Justin’s Notes

The Empirical Paper Confirms the Premise but Muddies the Picture

2026-07-21 · AI-generated insight

Winegard and Carl offer a tidy theoretical scaffold for race: humans dispersed, faced divergent selection pressures, and were kept apart by oceans, deserts, and mountains long enough to drift and adapt apart. It's a clean model — isolation plus differential selection equals divergence. The ancient DNA paper supplies exactly the kind of hard evidence the model predicts: not just drift, but pervasive directional selection, hundreds of alleles pushed by Darwinian forces, including trait-relevant changes in body fat, cognition, and disease risk.

But read closely, the empirical work complicates the philosophers' neat story in two ways.

First, timing. Winegard and Carl locate the action 75,000 years ago, in the great dispersal. The genomic time-series locates the strongest, most pervasive selection in the past ten millennia — long after the initial split, coinciding with agriculture and dense settlement. Selection isn't mainly the ancient sorting that produced continental groups; it's ongoing, recent, and local. The forces shaping populations kept working after the map was drawn.

Second, and more pointedly, the whole methodological burden of the ancient DNA paper is distinguishing directional selection from "migrations, population structure, or non-adaptive" change. That is precisely the confound the isolation model waves away. West Eurasia, the paper reminds us, is not a sealed compartment behind a mountain range — it's a palimpsest of repeated migration and admixture, and separating adaptation from gene flow required 15,000 genomes and careful statistics.

So the two notes rhyme and grate at once. The empirical paper vindicates the premise that populations diverge under selection, giving Winegard and Carl more than they asked for. Yet it also shows the process is messier, more recent, and more entangled with movement than a story built on stable barriers can accommodate. If selection is this vigorous and this recent even within one region, the boundaries the isolation model treats as fixed are better read as momentary snapshots of a system that never stopped churning.

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

A more plausible conception of race, one that is consistent with how careful philosophers and geneticists use the term, recognises that: When humans began leaving Africa around 75,000 years ago, they dispersed across a much greater range of environments than they had previously inhabited. The humans that settled in different geographic regions subsequently came under different selection pressures (e.g. temperature, seasonality, altitude). Natural barriers such as oceans (e.g. the Atlantic), deserts (e.g. the Sahara) and mountain ranges (e.g. the Himalayas) impeded gene flow between different populations for substantial periods of time. When there is limited gene flow between populations that have come under different selection pressures, we would expect them to gradually diverge from one another over via the processes of genetic drift and natural selection.

— Bo Winegard, Noah Carl Superior: The Return of Race Science—A Review