The main driving force of bacterial evolution is the ability to
acquire multiple genes in a single recombination event called
horizontal gene transfer(HGT).
The main three mechanism of lateral gene transfer among
bacteria are transformation, conjugation and bacteriophage
transduction.
Gene gensis , horizontal gene transfer, gene loss are the
principal driving force that shape prokaryotic genomes and
influence gene content.
Gene content was first thought to be effective by gene genesis,
particular, duplication and divergence of single genes or even
entire genomes.
Many pathogens were evolved by reductive evolution , involving
excessive gene loss and contribution of HGT are also
significant.However relative contribution of each remain
unknown.
Gene gain and gene loss in prokaryotes are balanced, thus
average bacterial genome remain constant.