Evolving Concepts of Bacterial Species

The same evolutionary forces that cause diversification in sexual eukaryotes are expected to cause diversification in bacteria. However, in bacteria, the wider variety of mechanisms for gene exchange (or lack thereof) increases the range of expected diversity patterns compared to those of sexual organisms. Two parallel concepts for bacterial speciation have developed, based on ecological divergence or barriers to recombination in turn. Recent evidence from DNA sequence data shows that both processes can generate independently evolving groups that are equivalent to sexual species and that represent separate arenas within which recombination (when it occurs), selection and drift occur. It remains unclear, however, how often different processes act in concert to generate simple units of diversity, or whether a more complex model of diversity is required, specifying hierarchical levels at which different cohesive processes operate. We advocate an integrative approach that evaluates the effects of multiple evolutionary forces on diversity patterns. There is also great potential for laboratory studies of bacterial evolution that test evolutionary mechanisms inferred from population genetic analyses of multi-locus and genome sequence data.