May 08, 2016

Natural selection in Britain during the last 2,000 years

The latest ancient DNA studies from the British Isles (Schiffels et al and Martiniano et al. and Cassidy et al.) support continuity over the last 2,000 years. Sure, there were continued migrations like the arrival of the Anglo-Saxons, but these were very similar groups in the grand scheme of things.

But, while ancestrally the modern Briton is probably a descendant of the Britons of 2,000 years ago with some admixture from similar continental European groups, he is not the same, as (apparently) substantial genetic adaptation has continued to operate in Britain over the same period. A new preprint by Field, Boyle, Telis et al. makes the case for adaptation in a variety of traits in the ancestors of Britons over this period. Mind you, the genetic underpinnings of many important human traits known to have high heritability are currently unknown, but there is little doubt that selection would have affected traits beyond those detected in this study. I am quite curious to see whether the striking efflorescence of cultural achievement in Britain over the last half millennium could have (at least in part) a genetic underpinning.

Depigmentation is a trait whose genetic architecture is as well as understood as any. The results of this study might surprise writers of decades and centuries past who supposed that the spectrum of pigmentation of modern Europeans was the result  of admixture-in varying measure- between Xanthochrooi and Melanchrooi races of primordial antiquity. All indications seem to be that depigmentation of hair, skin, and eyes did not co-occur in such a hypothetical race, but rather in different parts of the Caucasoid range, only reaching a high combined frequency in northern Europe to form the distinctive physical type that is distinctive of the natives of that region. It would be quite interesting to see how these traits evolved in Fennoscandia and the Baltic, regions that sport an even higher depigmentation than the British Isles. Traditionally, these areas were viewed as refuges of the Xanthochrooi but it may very well turn out to be that for whatever reason selection has acted in that area as well, as it did in the Eastern European plain where rather dark Bronze Age steppe groups gave way to rather light pigmented living eastern Slavs.

bioRxiv doi:

Detection of human adaptation during the past 2,000 years

Yair Field, Evan A Boyle, Natalie Telis, Ziyue Gao, Kyle J Gaulton, David Golan, Loic Yengo, Ghislain Rocheleau, Philippe Froguel, Mark I McCarthy, Jonathan K Pritchard

Detection of recent natural selection is a challenging problem in population genetics, as standard methods generally integrate over long timescales. Here we introduce the Singleton Density Score (SDS), a powerful measure to infer very recent changes in allele frequencies from contemporary genome sequences. When applied to data from the UK10K Project, SDS reflects allele frequency changes in the ancestors of modern Britons during the past 2,000 years. We see strong signals of selection at lactase and HLA, and in favor of blond hair and blue eyes. Turning to signals of polygenic adaptation we find, remarkably, that recent selection for increased height has driven allele frequency shifts across most of the genome. Moreover, we report suggestive new evidence for polygenic shifts affecting many other complex traits. Our results suggest that polygenic adaptation has played a pervasive role in shaping genotypic and phenotypic variation in modern humans.



Krefter said...

Well put. Our Iron Age/Roman-era Briton genomes had a modern-like high frequency of the lactose persistence but the they had a much higher frequency of Brown eye alleles(67% vs 22%) than modern Brits. I don't think that's a coincidence. Ancient DNA(also from other parts of Europe) suggest 2000 BC-0 AD is when Lactose persistence was selected for the most.

The SDS method is interesting. They found the highest selection for the same traits as ancient DNA does. That's pretty amazing because the SDS method relies on modern DNA.

Athena said...

The Scottish are as depigmented as the Scandinavians (albeit - I would guess - more on the red hair end of depigmentation than blonde. Still good for vit D synthesis, and perhaps sexually selected...).

Clay said...

Sexual selection seems like a likely culprit for the increase of blonde hair. At least it would be rare and therefore valued. How does this study distinguish between sexual and natural selection?

pconroy said...

IMO, Most Irish and Scots are more depigmented than the average Scandinavian.
Like, I cannot tan at all, only burn. I have gotten sunburnt in March in NYC a few times.

apostateimpressions said...

Is the tendency toward depigmentation driven by northern climate? If so, is the predominance of depigmentation in the north a foregone conclusion? Like, even if Asians and Africans are added to the British gene pool through mass immigration during the late capitalist period, eventually over centuries and millennia, depigmentation will continue to spread and those who lack the adaptations will simply die out? Would we then be left with a depigmented Afro-Asiatic-European population or would Afro-Asiatic genes gradually be got rid of, like Neanderthal genes?

Tobus said...

I think the initial environmental selection for depigmentation in Northern climes is no longer active - improvements in food, clothing, health, habitat, medicine etc. in the last few hundred years have replaced any survival benefit in being lighter skinned in the cold. Dark skin still offers a few advantages - it's harder for germs to penetrate (epidermal barrier), it reduces skin cancer rates by a factor of 20x and it ages better.

I doubt though any of these factors will affect selection in a modern environment (selection relies on death before children), and I'd imagine that people will tend towards an average (ie brown) skin colour over the next few thousand years as the various alleles spread evenly throughout the population. Everyone will probably end up "mixed race" although I could invisage that the very pale, non-tanning phenotype might disappear.

Joyce said...

Seriously doubt sexual selection, cows probably helped the very light skinned people with Vitamin D, because they do not take sun well. For the same serum concentration of D, different people might well have different D metabolic efficiency. If low light was the selective pressure for very pale skin, Arctic people would have become transparent. Besides sunburn, snow reflects sunlight a lot and the retina is sensitive to UV damage. Any research ever done about Vitamin D deficiency? Men are not known to be very picky, now and then, unless they are the 0.1%. If they have done Neanderthals and Denisovans, brown eyed women would not have stopped them.

Kurti said...


"Would we then be left with a depigmented Afro-Asiatic-European population or would Afro-Asiatic genes gradually be got rid of, like Neanderthal genes?"

As far as I remember Caucasoids in general have the depigmentation genes around the 80 to 100% range.

No one knows what would happen to those who don't have the gene. East Eurasians have their own depigmentation variances. And improvements in medicine will probably help those who don't have it to survive.

However I don't believe the adaption process is inactive. IMO it isn't as active as in the past but still active. It's not like 2000 BC to 0 AD the people didn't had clothes. They sure had also they had access to food with Vitamin D however there seems to be a something special about the Vitamin D we get through UV radiation. For example My sister was diagnosed with some slight Vitamin D deficiency, despite as the doctor said she should have enough from her daily diet. I have my own theory that especially people from more southern countries are used to higher Vitamin D admisson through UV-radiation and the sudden lack of this excessive radiation triggers something. So some people are genetically more vulnerable to Vitamin D deficiency

Kurti said...

And the strange thing is, my sister is quite pale pigmented, yet she doesn't seem to get enough Vit D what made the Doctor (who is rarely on duty cause he is Prof. at a bigger medical institute)even more curious about her case.