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: http://dx.doi.org/10.1101/052084
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.