Any cause that reduces reproductive success in a proportion of a population, potentially exerts evolutionary pressure or selection pressure.[1] 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.
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Drug resistance in bacteria is an example of the 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. Because of this, the drug resistance increases over generations. 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.[2]
The Malaria parasite can exert a selective pressure on populations. This pressure has led to natural selection for erythrocytes carrying the sickle cell hemoglobin gene mutation (Hb S)—causing sickle cell anaemia—in areas where malaria is a major health concern, because the condition grants some resistance to this infectious disease [3]
Just as with the development of antibiotic resistance in bacteria, resistance to pesticides and herbicides has begun to appear with commonly used agricultural chemicals. For example:
For more information see Pesticide resistance.