I am struggling to think why horizontal gene transfer between bacteria would have persisted during the course of evolution as surely it puts the 'donor' at a disadvantage?
For example, consider a hypothetical situation where only one species of bacteria has a gene to resist a new (almost) omnipotent antibiotic. When growing on this medium, this species of bacteria has nil inter-specific competition for the resources it needs. Yet if it passes on the plasmid containing the resistance gene via conjugation to individuals of a different bacterial species, they then too are able to grow on the medium and potentially out-compete the first species - thus the conjugation could possibly be extremely detrimental to the survival of the original population.
I'm obviously missing something as the feature has persisted, so what is the advantage to the donor species of conjugation?