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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.
Natural selection in humans Edit
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 
Herbicide/Pesticide Resistance Edit
Just as with the development of antibiotic resistance in bacteria, resistance to pesticides and herbicides has begun to appear with commonly used agricultural chemicals. Some examples include:
- In the US, studies have shown that fruit flies that infest orange groves were becoming resistant to malathion, a pesticide used to kill them.
- In Hawaii and Japan, the diamondback moth developed a resistance to Bacillus thuringiensis, which is used in several commercial crops including BT Corn, about three years after it began to be used heavily.
- In England, rats in certain areas have developed such a strong resistance to rat poison that they can consume up to five times as much of it as normal rats without dying.
- DDT is no longer effective in preventing malaria in some places, a fact which contributed to a resurgence of the disease.
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