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

Population genetics

Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...

, selection pressure is usually expressed as a selection coefficient

Selection coefficient

In population genetics, the selection coefficient is a measure of the relative fitness of a phenotype. Usually denoted by the letter s, it compares the fitness of a phenotype to another favored phenotype, and is the proportional amount that the considered phenotype is less fit as measured by...

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Antibiotic Resistance

Drug resistance

Drug resistance

Drug resistance is the reduction in effectiveness of a drug such as an antimicrobial or an antineoplastic in curing a disease or condition. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance. More commonly, the term is used...

 in bacteria is an example of the outcome of natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

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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.

Natural selection in humans

The Malaria

Malaria

Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 parasite can exert a selective pressure on populations. This pressure has led to natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 for erythrocytes carrying the sickle cell hemoglobin

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 (Hb

HB

HB may refer to:* HB , a 1920s automobile* HB , a German brand of cigarettes* Hofbräuhaus, a brewery in Munich* HB Ice Cream, an Irish brand* HB , a Finnish symphonic metal musical group...

 S)—causing sickle cell anaemia—in areas where malaria

Malaria

Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

 is a major health concern, because the condition grants some resistance to this infectious disease

Herbicide/Pesticide Resistance

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
  Malathion
  Malathion is an organophosphate parasympathomimetic which binds irreversibly to cholinesterase. Malathion is an insecticide of relatively low human toxicity, however one recent study has shown that children with higher levels of organophosphate pesticide metabolites in their urine are more likely...
  
  , a pesticide used to kill them.

• In Hawaii and Japan, the diamondback moth
  Diamondback moth
  The diamondback moth , sometimes called cabbage moth, is a European moth believed to originate in the Mediterranean region that has since spread worldwide. The moth has a short life cycle , is highly fecund and capable of migrating long distances...
  
   developed a resistance to Bacillus thuringiensis
  Bacillus thuringiensis
  Bacillus thuringiensis is a Gram-positive, soil-dwelling bacterium, commonly used as a biological pesticide; alternatively, the Cry toxin may be extracted and used as a pesticide. B...
  
  , 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
  DDT
  DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
  
   is no longer effective in controlling mosquito
  Mosquito
  Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
  
  es that transmit malaria
  Malaria
  Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
  
   in some places, a fact which contributed to a resurgence of the disease.

• In the southern United States, the weed Amaranthus palmeri
  Amaranthus palmeri
  Amaranthus palmeri is a species of edible flowering plant in the amaranth genus. It has several common names, including Palmer's amaranth, Palmer amaranth, Palmer's pigweed, and carelessweed. It is native to most of the southern half of North America. Populations in the eastern United States are...
  
  , which interferes with production of cotton, has developed widespread resistance to the herbicide Roundup.

For more information see Pesticide resistance

Pesticide resistance

Pesticide resistance is the adaptation of pest population targeted by a pesticide resulting in decreased susceptibility to that chemical. In other words, pests develop a resistance to a chemical through natural selection: the most resistant organisms are the ones to survive and pass on their...

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