Bacterial genomes are highly dynamic in both size and composition. The extensive variation in gene repertoires that characterizes prokaryotic genomes can be caused by genome expansion via horizontal gene transfer and gene duplication or, alternatively, contraction due to gene loss. ~ German microbiologists Glen D’Souza & Christian Kost
Bacteria can be genic packrats: soaking up genetic material from the environment, including long-dead organisms. DNA bits are only added if they may be of potential use.
Bacteria shed genetic material they feel they no longer need. For example, in the instance of living in an environment rich in a certain nutrient previously biosynthesized, a bacterium may discard the ability to fabricate the essential metabolite. Such economic gene shedding is how bacteria become dependent upon their situation.
Nutrient-containing environments drive the loss of biosynthetic genes from bacterial genomes and facilitate the establishment of metabolic cross-feeding interactions among bacteria. ~ Glen D’Souza & Christian Kost
Besides genic scavenging, bacteria swap genes among themselves. Many bacteria shed and uptake plasmids, which are independent DNA molecules. These transfers enable bacteria to quickly evolve so as evade destruction by antibiotics and toxic compounds, sometimes by alchemic genes that can transform mercury or other heavy metals into less noxious forms.
Prokaryotes come by 88% to 98% of new DNA through pickup, particularly horizontal gene transfer (HGT), which lets microbes acquire preexisting adaptations from other microbes: ready-made evolution. This is one reason bacteria can acquire antibiotic resistance so quickly. Another is that highly resistant bacteria try to shelter less resilient members of the population to preserve their society.
Self-evaluative genetic modification is the dominant force in microbial evolution. Bacteria populations intelligently adapt via HGT.
Microbial gene pickup played a key role in the evolution of more complex life forms. It continues.
Horizontal gene transfer is rampant, and occurs among distantly related organisms: viruses, bacteria, fungi, plants, and animals. The microbiome within complex eukaryotes is a largely invisible driver of host adaptation by way of selective genic exchange.