Microbial evolution refers to the genetically driven changes that occur in microorganisms and that are retained over time. Some microbial changes can be in response to a selective pressure. The best examples of this are the various changes that can occur in bacteria in response to the presence of antibiotics. These changes can make an individual bacterium less susceptible or completely resistant to the killing action of one or more antibiotics.
Other microbial changes can occur randomly in the absence of any selective pressure. These changes, which often are due to a change in the sequence of the units (nucleotides) that comprise an organism's genetic material, can confer an advantage to the organism, as compared to unaltered organisms. In the classic scenario of evolution, such as advantageous trait will be retained and can be passed on to future generations of the organism.
In contrast to Darwinian evolution, which takes place over millions of years, microbial evolution can occur within hours. This is because some bacteria are capable of growing and dividing in about 20 minutes under ideal growth conditions. A bacterium containing an altered gene that confers a survival advantage can, over 24 hours, give rise to thousands of progeny that carry the same gene. Each new bacterium can in turn give rise to thousands of progeny by the next day. Thus, a mutation can rapidly spread in a bacterial population and, because the trait is capable of being transferred to unrelated bacteria, to other bacterial populations as well.
There are clearly instances of lateral transfer, where viruses, for example, carry genes from one species to another or insert themselves in the middle of a gene making substantial changes. That may further cloud the picture at times, and it clearly plays a far greater role in complexities of microbial evolution. The current sequencing of large numbers of microbial genomes is facilitating rapid growth in our understanding of that process, presenting very different pictures of the early stages of cellular evolution and the development of the three kingdoms than those based primarily on ribosomal RNA data. The possibility has been raised that viruses may even provide windows into some of the ancient organisms that disappeared in the bottleneck of the ‘last common ancestor’ of the three kingdoms; only a fraction of the genes of the large viruses look like anything seen to date in cellular organisms, and a significant number of similarities have been seen between genes of bacteriophages and eukaryotic viruses. There is much evidence, at least, that most families of viruses are very ancient in origin and have coevolved with their various hosts.
Microbiology: Current Research
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