K. Harris1,2 ; R. Nielsen2,3
1) Stanford University, Stanford, CA; 2) University of California Berkeley, Berkeley, CA; 3) Center for Bioinformatics, University of Copenhagen, Copenhagen, Denmark.
Founder effects and bottlenecks can damage fitness by letting deleterious alleles drift to high frequencies. This almost certainly imposed a burden on Neanderthals and Denisovans, archaic hominid populations whose genetic diversity was less than a quarter of the level seen in humans today. A more controversial question is whether the out-of-Africa bottleneck created differences in genetic load between modern human populations. Some previous studies concluded that this bottleneck saddled non-Africans with potentially damaging genetic variants that could affect disease incidence across the globe today (e.g. Lohmueller, et al. 2009; Fu, et al. 2014), while other studies have concluded that there is little difference in genetic load between Africans and non-Africans (e.g. Simons, et al. 2014; Do, et al. 2015). Although previous studies have devoted considerable attention to simulating the accumulation of deleterious mutations during the out-of-Africa bottleneck, none to our knowledge have incorporated the fitness effects of introgression from Neanderthals into non-Africans. We present simulations showing that archaic introgression may have had a greater fitness effect than the out-of-Africa bottleneck itself, saddling non-Africans with weakly deleterious alleles that accumulated as nearly neutral variants in Neanderthals. Assuming that the exome experiences deleterious mutations with additive fitness effects drawn from a previously inferred gamma distribution, we predict that the fitness of the average Neanderthal was about 50% lower than the fitness of the average human, implying the existence of strong selection against early Neanderthal-human hybrids. This is a direct consequence of mutation accumulation during a period of low Neanderthal population size that is thought to have lasted ten times longer than the out-of-Africa bottleneck (Pruefer, et al. 2014). Although our model predicts some transmission of deleterious Neanderthal variation to present-day non-Africans, it also predicts that many Neanderthal alleles have been purged away, depleting conserved genomic regions of Neanderthal ancestry as observed empirically by Sankararaman, et al. (2014). Our results imply that the deficit of Neanderthal DNA from functional genomic regions can be explained without the action of epistatic reproductive incompatibilities between human and Neanderthal alleles.