Examples of Evolution: Disease

Human diseases are excellent examples of evolution in action. Pathogens must evolve rapidly to avoid the human immune response and medical interventions, such as drugs. Because bacterial and viral pathogens have short, and generally quick, life cycles, evolution can be observed in a few days or months. On the opposite side of this equation, several human mutations appear to provide resistance to pathogens, explaining the persistence of these mutations in the human population.

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  • HIV: The Human Immunodeficiency Virus (HIV) evolves so rapidly in response to the selective pressure of anti-viral drugs that the viral population in each infected individual is a case study in evolution. Different HIV strains can be observed to rise in number and fall again in response to different drugs. Multiple drug therapies are designed to thwart HIV’s adaptive abilities. The evolutionary patterns of HIV in an individual are so well documented that they have been used to identify sources of HIV infection (see Forensics and Molecular Epidemiology). For more information try these sites:

 

  • Avian Flu: Every year we are exposed to a new version of the flu virus, because each year the flu virus evolves. Like other flu viruses, the avian flu also evolves through mutations and picking up pieces of other viruses (recombination). Humans have little resistance to this form of flu virus. Currently the virus passes between birds, and only occasionally infects a human. Usually these humans are in close contact with infected birds, so this virus remains limited in its ability to spread through human populations. However, if the avian flu evolves the ability to pass from human to human, it could spread around the world rapidly. For more information try these sites:
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  • Antibiotic Resistance: Bacteria and viruses evolve rapidly because they have a short generation time. E. coli generates offspring by binary fission; a process which takes only 20 minutes under favorable circumstances. When populations of bacteria or viruses are exposed to selective pressures, such as drug treatment, variation within the population ensures that some bacteria are resistant to the drugs while others are not. The resistant bacteria often survive the treatment and reproduce, resulting in a resistant population. For more information try these sites:

 

  • Lactose Intolerance: Lactose intolerance is found in some human populations, but not others. Cultures that have depended on milk and milk products for major dietary contributions have aquired mutations that allow retention of childhood tolerance for milk. In cultures in which milk is not part of adult diets these muations are not found, and adults are lactose intolerant. For more information try these sites:

 

  • Malaria and Sickle Cell Anemia: Malaria is a deadly parasitic disease which results in over 1 million deaths annually. It is carried by mosquitoes. Humans have tried to control the disease by attacking the parasite, P. falciparium, with drugs or by using pesticides to control the mosquitoes. In both cases, the organisms have developed resistance. Malaria is common in Africa and many people of African descent have either sickle cell anemia or sickle cell trait. . Sickle cell anemia is a deadly condition itself, however, the more commonly occurring sickle cell trait reduces the occurrence of malaria by making infection by P. falciparium less likely, hence this adaptation has been selected for and retained in human populations. For more information try these web sites:
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  • Cystic fibrosis and cholera: Cystic fibrosis is another example of a disease prevalent in one particular population of humans which may be linked to disease resistance. Cholera is a deadly disease common in areas with poor sanitation. It was rampant in Europe prior to adoption of sanitation measures and in people of European descent a genetic mutation which confers some resistance to infection by Vibrio cholerae is fairly common. A double dose of this gene results in cystic fibrosis, a deadly disease which typically kills its victims before the age of 20. For more information try these web sites:

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