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Humans have about 2 new deleterious mutations per birth. We wanted to know what happens when deleterious mutation rates are this high. We found that background selection isn’t very important at low mutation rates, and was far more important than we expected at high mutation rates. 2/5

The usual theory assumes that background selection is only relevant to sites that are closely linked to the deleterious mutations along the chromosome. Our results are better explained by a model in which the sheer volume of deleterious mutation load rules too many individuals out of the “effective” population size, with that load mostly occurring at unlinked sites. 3/5

The linked sites theory predicts weaker background selection with large selection coefficients. We found the opposite, as predicted by the unlinked sites theory. Results do not depend on the census population size. This means there are issues with the common pop gen practice of “scaling” parameters to be more convenient while keeping products like sN constant. 4/5

Background selection is often studied by simulating just the linked section of the genome. Our findings show that this can give the wrong results. Simulating the whole genome is computationally expensive, but possible. 5/5