Of course, we know that this isn't always true; people of European and African descent didn't get genetically swamped by the native populations of North and South America, for example. But it appears that it most frequently did. When Sanskrit and Indic culture came to India, it didn't make a huge dent in the population genetics of the area, even though it clearly made a major change in the political, cultural and linguistic map of the subcontinent. Similarly, the Mongols and the Manchus are genetically invisible across the face of most of China.
So, thinking about this, I decided to make a slightly more expanded, albeit sketchy, map of the lateral hemisphere of my fantasy world, with the more "local" Mezzovian Sea region that is really the heart of my setting shown in context with the greater continental picture around it. And, for fun, I thought I'd speak in very broad and vague patterns about the migration of humanity across the face of these continents; where they came from and how they diversified and migrated across the face of the land in ancient times, tens of thousands of years ago, long before anything like the current civilizations were a spark in anyone's eye. These broad patterns speak more to physical type than they do to culture or linguistics, clearly. Cultures and linguistics are much more recent and transient than biology.
Anyway, this'll be helpful to me when I need to come up with slightly more far-flung representatives of humanity, or when I want to describe what humanity really looks like in a given area. It occured to me that I really had no idea what the substrate population of Qizmir or Kurushat was like, even though I said that there was a healthy substrate population; in Qizmir in particular, where I said the ruling caste of jann was a minority. I can use the broad patterns established in this post to extrapolate other population groups as needed.
To me, this is important because I don't want to actually develop them in detail--but I want to suggest that they are out there; that the world is bigger, more complex, and more realistic (or verisimilitudinous, to be nitpicky) than what I can reasonably come up with on my own. Plus, sketching out this very broad, handwavy prehistory of population movement gives me something that I can use as a backbone to maintain consistency as needed as well.
The large landmass that makes up the central (and slightly to the east and south) portion of this map is about the size of Africa or North America (10 million square miles or so) although geographically very different. The developed portion of the world is centered on the area where the labels are; from the equator southwards to Kurushat, the most southerly labeled area (but not as far as the southern coast; my "regional" map shows no southern coastline.) Laterally, it also does not cover the most westerly or most easterly portions of the continent, and the other shown continents, microcontinents, and islands are not shown at all, of course. Although the inhabitants don't have a name for this continent as a whole (nor do most of them suspect the existance of this geography, frankly; the entire concept of a continent would be foreign to them) for ease of use, I'm going to call this the Mezzovian continent, named for the gigantic sea that floods most of its central portion, and which is the central feature of the detailed area of the map.
You can also see a much larger continent--a supercontinent, really, made up of several continental plates and terrenes that have been united together for millions of years--looming across the ocean. Nobody in my area of the map even suspects the existance of this continent, really. This large continent is (roughly) the size of Eurasia and Australia jammed together, so about 25 million square miles--more than twice the size of the Mezzovian continent. The supercontinent is also the homeland of humanity who came in daring mariner waves, not unlike the Polynesians or the Vikings. Subsequently, much of their technology was lost both on the supercontinent and the Mezzovian continent, as ecological disasters associated with glacial stages destroyed the existing civilizations and introduced a prolonged global dark age.
The first wave, perhaps the most daring, crossed the ocean south of the equator, stopping off to populate the islands, then moving on to land on the southern shores of what is today known as the d'Acs Sea. From there, they spread southward, eastward and northward. Initially, this group can be described as "Med1"--Mediterranean in nature; on average, shorter and more gracile than some other groups, with dark hair and eyes, and a bronzed skin coloration. As they spread, they encountered groups of Neanderthal humans still living in some areas. Today, these Neanderthals have been pushed back to the far southern limits of the continent, and to the steppes of Cavusto, while the Med1 physical type spread over much of the continent except the far north and far west, especially north of the Mezzovian itself, where the Forbidden Lands are; places that even tens of thousands of years ago were of ill-repute and were carefully avoided.
Possibly some groups of this first wave intermingled with the Neanderthals, or possibly it was just diversity starting to pop up as the initially homogenous population wave separated into smaller and more isolated groups, split by natural barriers such as mountain ranges, rivers, dense forests, deserts, and other hazards. To the north of the Mezzovian sea, the population developed paler skin and became Med2--the basis of what was later called the Balshatoi ethnic group and the Tarushans, and others. On the shores of the vast Lake Karkose a genetically isolated population (Med3) grew taller on average and their eyes turned an icy pale gray, nearly white--the antecedants of the Kurushi groups. Their skin tone remained bronze, and some of them developed narrow eyes marked by modest epicanthic folds, as well as aquiline noses. Along the southern shore of the Mezzovian, all the way to the oceanic coastline and even the Qizmiri islands beyond, the original Med1 physical type did not radically alter, however, and a physically (if not culurally and linguistically) homogenous population spread and grew, giving rise to the Qizmiri substrate, the Terrasans, and other, smaller ethnic groups that still remain in the area today.
