“First, the smaller effective population sizes of non-African groups results in greater efficacy of background selection. As random genetic drift tends to increase the frequency of deleterious alleles, powerful negative selection is given opportunity work against that region of the genome. This results in more background selection as adjacent genomic regions are impacted.”

In larger populations, natural selection is more effective (there are fewer effectively neutral mutations — those with -1 less than Ns less than 1), which will lead to more fixations of slightly beneficial mutations and loss of slightly deleterious mutations due to selection. The background selection you’re talking about here (i.e., what’s occurring in small populations) won’t leave as large of a signature around the deleterious allele because it’s an older variant, and recombination would have decreased LD around that site. Is this idea expanded on in the paper? The fact that larger populations have smaller haplotype blocks, more segregating variation, etc is what’s important, right?