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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.
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. For example, in hospitals, environments are created where pathogens such as C. difficile have developed a resistance to antibiotics. 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.
Natural selection in humans
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 
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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:
- 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 controlling mosquitoes that transmit malaria in some places, a fact that contributed to a resurgence of the disease.
- In the southern United States, the weed Amaranthus palmeri, which interferes with production of cotton, has developed widespread resistance to the herbicide glyphosate.
- In the Baltic Sea, decreases in salinity has encouraged the emergence of a new species of brown seaweed, Fucus radicans.
For more information see Pesticide resistance.
- "Selection Pressure". Iscid.org. Retrieved 2011-11-15.
- "Clostridium difficile- a continuing evolving pathogen". sciencedirect.com. 2009-06-19. Retrieved 2014-10-01.
- "CDC - Healthcare-associated infections - HAI". Cdc.gov. Retrieved 2011-11-15.
- Kenneth R. Bridges, M.D. (2002-04-02). "Malaria and the Sickle Hemoglobin Gene". Sickle.bwh.harvard.edu. Retrieved 2011-11-15.
- Pereyra1, R.T.; L. Bergströ;, L. Kautsky and K. Johannesson (2009). "Rapid speciation in a newly opened postglacial marine environment, the Baltic Sea". BMC Evolutionary Biology 9 (70). doi:10.1186/1471-2148-9-70.
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