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Don't confuse genetic ancestry with racial types. They correlate, but are not necessarily the same.
Just imagine a group of Slavic peasants which had a percentage of just 10 percent Osteuropids, whereas the rest was Nordoid, Nordoid-Mediterranid and Nordoid-Cromagnid, like in most Indo-Europeans, for convenience and because that was dominant, let's just speak of Nordoid vs. Osteuropoid, like I. Schwidetzky did in her work about the ancient Slavs.
Now the peasantry became more and more dependent, sedentary and began to live more peacefully, yet in larger settlements, with worse hygiene, nutrition and more diseases.
Baltisation starts and certain traits which make up the Osteuropoid variant in comparison to the Nordoid are favoured. From one generation to the next, slowly but steadily, the Osteuropoid element proportions will grow, while the Nordoid will shrink.
Probably not even because the Nordoids have less children than they had before, but because the Osteuropoid HAVE EVEN MORE and even under deteriorating conditions!
So all of them would be, when the Nordoid would be only 50 percent and Osteuropoid 50 percent too, still be descendents of the very same people, just the racial proportions changed, because people with this traits (Osteuropoid) had more offspring relative to those with the other traits (Nordoid).
It is a change in the genpool which might not even be related in all instances to massive immigration, it is natural selection at work, just under unfavourable conditions in which the more frugale people which procreate faster under bad, but rather stable conditions, had the upper hand on the longer run, especially in the lower classes.
Some assume that the introduction of Christianity had an influence on this too, because now certain limitations to mixture and marriages etc. were lifted, while at the same time all people became subjugated as peasantry, with just the nobility being really free and old high level selection being at its end, because social success and positive risk taking, Idealism and group orientation might have been still important, especially for the group, but did no longer pay off biologically for the practising individuals!
So a sturdy, more cold resistent and frugal peasant type became more dominant, which is the classic Osteuropid, which descends from archaic hunters, fishers and gatherers of the North East, which became Finno-Ugrians at one point in prehistory, and Balticised and Slavicised later, which was the way they came into the Slavic populations genpool...
That process of Baltisation might be older though, since the Cromagnoid elements became more reduced and gracile over time, resulting without Mongoloid tendencies in Westbaltid and Baltid variants, with Mongoloid tendencies in Eastbaltids.
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