I drew the following grossly oversimplified map to give myself a mental picture of the seven major dialect groups that make up the Chinese language family. 

The linguistic map of China is a manifestation of its geography. The great plains of the north facilitated the dominance of a single dialect family. The South, with its numerous mountains and rivers  making communication difficult, is thriving with numerous tones.

It is also a reflection of history. For example, the Min dialect retains a lot of old Wu characteristics because many of its early immigrants were from the Wu-speaking region.

The Xiang and Wu dialect regions were once joined and considered a single language family by some. However, beginning around 300 AD, due to wars and invasions by Altaic-speaking nomadic tribes from the north, the thousand-year long southward migration of people from central China drove a wedge between the Wu/Xiang speaking regions and eventually split them apart completely. Along this migration path, a new dialect was formed -- Gan is thus a transitional language that share many similarities with its neighbors. This Great Migration also played a central role in the formation of the Cantonese and Hakka dialect groups.