Recently, microbiologists have established the existence of biogeographic patterns among a wide range of microorganisms. The focus of the field is now shifting to identifying the mechanisms that shape these patterns. Here, we propose that four processes - selection, drift, dispersal and mutation - create and maintain microbial biogeographic patterns on inseparable ecological and evolutionary scales. We consider how the interplay of these processes affects one biogeographic pattern, the distance-decay relationship, and review evidence from the published literature for the processes driving this pattern in microorganisms. Given the limitations of inferring processes from biogeographic patterns, we suggest that studies should focus on directly testing the underlying processes.