It must be safe to say that all modern Slavic languages are derived from a common ancestor - праславянский язык (this is one of the well established theories). To use the term from the lecture in the link, this process is known as дивергенция.

The major point of interest in that lecture is development of the Russian language, i.e. the speaker suggests (largely based on his study of берестяные грамоты in Novgorod and Pskov) that the Russian language might be the result of конвергенция (the opposite process compared to дивергенция) of the two large variations, namely the north-western Novgorod and Pskov variation and central/eastern/southern variation.