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.
Read more about Evolutionary Pressure: Antibiotic Resistance, Natural Selection in Humans, Herbicide/Pesticide Resistance
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