White Croats
It is considered that the Czech-Polish Croatian tribes were related to the Croatian tribes from Zakarpattia and Prykarpattia in Ukraine. They became separated during the migration period and seemingly formed one large Proto-Slavic tribe. It is theorized that the migration of those (White) Croatian tribes in the 7th century was the second and final Slavic migratory wave to the Balkans, which is related to the thesis by Bogo Grafenauer about the double migration of Slavs. According to this thesis, although it is possible that some Croatian tribes were present among Slavs in the first Slavic-Avar wave in the 6th century, it is argued that the Croatian migration in the second wave probably was not equally numerous to make a significant common-linguistical influence, while others considered they arrived in a significantly larger number. Zdenko Vinski and argued that the double migration thesis is supported by the rare findings of objects and ceramics of Prague-Korchak culture on the territory of Croatia, Bosnia and Herzegovina, and Serbia which were dated to the end of 6th and beginning of the 7th century (indicating first wave Slavs), and did not make a continuity with Slavic archaeological findings from the 8th-9th century, indicating assimilation. The Croatian tribes are also seen as a warrior group who assimilated into already present Slavs. In the territory of present-day Croatia they also gradually assimilated with the Pre-Slavic population because archaeology indicates a significant continuity of late antiquity population which have partially withdrawn to the mountains, coastal cities and islands.
Slavic languages
The migration of Slavic speakers into the Balkans in the declining centuries of the Byzantine Empire expanded the area of Slavic speech, but the pre-existing writing (notably Greek) survived in this area. The arrival of the Hungarians in Pannonia in the 9th century interposed non-Slavic speakers between South and West Slavs. Frankish conquests completed the geographical separation between these two groups, also severing the connection between Slavs in Moravia and Lower Austria (Moravians) and those in present-day Styria, Carinthia, East Tyrol in Austria, and in the provinces of modern Slovenia, where the ancestors of the Slovenes settled during first colonisation.
In September 2015, Alexei Kassian and Anna Dybo published, as a part of interdisciplinary study of Slavic ethnogenesis, a lexicostatistical classification of Slavic languages. It was built using qualitative 110-word Swadesh lists that were compiled according to the standards of the Global Lexicostatistical Database project and processed using modern phylogenetic algorithms.