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Evolutionary pressure
Any cause that reduces reproductive success in a proportion of a population potentially exerts evolutionary pressure or selection pressure. With sufficient pressure, inherited traits that mitigate its effects—even if they would be deleterious in other circumstances—can become widely spread through a population. It is a quantitative description of the amount of change occurring in processes investigated by evolutionary biology, but the formal concept is often extended to other areas of research.
In population genetics, selection pressure is usually expressed as a selection coefficient.
== Antibiotic resistance ==
Drug resistance in bacteria is an example of an outcome of natural selection.
When a drug is used on a species of bacteria, those that cannot resist die and do not produce offspring, while those that survive potentially pass on the resistance gene to the next generation (vertical gene transmission). The resistance gene can also be passed on to one bacterium by another of a different species (horizontal gene transmission). Because of this, the drug resistance increases over generations. For example, in hospitals, environments are created where pathogens such as ''C''. ''difficile'' have developed a resistance to antibiotics.〔Dawson, L.F., E. Valiente, and B.W. Wren. 2009. ''Clostridium'' ''difficile''—A continually evolving and problematic pathogen. Infections, Genetics and Evolution. 9:1410-1417.〕Antibiotic resistance is made worse by the misuse of antibiotics. Antibiotic resistance is encouraged when antibiotics are used to treat non-bacterial diseases, and when antibiotics are not used for the prescribed amount of time or in the prescribed dose.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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