acquire a different shade of color that enables it to avoid being seen by predators; or a fly might have a difference in its wing patterns or courtship behaviors that more successfully attracts mates.

If a mutation increases the survivability of an organism, that organism is likely to have more offspring than other members of the population. If the offspring inherit the mutation, the number of organisms with the advantageous trait will increase from one generation to the next. In this way, the trait — and the genetic material (DNA) responsible for the trait — will tend to become more common in a population of organisms over time. In contrast, organisms possessing a harmful or deleterious mutation are less likely to contribute their DNA to future generations, and the trait resulting from the mutation will tend to become less frequent or will be eliminated in a population. Evolution consists of changes in the heritable traits of a population of organisms as successive generations replace one another. It is populations of organisms that evolve, not individual organisms.

The differential reproductive success of organisms with advantageous traits is known as natural selection, because nature “selects” traits that enhance the ability of organisms to survive and reproduce. Natural selection also can reduce the prevalence of traits that diminish organisms’ abilities to survive and reproduce. Artificial selection is a similar process, but in this case humans rather than the environment select for desirable traits by arranging for animals or plants with those traits to breed. Artificial selection is the process responsible for the development of varieties of domestic animals (e.g., breeds of dogs, cats, and horses) and plants (e.g., roses, tulips, corn).

[Natural selection: Differential survival and reproduction of organisms as a consequence of the characteristics of the environment.]

Evolution in Medicine: Combating New Infectious Diseases

In late 2002 several hundred people in China came down with a severe form of pneumonia caused by an unknown infectious agent. Dubbed “severe acute respiratory syndrome,” or SARS, the disease soon spread to Vietnam, Hong Kong, and Canada and led to hundreds of deaths. In March 2003 a team of researchers at the University of California, San Francisco, received samples of a virus isolated from the tissues of a SARS patient. Using a new technology known as a DNA microarray, within 24 hours the researchers had identified the virus as a previously unknown member of a particular family of viruses — a result confirmed by other researchers using different techniques. Immediately, work began on a blood test to identify people with the disease (so they could be quarantined), on treatments for the disease, and on vaccines to prevent infection with the virus.

An understanding of evolution was essential in the identification of the SARS virus. The genetic material in the virus was similar to that of other viruses because it had evolved from the same ancestor virus. Furthermore, knowledge of the evolutionary history of the SARS virus gave scientists important information about the disease, such as how it is spread. Knowing the evolutionary origins of human pathogens will be critical in the future as existing infectious agents evolve into new and more dangerous forms.

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