Evo paralelne evolucije jedne vrste na delu
http://evolution.berkeley.edu/evolibrary/news/080401_mrsaSuperbug, super-fast evolution Where's the evolution?
MRSA is resistant not only to the antibiotic methicillin, but also to whole other suites of our drugs, making it very difficult to treat and, occasionally, deadly. Modern strains of MRSA did not, however, show up out of the blue. In the early 1940s, when penicillin was first used to treat bacterial infections, penicillin-resistant strains of S. aureus were unknown — but by the 1950s, they were common in hospitals. Methicillin was introduced in 1961 to treat these resistant strains, and within one year, doctors had encountered methicillin-resistant S. aureus. Today, we have strains of MRSA that simultaneously resist a laundry list of different antibiotics, including vancomycin — often considered our last line of antibacterial defense.
How did S. aureus morph from a minor skin infection to a terror? When the media report on MRSA and other drug resistant pathogens, they often say that such pathogens have recently "emerged" — that they've "developed" resistance or "learned" to evade our drugs. In fact, it's more accurate to say that these bugs have evolved resistance. It's particularly ironic that newspapers might shy away from describing bacterial evolution as such because, when it comes to evolution, bacteria have most of the rest of us beat.
Bacteria are great evolvers for many reasons. For example, their short generation times and large population sizes boost the rate at which they can evolve. In addition, one quirk of bacterial genetics is particularly salient to the evolution of antibiotic resistance: horizontal transfer. Here’s a quick explanation:
Evolution with vertical transmission. In most familiar organisms, new gene variants arise in a population through random mutation — that is, one individual experiences a genetic mutation and if that mutation ups the individual's ability to survive and reproduce, it is favored by natural selection. Mutant gene variants are passed from parent to offspring, and advantageous mutations spread through future generations in that way. Over time, additional beneficial mutations that build on the first may occur and begin to spread in the population, allowing more complex traits to evolve as mutations accumulate. This standard picture of evolution is at work in all organisms — whether they are humans that eventually evolve the ability to digest milk or a plant species that adapts to the presence of heavy metals in its environment. The same mechanism also works on bacteria. In fact, biologists have observed the MRSA strain infecting a single patient evolving through random mutation and selection. The patient was being treated with vancomycin, and slowly, over the course of a few months and 35 separate mutations, the bacteria evolved into a vancomycin-resistant MRSA strain.
Evolution with horizontal transfer. So bacteria acquire genetic variation through random mutation, but, unlike humans or oak trees, they also regularly get new gene variants through the process of horizontal transfer — that is, they can pass DNA back and forth to one another directly. For example, bacterial genes can be incorporated into small self-replicating circles of DNA called plasmids, which can be "injected" into other bacteria. The receiving bacterium may even incorporate some of the new DNA from the plasmid into its own genome and pass those genetic sequences on to its descendents. Importantly, bacteria do not have to be closely related to share DNA. Horizontal transfer can occur across even distantly related species — which would be a bit like you picking up the family pet and winding up with a few cat genes in your genome. In terms of evolution, this means that bacteria do not have to rely on random mutation to produce a beneficial gene variant. One species might pick up an advantageous gene from another species, and the process of natural selection could begin to act right away, spreading the new variant through future generations.
