Are Population Differences Clinal?

Population differences are not perfectly continuous. Genetic distances between clusters are larger than what would be predicted from geographic distance:
https://sci-hub.ru/https://doi.org/10.1371/journal.pgen.0010070
The boundaries also line up with geographic features like mountains + deserts + bodies of water + etc:
https://sci-hub.ru/https://doi.org/10.1093/molbev/msz280

How large are the barriers? In the first paper, when predicting genetic distances from geographic distance, incremental R^2 is .0153 when adding either the sahara or himmalaya migration barriers. Keep in mind, this is incremental R^2, and there is collinearity between distance and barrier crossings, as the correlation between geographic and genetic distance is worldwide and benefits from said migration barriers (i.e. is not within-region). How much is this effect worth in terms of raw units? Authors report that crossing either barrier is worth 3,100 KM, roughly the distance between Moscow and Granada. Now, this 3100 KM figure has the same problem to it. When we look at figure 6, we can clearly see that this is not the effect in terms of within-population KM; properly corrected, crossing the sahara is worth about 10,000 KM (red dots are distances from within-population comparisons, blue dots are between-population comparisons; extrapolate a regression line from the red dots until it reaches the genetic distance of the lowest blue dots, and it's about 10,000 KM of geographic distance that this happens).

Or, in other terms, crossing the Sahara is worth roughly half the earth's circumference in terms of genetic distance.