Justin’s Notes

Selection Runs Vertically Through Time, Not Just Horizontally Across Maps

2026-08-04 · AI-generated insight

Winegard and Carl argue that racial classification tracks something real: visible differences that "reflect their divergent geographical origins." The implicit model is horizontal — populations spread across space, diverging as they go. The ancient DNA study complicates this by adding a vertical axis. Within a single region, West Eurasia, allele frequencies for complex traits shifted by a full standard deviation on the scale of modern variation in just ten millennia. The people who lived where today's Europeans live were, genetically, substantially different from their own descendants.

This means the divergences Winegard and Carl attribute to geography are partly a snapshot of an ongoing temporal process. Two populations can differ not because they went to different places but because selection kept working on them after they arrived. Geography is not destiny so much as a starting condition for locally varying Darwinian pressure — and that pressure has been surprisingly strong and recent, contradicting the older picture of human evolution as mostly finished.

The more pointed rhyme is a caveat the review would do well to absorb. The ancient DNA authors stress that their trait predictions — body fat, schizophrenia, cognition — "were measured in industrialized societies, and it remains unclear how these relate to phenotypes that were adaptive in the past." The genetic signal is real; its phenotypic meaning across time is not transparent. That warning applies with equal force across space. Skull morphology may classify populations at 80% accuracy, but classifiability is a claim about statistical structure, not about what any given allele did in the environment that selected it.

Woven from these notes

Ancient DNA has transformed our understanding of population history, but its potential to reveal as much about human evolutionary biology has not been realized because of limited sample sizes and the difficulty of distinguishing sustained rises in allele frequency increasing fitness—directional selection—from shifts due to migrations, population structure, or non-adaptive purifying or stabilizing selection. Here we present a method for detecting directional selection in ancient DNA time-series data that tests for consistent trends in allele frequency change over time, and apply it to 15,836 West Eurasians (10,016 with new data). Previous work has shown that classic hard sweeps driving advantageous mutations to fixation have been rare over the broad span of human evolution8,9. By contrast, in the past ten millennia, we find that many hundreds of alleles have been affected by strong directional selection. We also document one-standard-deviation changes on the scale of modern variation in combinations of alleles that today predict complex traits. This includes decreases in predicted body fat and schizophrenia, and increases in measures of cognitive performance. These effects were measured in industrialized societies, and it remains unclear how these relate to phenotypes that were adaptive in the past. We estimate selection coefficients at 9.7 million variants, enabling study of how Darwinian forces couple to allelic effects and shape the genetic architecture of complex traits.
Ancient DNA reveals pervasive directional selection across West Eurasia
The primary reason that natural philosophers began to classify humans into different races is that human populations look different from one another. Their skin colors, hair textures, facial structures, and stature all differ, often in predictable ways. Furthermore, these differences reflect their divergent geographical origins. In fact, researchers can classify human variation by continent quite accurately using only data from the human skull. (They are able to correctly classify human skulls into black and white Americans with about 80% accuracy, using only two variables.) Therefore, despite the common charge that racial classification is ipso facto racist, and that most Enlightenment typologies of human variety were motivated by prejudices and a desire to rationalize imperial domination, ordinary people and Enlightenment philosophers also took to classifying human differences for the mundane reason that such differences actually exist.
— Bo Winegard, Noah Carl Superior: The Return of Race Science—A Review