From somewhere amongst the Med2 population sprouted the vucari or wildmen--individuals cursed with lycanthropy who fled their people to live in the wilds of the vast forest that stretched over much of the northern portion of the continent in those days. These groups grew, while the curse of lycanthropy was still potent and strong, and others fled their homes to live with other poor werewolves as well. Over generations, the curse weakened. Fewer individuals came by it, and those who did had descendents who were only lightly touched by its ravages. These became the first wildmen, and while their original homeland was deep in the Shifting Forest, in the years before the rise of Terrasa, a major diaspora and exodus from that region led to their spreading into various areas, including the population of the Erau river basin, and the minority populations throughout the Terrasan city-states.
Two other waves of humans came from the supercontinent. The first of these came from the equatorial regions of the supercontinent, went north to the microcontinents and islands (which were larger and less separated, since glacial maximums would have lowered sea levels globally). This physical type, which I call Nor1 (Nordic), were pale-skinned and flaxen haired, with blue and green eyes--not too unlike the population of modern Northern Europe in our world (despite their tropical origin). When they arrived, the earlier wave of Med1 immigrants were already fairly well esablished all around the Mezzovian sea region itself as well as the westernmost coastal regions. While there was significant admixturing and hybridization of the two waves of population, a large group of this Nor1 wave said, "Screw it" and settled northward where the land was uninhabited. They quickly covered most of this territory, spreading across the coastline until they came to the northern edges of the Forbidden Lands. Some few of them bravely attempted to colonize this area, but they were uniformly either killed or corrupted by the forces that lurked in those dark lands. This may be the origin of the cannibal inhabitants of the offshore islands, although physically there is no longer any resemblance to the Nor1 population group.
The final wave of settlers to the Mezzovian continent came from the far north of the supercontinent, and crossed on the northern edges of the microcontinents and islands, completely unaware of the meager population that was settling in on the southern shores. This group, called Pol1, were more robust than most other inhabitants of the Mezzovian continent, with dark hair, eyes, and brown skin. They would have appeared to us not unlike Polynesians: Maori, Hawaiians, Samoans, etc. When they arrived on the continent, young settlements of Nor1 peoples already ringed the coastal areas, and they came into immediate contact. In fact, the numbers of Nor1 settlers had grown to the point where they outnumbered the Pol1 significantly, and the hybrid cultures that grew along the northwest coast lost much of their physical appearance to both; many of them became much more lithe; tall and rangy. The paler skin, hair and eyes proved to be genetically recessive, although Pol2, the population that grew out of this admixture was sometimes lighter in most instances than the original Pol1 peoples had been. In comparing Pol2 to populations here on earth, they probably would have looked somewhat like North American Indians. Close enough, anyway.
Pol1 and especially Pol2 physical types penetrated deep into the interior of the continent, stopping only when they came to well established settlements of Med2 peoples, with which they further mixed and hybridized. The various tribes of the Hamazi-lands; Tazitta, Untash, etc., as well as the Hamazin themselves (after making a literal deal with the devil) are descended from these groups, as are other, undescribed peoples to the north.
Remember again; this is physical type only. This is meant to describe how various population clusters of folks with similar genetics and physical appearance got to be where they are, literally tens of thousands of years prior to the campaign setting as it's presented. Two individuals of, say, Pol2 physical appearance who speak completely unrelated languages and practice completely disaparate cultures might not recognize any kinship with each other at all, even though they do share some gross physical similarities that suggest that in very ancient times their ancestors came to the Mezzovian continent as part of the same wave of prehistoric settlers.
Shortly after this third wave arrived, the glaciers receded, sea levels rose, the technolgical "dark age" globally occurred, and both continents lost contact with each other (assuming that contact was even maintained in the first place.) No additional waves of migrants arrived on the shores of the Mezzovian continent until the jann did so in historic times, this time from the unknown east instead of the west--the first such migration to come from that direction. Groups mixed, mingled, and developed locally into very divergent and diverse cultures and ethnic groups. So many generations passed that there was no hope of recovering evidence of common languages anymore. Time, evolution (both culturally and physically in some cases) eliminated the evidence of the three waves of migration, leaving only the cultures that grew from those roots to become the new native cultures, societies and ethnicities of the Mezzovian world.