Strong reproductive isolation between humans and Neanderthals inferred from observed patterns of introgression

Recent highly publicized studies suggest that interbreeding between modern humans and Neanderthals accounts for 1 to 4% of the genome of present-day non-Africans. Using the simulation program SPLATCHE2, Currat and Excoffier modeled differing scenarios of early human expansion in order to investigate the demographic history of early humans and Neanderthals. They found that 2-3% Neanderthal ancestry in non-Africans can be accounted for by interbreeding rates of less than 2%, and suggest that this low rate may be an effect of such gene flow barriers as avoidance of interspecific mating and/or low fitness of hybrids. One scenario with a hybrid success rate of only 5% led to 80% Neanderthal introgression in modern Eurasians, further suggesting that interbreeding was very limited. The authors also suggest that different populations of Neanderthals may have interbred with Europeans and Asians after their division, which could be tested by comparing the specific genomic elements shared between Europeans and Neanderthals versus Asians and Neanderthals.