  We model the competition between recombination and point mutation in
microbial genomes, and present evidence for two distinct phases, one uniform,
the other genetically diverse. Depending on the specifics of homologous
recombination, we find that global sequence divergence can be mediated by
fronts propagating along the genome, whose characteristic signature on genome
structure is elucidated, and apparently observed in closely-related {\it
Bacillus} strains. Front propagation provides an emergent, generic mechanism
for microbial "speciation", and suggests a classification of microorganisms on
the basis of their propensity to support propagating fronts.
