Directional Selection

Directional - due to soot The once rare dark-colored moths became more prevalent while the once-common light-colored moths became increasingly rare

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Directional Selection Directional selection tends to favor phenotypes at one extreme of the range of variation. Insecticide resistance is an example. DDT was a widely used insecticide. After a few years of extensive use, DDT lost its effectiveness on insects. Resistance to DDT is a genetic trait that the presence of DDT in the environment made into a favored trait. Only those insects resistant to DDT survived, leading over time to populations largely resistant to DDT. Another example is the peppered moth ( Biston betularia ). Before the Industrial Revolution in the 18th and early 19th centuries, only light-colored moths were collected in light-colored woodlands in England. There was a rare, dark form. With the pollution caused by the buring of coal, the light-colored tree trunks became darker
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Unformatted text preview: due to soot. The once rare dark-colored moths became more prevalent, while the once-common light-colored moths became increasingly rare. Reason: predation by birds. The color that had the greatest contrast with the background (tree trunk) was at a disadvantage. Cleanup of the forest during the 1950s caused the allele frequencies of light and dark moths to reverse to pre-Industrial Revolution levels, dark moths are now rare, light moths are now common. The resistance of many bacterial species to antibiotics ia another example of directional selection. Over 200 speciews show some degree of antibiotic resistance, necessitating the development and more prudent use of a new generation of antibiotic medicines....
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This note was uploaded on 11/29/2011 for the course BIO BSC1010 taught by Professor Gwenhauner during the Fall '10 term at Broward College.

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