Changes in Life History and Population Size Can Explain the Relative Neutral Diversity Levels on X and Autosomes in Extant Human Populations
Changes in life history and population size can explain the relative neutral diversity levels on X and autosomes in extant human populations Guy Amstera,1, David A. Murphya, William R. Milligana, and Guy Sellaa,b,c,1 aDepartment of Biological Sciences, Columbia University, New York, NY 10027; bDepartment of Systems Biology, Columbia University, New York, NY 10032; and cProgram for Mathematical Genomics, Columbia University, New York, NY 10032 Edited by Brian Charlesworth, University of Edinburgh, Edinburgh, United Kingdom, and approved July 7, 2020 (received for review October 22, 2019) In human populations, the relative levels of neutral diversity on generally—the effects of selection at linked sites should be stronger the X and autosomes differ markedly from each other and from on the X (ref. 18, but see ref. 3). To evaluate these effects empiri- the naïve theoretical expectation of 3/4. Here we propose an ex- cally, several studies have examined how diversity levels on the X and planation for these differences based on new theory about the autosomes vary with genetic distance from putatively selected re- effects of sex-specific life history and given pedigree-based esti- gions, for example from coding and conserved noncoding regions mates of the dependence of human mutation rates on sex and (11, 13–15, 19, 20). In most hominids, including humans, such age. We demonstrate that life history effects, particularly longer comparisons confirm the theoretical expectation that selection at generation times in males than in females, are expected to have linked loci reduces X:A diversity ratios (11, 15, 20). They further had multiple effects on human X-to-autosome (X:A) diversity ra- suggest that the effects are minimal sufficiently far from genes (11, tios, as a result of male-biased mutation rates, the equilibrium X:A 19), thereby providing an opportunity to examine the effects of ratio of effective population sizes, and the differential responses other factors shaping X:A diversity ratios in isolation by considering to changes in population size.
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