This is a discussion of Replacement, perhaps more correctly called Genetic Replacement. It can occur between between tribes, within tribes and perhaps even within families. It is especially important now, because one of the changes occurring now is the meeting of different peoples. Replacement in terms of tribes common enough in the past, but it was slower and so it actually often resulted in hybridization. The danger now is that the natural barriers that prevented it before are gone, so it presents a risk of losing valuable genetic variability. Also, it presents a moral problem.
Nature is quite amazing, but not very smart. Natural selection is a blunt instrument without much strategy to it so it wastes a lot of opportunities. In that humans need to compensate for having removed Natural Selection in the name of progress, we need to compensate in a smart way. In my discussion of needing to use artificial selection to replace the natural selection that we have removed, it is in the context of a family. It is about applying artificial selection within a family to have healthy children. The same situation applies though in larger groups. As said above, there are at least two reasons we don't want to follow nature's course right now.
Vilfredo Pareto was a sociologist that why the majority of rewards always seemed to go to a minority of people. He is credited with the 80/20 rule that in society 80% of the wealth typically goes to 20% of the population. The values vary, but the situation is common in nature. In a forest, there may be a great diversity of species, but most of the forest is just made up of a couple species. This seems to apply to most things - a few hazards cause most accidents, most traffic is during certain times, the hardest 20% of the code in a software application takes 80% of the time, 20% of patients use 80% of health-care and lets not even mention who spreads disease.
The reason this occurs this way is also known and is referred to as "cumulative advantage". What begins as a small advantage gets bigger over time. Also, since many situations in nature, such as survival are winner take all or nearly so, the benefit of even a small advantage grows quickly and often that increases the advantage even more. It can apply to disadvantage too such as in the case of genetic load, but those do not last and so are uncommon.
So where this becomes important to humans is that in biology this often leads to replacement. Put it this way, if you put two ethnically (genetically) different populations together in the same place (niche or ecology), one would be better adapted to that niche than the other was. In simple terms, because of "cumulative advantage", one tribe would soon replace the other. Hybridization would perhaps modify this, but still, before long (in terms of generations), there would be one group remaining in the niche. This is a problem for the reasons mentioned above. There is a risk of losing ethnic (genetic) diversity when we especially don't want to. Also, groups that feel existentially threatened may attack groups that they feel threatened by, thereby endangering the civilization. In moral terms, this relates to two points as well. Part of "the moral system" we are using includes an agreement that civilization offers great potential for survival, so if you (a group) agrees to embrace that moral strategy of husbanding a civilization, they will be given an opportunity to survive and offered a place in the future.
There are many caveats to this. An individual or group might identify as an ethnic group, but you can be pretty sure that they are hybridized with one of the main ancient civilizations or they would be in great danger of diseases common to the larger populations of civilization. Also they would have trouble coping with the greater population density of a city. There are though what are called "core populations" that may have less external genetic sources, especially in their stories. In most cases though, they have had many members "leave" and are now part of the larger civilization. More importantly, the larger civilization contains their genetic potentials.
The idea is that ethnic groups would be able to adapt to civilization by hybridization and artificial selection. The civilization gains genetic potentials and constructive support, while not making enemies. This is also part of the larger integrated moral strategy that supersedes the morality created by nature that we used in the tribe. In tribal morality, it would be right for the one tribe to replace the other. Civilization is not resource limited the same way that the ecology of the tribes was. This must be a win-win situation rather than win-lose where the win for one group means a loss for another group. Like natural selection, competition will never go away and we don't want it to, but we want it highly regulated so that the outcome is what is desired in the long run, the potential for long-term survival, based on husbanding genetic and cultural diversity. Nature doesn't do that. Cultural diversity is going to vanish extremely fast in any case and while it can cause difficulty or even conflict in the short term, in the long term it will be irrelevant as long as the basic moral support for the ecology that is civilization exists.
There is a third case of Replacement, that is within a tribe and is more common in nature. Part of the description of a tribe is the high degree of relatedness between all individuals in the tribe. A tribe is an extended family to some extent or another. They can even suffer from inbreeding if they are isolated for too long. The thing is though that even though tribal members are closely related, they are not the same. Luck, good and bad, means some individuals in the tribe will have greater genetic wealth than others. Over time, those individuals and families in the tribe, will replace the less gifted members. The thing is though that in genetic terms, because of their relationship, the losers do not lose in genetic terms. They even win in a way, because not only do their genes continue, but they are now associated with the genes that made the "winners" more gifted, that is better survivors, then themselves.
This is a weird perspective that applies up and down in group size from family to specie, but it is relevant and it is part of what will make an understanding in terms of genetics and moral strategy.