The importance of physical isolation, defined as the spatial separation of two or more populations, to the evolution of organisms has been well studied in plants and animals yet its significance regarding microbial evolution has not been fully appreciated. Here we review the theoretical paradigm of physical isolation for the diversification of organisms in general and then provide a variety of evidence indicating that microbial populations also fit into a similar evolutionary framework.

(Keywords: Microorganisms; Allopatric speciation; Geographic isolation; Endemism; Genetic drift; Everything is everywhere)