We are interested in the cohesive forces that bind a bacterial species together. Genetic exchange is usually assumed to be the primary force of cohesion within eukaryotic species, but this is not the case for bacteria. What holds a bacterial species together is a particularly interesting issue, since genetic exchange in bacteria is rare yet also promiscuous. We have modeled the effects of rare but promiscuous genetic exchange on the population structure of bacteria. Here are our major conclusions.
Recombination is too rare to hinder adaptive divergence between bacterial populations.
Therefore, speciation in bacteria does not require evolution of barriers to genetic exchange. Recombination is not a significant force of cohesion within or between species in the bacterial world. Bacterial species are held together instead by natural selection favoring each new adaptive mutation. In the absence of recombination, the original adaptive mutant and its clonal descendants will replace all the other cells in the population. Because the entire genome originally associated with the adaptive mutation remains intact as it sweeps through the population, the population loses its genetic diversity at all loci.