Recent research on Neanderthal DNA in (non-African) humans suggests that human-Neanderthal interbreeding showed characteristics of inter-species breeding, with probable sterility for male offspring and systematic selection against Neanderthal DNA in the descendants. (This is very unlike the situation for interbreeding in modern humans.)
E.g., from Were Neanderthals a Different Species?:
… this suggests that the male hybrids might not have been fertile, whereas the females might have been fully fertile,” Svante Pääbo told Richard Harris of National Public Radio…. We might have inherited most of our Neanderthal genes through hybrid females, he said.
Another author, David Reich of Harvard Medical School, told reporters that we and Neanderthals “were at the edge of biological compatibility.”
“This underlines that modern humans and Neanderthals are indeed different species,” Fred Spoor told New Scientist. Spoor is also at the Leipzig Max Planck but was not a part of the Neanderthal research. Other scientists are more cautious about making so firm a declaration, but it’s clear that many lean toward that same conclusion, that Neanderthals were not Homo sapiens neanderthalensis but, rather, Homo neanderthalensis.
and from Neanderthals and humans: an interspecies affair to remember:
Many of our living primate cousins interbreed naturally in the wild: estimates show more than 10% of primate species do it!
Doesn’t this run counter to the very definition of a species? Not unless you take an overly simplistic, or iconoclastic, approach to defining them. Even the man credited more than anyone with developing the “biological species concept” based on reproductive isolation – the late Ernst Mayr – recognised that interbreeding sometimes occurs between them.
The latest findings from genome comparisons reinforce the status of Neanderthals and modern humans as distinct species.