The human and chimpanzee X chromosomes are less divergent than expected based on autosomal divergence. This has led to a controversial hypothesis proposing a unique role of the X chromosome in human-chimpanzee speciation. We study incomplete lineage sorting patterns between humans, chimpanzees and gorillas to show that this low divergence is entirely due to megabase-sized regions comprising one-third of the X chromosome, where polymorphism in the human-chimpanzee ancestral species was severely

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... educed. Background selection can explain 10% of this reduction at most. Instead, we show that several strong selective sweeps in the ancestral species can explain this reduction of diversity in the ancestor. We also report evidence of population specific sweeps in extant humans that overlap the regions of low diversity in the ancestral species. These regions further correspond to chromosomal sections shown to be devoid of Neanderthal introgression into modern humans. This suggests that the same X-linked regions that undergo selective sweeps are among the first to form reproductive barriers between diverging species. We hypothesize that meiotic drive is the underlying mechanism causing these two observations.