R1a is more complicated, as there are a lot of branches and not all of them seem to be connected to the Slavs.
I2a is easy because literally 99% of it in Southeast Europe falls under Y3120, which is linked to the period of Slavic migrations.
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First of all you should learn the difference between I2 and subtypes of I2, because we are talking about the last one and these are not same. Indeed the pure version of I2 is paleo-european, but this type doesn't exist nowadays only it's subtypes and these subtypes (like I2a-din) is not so old and connected to the slavic ethnogenesis, other subtypes connected to others ethnogenesis for example r1b-u106 is germanic, n1c1 is uralic etc.
By the way, for the sake of truth, Slavs didn't invade Illyrians, they migrated in numbers after the devastating Justinian pandemics which weakened the Illyrian population. Otherwise, on their best i seriously doubt Illyrians will allow Slavs to migrate en masse.
Quote:
The plague pandemic in 541–543 and successive outbreaks of the disease till the latter half of the 8th century caused a deep demographic crisis in the Eastern Roman Empire. The most important effects of the plague were a shortage of manpower and a growing importance of marginal barbarian populations, which had suffered less or not at all from the disease. Demographic, political and economic consequences of the pandemic likely caused or at least facilitated Slavic expansion in the Balkans between the 6th and 8th century. The Slavs began to raid intensively and then settle the European provinces of the Roman Empire soon after the first outbreak of the plague and available textual evidence suggests that this region was depopulated by the disease and neglected by the government. During the 7th century, the Empire’s administration and economy collapsed due to the effects of the plague and the existing system of land taxation and central provisioning of professional armies must have been replaced by regional organization of territorial troops recruited from free peasant farmers. In the new circumstances, the Slavs, who had in the meantime re-populated the Balkans, constituted an abundant source of manpower for a restored Empire.
https://www.researchgate.net/publica...-8th_centuries