I'm tired but I'll just point something out before I go to sleep.

In the same event, replacement and absorption can both be present. If we look at historical population turnovers such as the invasion of the new world by Europeans and the change in Aleutian islanders from the article I mentioned last post, we see that, since the two populations are of the same species, at least some degree of hybridization occurs. The colonization of the Americas mostly replaced the indigenous population, as native American population numbers from before until today will attest, but they were not eradicated and still form a part of the population.

As for prehistoric cases, such as Homo Erectus in asia, it seems the natives were exterminated, as there are no further living H. Erectus today, and they haven't contributed to the modern DNA pool. Since H. Sapiens and H. Erectus are different species and have noticeably differrent morphology, it isn't a far leap to imagine the two groups regarding each other as sub-human (or sub-erectus in our case) and causing a complete replacement to occur. How can hybridization even happen if they are different species?

I don't think a complete replacement event can occur anymore, at least on a planetary scale, due to modern technology making the effective distance between any two points on the earth to a human much much smaller. Whereas before ~2kya a human living in Africa, for example, would never have been able to find himself among an Alaskan village in his lifetime. Today, there is continual mixing of population from any point on the globe to another, so I think that differentiation of humanity into separate species is highly unlikely. The isolation required for specialization is just not there. If humanity colonizes planets light years away perhaps enough isolation will be present for the different lineages to diverge and eventually become separate species. Until then, barring a drastic technological reduction, complete replacement seems impossible today because the two populations involved will be of the same species and have a degree of mixing. (even in something barbaric like nuclear extermination, there will have been some of the victim population who were traveling, even some living in the aggressor's area)

time to sleep~