Bacterial evolution and the nature of species

Taxonomists classify bacteria into species, even though the genetic systems and ecology of bacteria are very different from those of higher organisms. Do bacteria have species in the conventional sense? This chapter considers the likely impact of the various recombination mechanisms available to bac teria, and reviews some of the relevant evidence based on genetic variation at the molecular level. It is clear that bacterial species vary widely in their degree of intraspecific recombination, and that in some groups there is also considerable genetic exchange between related species. There appear to be clearly separated species even in some bacteria which are strongly c lonal. It is likely that gene exchange is a major cohesive mechanism in som e species, but that for others the main force is periodic selection, which depends on ecological equivalence. A consideration of the question of speci es in bacteria can offer a new perspective on the general issues surroundin g the nature of species.