Studies on the experimental evolution of microorganisms, on their in vivo evolution (mainly in the case of bacteria producing chronic infections), as well as the availability of multiple full genomic sequences, are placing bacteria in the playground of evolutionary studies. In the present article we review the differential contribution to the evolution of bacterial genomes that processes such as gene modification, gene acquisition and gene loss may have when bacteria colonize different habitats that present characteristic ecological features. In particular, we review how the different processes contribute to evolution in microbial communities, in free-living bacteria or in bacteria living in isolation. In addition, we discuss the temporal constraints in the evolution of bacterial genomes, considering bacterial evolution from the perspective of processes of short-sighted evolution and punctual acquisition of evolutionary novelties followed by long stasis periods. [ABSTRACT FROM AUTHOR]