A. I. Kushniarevich, Russian Journal of Genetics: Applied Research, 2012, Vol. 2, No. 2, pp. 114-121, about R1a-M458 in Belarus:
"The migration processes of the later period could enhance the regional differences in the distribution of the studied molecular marker [subclade M458 of R1a] in the population of modern Belarusians. In particular, a significantly high frequency of the R1a-M458 haplogroup in Ponemanye and Eastern Polesie regions of Belarus can reflect a movement of the tribal communities with genetic characteristics close to those in the local population into the Neman River and Pripiat River basins. Such communities could be for example Polabian Slavs who left the territory between Elbe River and Oder River at the end of the First and beginning of the Second Millennia under the influence of the expansion of Germanic tribes [Saxons, etc.]. Toponymic parallels, as well as previous results of studies of linguistic and archaeological science, indicate the relationship between the Polabian Slavs and the population of modern Belarus (Jezowa M., 1962; Sedov, 1982; Perkhavko, 1983)."
Polish (Masovian etc.) immigration could also explain higher levels of M458 in Ponemanye (= Grodno Region), but we cannot say the same thing about Eastern Polesie, which never had any significant Polish population. But the
Radimiches lived there, and they are considered to be of West Slavic origin:
https://en.wikipedia.org/wiki/Radimichs
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