticism
This
evolutionary change may be related to the demographic success of the middle
class and associated mental and behavioral traits, particularly lower time
preference and higher future orientation. The nascent English middle class
valued being “calm, cool, and collected,” as opposed to reacting emotionally to
negative outcomes.
References
Béteille,
A. (1967). Race and descent as social categories in India. Daedalus 96(2): 444-463.
Chen,
C., R.K. Moyzis, X. Lei, C. Chen, and Q. Dong. (2016). The encultured genome:
Molecular evidence for recent divergent evolution in human neurotransmitter
genes. In: J.Y. Chiao, S.-C. Li, R. Seligman, and R. Turner, Eds, The Oxford handbook of cultural neuroscience.
New York, NY: Oxford University Press, 315-336.
Clark,
G. (2007). A Farewell to Alms. A Brief
Economic History of the World, 1st ed.; Princeton University Press:
Princeton.
Cochran,
G., and H. Harpending. (2009). The 10,000
Year Explosion: How Civilization Accelerated Human Evolution. Basic Books.
Darwin,
C. (1936 [1888]). The Descent of Man and
Selection in relation to Sex. reprint of 2nd edition, The Modern Library,
New York: Random House.
Hawks,
J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent
acceleration of human adaptive evolution. Proceedings
of the National Academy of Sciences, 104(52), 20753-20758.
Hulse,
F.S. (1967). Selection for skin color among the Japanese. American Journal of Physical Anthropology 27(2): 143-156.
Hysi,
P.G., A.M. Valdes, F. Liu, N.A. Furlotte, D.M. Evans, V. Bataille, et al.
(2018). Genome-wide association meta-analysis of individuals of European
ancestry identifies new loci explaining a substantial fraction of hair color
variation and heritability. Nature
Genetics 50(5): 652-656.
Morgan,
M.D., E. Pairo-Castineira, K. Rawlik, O. Canela-Xandri, J. Rees, D. Sims, A.
Tenesa, and I.J. Jackson. (2018). Genome-wide study of hair colour in UK
Biobank explains most of the SNP heritability. Nature Communications 9: 5271
Neel, J. V. (1962). Diabetes mellitus:
a 'thrifty' genotype rendered detrimental by 'progress'? American Journal of Human Genetics 14: 353-362.
Stern,
A.J., L. Speidel, N.A. Zaitlen, and R. Nielsen. (2020). Disentangling selection
on genetically correlated polygenic traits using whole-genome genealogies
bioRxiv 2020.05.07.083402
