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Any cause that reduces reproductive success in a proportion of a population potentially exerts Evolutionary pressure or selection pressure.citation needed 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.
Examples
Drug resistance in bacteria is an example of the outcome of evolutionary pressure.
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. Therefore, the concept can be described as the application of Charles Darwin's principle of "survival of the fittest" (which actually should be understood as "extinction of the un-fittest") via some selection mechanism.
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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