Discussion of everything related to the Theory of Evolution.
That is exactly what would happen: if we stopped using penicillin for a decade, it would be much more effective when taken into use again. However, quite a few species of bacteria would retain their penicillin resistance, because it is a naturally occuring antibiotic that certain fungi produce. Nevertheless, in clinical use any antibiotic would be considerably more effective if its use was stopped for a long enough time.
This very phenomenom is also the reason why many scientists say that the use of antibiotics should be reduced: currently the selection pressure is very high in advantage to resistant strains. Both humans and livestock are being fed so huge amounts of antibiotics that in certain cases previously very effective antibiotics are now next to useless.
Furthermore, the effect of selective pressure of antibiotics (or antiviral drugs in the case of viruses) can be seen in certain slow-growing, resilient bacteria such as the mycobacteria - or in fast mutating viruses such as the HIV. In these cases the treatment must be done by means of combination of drugs in order to prevent resistant mutant strains from emerging. It is much more likely that a microbe can develop a single protective mutation against one drug than it would be to develope three mutations protecting from three different drugs.
In some ares of the world e.g. the multi-drug resistant tuberculosis is a big problem (say, Russia for example). In such places the resistance of these microbes would become lower if the use of drugs was ceased for long enough time: the mycobacterium would have no use for their resistance genes and those bacteria that would not express the genes would survive better because of lower metabolic demands (they'd not need to produce enzymes or other molecules required to protect them). Alternatively, efficient use of combination drug treatment would also limit the level of resistant strains by killing the bacteria before they become multi-resistant, but unfortunately many people stop the drug treatment before the disease is cured and thus keep spreading resistant bacteria.
Some metabolic genes (typically coding some enzyme) in many organisms can be induced by the presence of their target molecule so as to cause metabolic load only when they are needed and otherwise being passive and not expressed. This reduces the chance that they would consume resources when not needed and also lowers the chance that these genes were lost when there is no pressure favouring their use. However, even such genes can and often will be lost if there is absolutely no use for them for a long time; they are likely to mutate and the mutants are as likely to survive as the "wild type" organisms. Some of these mutants with their new gene might even be better than their ancestors and eventually replace the original version completely.
I remember I have submitted the example of the white tiger to someone one day on another forum.
He told me that the mutation responsible for the white fur was baaaaad for the poor animal because its hunting ability is likely to be reduced by its new color (understand: camouflage is weakened). More generally, he said that because "any mutation is bad". The guy seemed to ignore that we are all mutants, in the strict sense of the term.
Plus, when I've read the storyabout the rabbits above, I thought of an all-snowy India, where Sher-Khan wouldn't have no reason to feel ashamed.
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