The most apparent genetic distance was found between the northern (Eastern and Western) and Southern Slavs, who at the end of the 9th century were separated by the invasion of Finno-
Ugric Hungarians [...] The observed northern Slavic Y-STR genetic homogeneity extends from Slovakia and Ukraine to parts of Russia and Belarus, but also involves Southern-Slavic populations of Slovenia and western Croatia, and is the most probably due to a homogeneous genetic substrate inherited from the ancestral Slavic population. However, due to the Y-STR proximity of linguistically and geographically Southern-Slavic Slovenes and western Croats to the northern Slavic branch, the observed genetic differentiation cannot simply be explained by the separation of both Slavic-speaking groups by the non-Slavic Romanians, Hungarians, and Germanspeaking Austrians [...] Thus, the contribution of the Y chromosomes of peoples who settled in the region before the Slavic expansion to the genetic heritage of Southern Slavs is the most likely explanation for this phenomenon. On the other hand, our results indicate no significant genetic traces of pre-sixth-century inhabitants of present-day Slovenia in the Slovene Y chromosome genetic pool.
Because Slavs unequivocally enter the records of history as late as the sixth century AD, when their expansion in Eastern Europe was already advanced, different theories concerning the Slavs’ geographic origin based on archaeological, anthropological and/or linguistic data have been formulated. Two such theories have gained the largest support among the scientists (Schenker 1995), one placing the cradle of Slavs in the watershed of the Vistula and Oder rivers (present-day Poland), and the other locating it in the watershed of the middle Dnieper (present-day Ukraine). Our results indicate that using the population-of-origin approach based on the AMOVA, as many as nine (P > 0.05) or ten (P > 0.01) populations can be traced back to the lands of present-day Ukraine, including Eastern-Slavic Russians and Belarusians, Western-Slavic Poles and Slovaks, and Southern-Slavic Slovenes and Croats.http://dienekes.blogspot.com/2007/03/origin-of-slavs-in-ukraine.html
All that being said, genetics does have something to say about this. It seems that to make a broad sweeping claim: Slavicization was not always a function of genetic replacement, but in part one of assimilative absorption of local substratum. There is data on this from the Balkans, which I will address at some point, but Dienekes points me to a new paper, Two sources of the Russian patrilineal heritage in their Eurasian context:
…we show that the patrilineages within the pre-Ivan the Terrible historic borders of Russia have two main distinct sources. One of these antedates the linguistic split between West and East Slavonic-speaking people and is common for the two groups; the other is genetically highlighted by the pre-eminence of haplogroup (hg) N3 and is most parsimoniously explained by extensive assimilation of (or language change in) northeastern indigenous Finno-Ugric tribes. Although hg N3 is common for both East European and Siberian Y chromosomes, other typically Siberian or Mongolian hgs (Q and C) have negligible influence within the studied Russian Y chromosome pool. The distribution of all frequent Y chromosome haplogroups (which account for 95% of the Y chromosomal spectrum in Russians) follows a similar north-south clinal pattern among autosomal markers, apparent from synthetic maps….http://blogs.discovermagazine.com/gnxp/2008/01/from-where-came-the-slavs/
Southern parts of present Poland were under Celtic influence. In the second century B.C., the Celts arrived in southern Poland via the Moravia and Bohemia regions, where they prevailed with their La Te`ne culture from the fifth century B.C. Therefore, it is probable that the R1a/R1b proportion varied in those regions according to the degree of influence of one population or another (i.e., Slavic or
The frequency of eastern-Eurasian (Mongoloid) mtDNA lineages in this population constituted 1.8% (haplogroups A, N9a, and M). African lineage (with the frequency of 0.4%) belonging to haplogroup L2a and marked by the +13803HaeIII variant was also detected.
Low frequency of Mongoloid mtDNA variant in Letts and Lithuanians suggests that Mongoloid component was probably not typical of Balto-Slavic protogene pool. Thus, it seems reasonable that accumulation of Mongoloid mtDNA lineages in Slavs and their ancestors was intensified only in the last 4000 years.
The highest frequencies of the Mongoloid component are typical of the Russian populations from Russian Pomor’e and Northwestern region. These populations, however, differ in the mtDNA haplogroup composition. It was established that assimilation of the indigenous pre-Slavic population of Eastern Europe by true Slavs was of great importance to the process of the development of Russian population.
It is suggested that after the decay of Avar Khaganate the populations included into it were assimilated by Slavic tribes . It is thereby suggested that rather high frequencies of eastern-Eurasian mtDNA lineages observed in the gene pools of some populations of Western and Southern Slavs (especially on the territoryof former Avar Khaganate) can be considered as a consequence of the process described.
Concerning the population of Eastern Europe, it should be noted that the forest zone of Eastern Europe was the area of intense population admixture . It seems likely, that formation of the complex of Mongoloid traits happened not later than in Upper Paleolithic. For this reason, it is suggested that East Siberian populations could have much time for migration to Eastern Europe . The number of such migrations still remains unclear, since in the northwest of Eastern Europe Mongoloid component is detected 10 000–8000 years ago; in Dnepr–Donetsk tribes, 7000–6000 years ago, and on the territory of Ivanovo oblast (Sakhtysh), 6000–5000 years ago [35, 36]. The data on mtDNA variation in Russian populations are consistent with anthropological data, since they point to the substantial differences in the frequencies of Mongoloid mtDNA lineages between the Russian populations of the Russian North, Northwest, and the central/southern regions of the European part of Russia (Table 3). http://dienekes.blogspot.com/2008/04/origins-of-mongoloid-mtdna-in-slavs.html
Y chromosome variation in 457 Croatian samples was studied using 16 SNPs/indel and eight STR loci. High frequency of haplogroup I in Croatian populations and the phylogeographic pattern in its background STR diversity over Europe make Adriatic coast one likely source of the recolonization of Europe following the Last Glacial Maximum. The higher frequency of I in the southern island populations is contrasted with higher frequency of group R1a chromosomes in the northern island of Krk and in the mainland. R1a frequency, while low in Greeks and Albanians, is highest in Polish, Ukrainian and Russian populations and could be a sign of the Slavic impact in the Balkan region. Haplogroups J, G and E that can be related to the spread of farming characterize the minor part (12.5%) of the Croatian paternal lineages. In one of the southern island (Hvar) populations, we found a relatively high frequency (14%) of lineages belonging to P*(xM173) cluster, which is unusual for European populations. Interestingly, the same population also harbored mitochondrial haplogroup F that is virtually absent in European populations - indicating a connection with Central Asian populations, possibly the Avars.http://anthrocivitas.net/forum/showthread.php?t=1769