I've started the current series of ADMIXTURE runs with an hypothesis I'm trying to test. I want to state it clearly and simply, because thats decisive in order for readers to understand the poles/experiments I'm guiding ADMIXTURE to do.
Here is the hypothesis put as a simple model:
1. The Neolithic Revolution was about a significant change in Humans. I happened due to selection over thousands of years in very few very special and uniquely rich and unstable environments around the world. This change influenced and was influenced mutually and gradually by technology and culture of the originally Forager people subjected to it.
2. Agricultural lifestyles allow for at least 10x higher population differences. Imagine for once this theoretical unreal situation: A land divided as a chess board into 10 squares; 5 inhabited by Neolithic people at 10ppl per square and 5 by Forager people at 1 person per square exclusively. Now imagine they mix. Total resulting population 10x5+5x1=55. Total resulting Forager contribution to gene pool: <10%. Once Neolithics dominate some regions, Foragers are a minority in their land even without wars and genocides (even though these last likely occurred).
3. Foragers don't invent or adapt to agricultural lifestyles because they do not possess such changes and can't develop them fast enough in most circumstances. They're predisposed to fight or flee instead. Their much lower densities make them prone to high-density disease they didn't evolve immunity to (but Neolithics did).
4. Foragers thus get swamped by agriculturalists, and populations become dominated by the Neolithic Core Area population. Since mostly men migrate, in ever larger numbers, almost all Y-DNA is Neolithic; much of the mt-DNA is forager (since early women were taken from forager populations, and later women descend from these); but autossomes are overwhelmingly Neolithic too.
5. Established Neolithic populations live at the Malthusian limit. Extra food means extra surviving children eating it.
6. The only major changes in such a setting involve invasion by populations with food producing advantages. Like better seeds, tools (techs); better organization for irrigation works (culture); major genetic advantages like better digestion of food products (lactose tolerance). Advantageous alleles still diffuse, but neutral genes remain overwhelmingly local.
7. "Invasions" and "Conquests" in such a setting are about militarized elites subjecting the peasants. Travelling is difficult and they are few compared to peasants. They live off rents collected from them, rather than join in the miserly peasant life. Since they despise peasants as serfs, they refer to themselves and their land by their minority identities. Thus we get "Roman" Tunisia; "Gothic" Ukraine; "Celtic" Anatolia, "British" Jamaica. This has no meaning as far as actual genetic constitution of the majority peasant population, but it's all contemporary authors talk about, as well as most contemporary luxury works.
8. If subjected populations live for long enough under the alien elite, they mix with it, appropriate their prestige tags, assimilate their prestige language with their substrate one, and much of material culture too. Thus French "Latins" with Celtic substrate, Bulgarian "Slavs", Anatolian "Turks", Egyptian "Arabs", etc. Elites do make a contribution on Y-DNA, since their societies transmit prestige patriarchally. But almost no mt-DNA. And little autossomical DNA, since elite Y-DNA bearers persist but successive wives are mostly local.
9. Slaves: slaves in settled agricultural societies do not have the impact they have in mostly unsettled Forager inhabited frontiers. Demographic success of agricultural slaves in the Americas is the exception not the rule for slave owning societies. Just like Agricultural minority success at Forager inhabited frontiers is the exception, and elite assimilation into settled populations the rule. African genes in the Americas expanded because they were able to join the early "Neolithic" gene pool there. They had high density disease immunity, agricultural knowledge, social and genetic adaptations, and the right crops versus Forager Amerindians in some regions. Slaves functioned as new rural "peasants" in untilled land from whom Barons extracted rents.
In Old World regions, where large peasant populations lived at the Malthusian limit, there was no advantage bringing slaves to till the land and replace the peasants. If a Baron could move the peasants from productive land so could he make them work as hard as slaves. Slaves were useful for house work, prestige, for city and mine labour. They had very severe social disadvantages. They generally died at far higher rates and reproduced a lot less than local peasants and so had to be continuously imported. They could not make large contributions to gene pools of very dense agricultural peoples, except if they carried food producing advantages, such as crops specially adapted to local circumstances. Slaves as a rule contribute a little mt-DNA, almost none Y-DNA and residual autossome DNA in such societies.
10. Over time, once a functioning advanced agricultural community is established, no matter how many elite invasions and conquests, or how many slaves brought; there might be small but significant changes to Y-DNA and mt-DNA respectively. But autossomes are likely to remain the same, with only residual contribution (except if food producing improvements are brought by migrants as said). At least until such times when machines till the soil, people don't live at the Malthusian limit, and they don't continue to reproduce even though food available.
Both presently, and in the Neolithic Revolutions, these assumptions don't hold. But in the intervening 10.000 years or so, they likely do.