The interplay of homologous recombination and horizontal gene transfer in bacterial speciation.

Bacteria experience recombination in two ways. In the context of the Biological Species concept, allelic exchange purges genic variability within bacterial populations as gene exchange mediates selective sweeps. In contrast, horizontal gene transfer (HGT) increases the size of the population's pan-genome by providing an influx of novel genetic material. Here we discuss the interplay of these two processes, with an emphasis on how they allow for the maintenance of genotypically cohesive bacterial populations, yet allow for the separation of these populations upon bacterial speciation. In populations that maintain genotypic similarity by frequent allelic exchange, horizontally transferred genes may initiate ecological barriers to genetic exchange. The resulting recombination interference allows for the accumulation of neutral mutations and, consequently, the imposition of a pre-mating barrier to gene transfer.