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EvolutionMechanisms160-page15

EvolutionMechanisms160-page15 - Directional Selection •...

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Evolutionary Mechanisms - 15 How does Natural Selection Affect General Population Patterns? Three basic patterns emerge in populations through time, as a result of selection pressures (or forces): stabilizing, directional and disruptive selection. Stabilizing Selection Directional Selection Disruptive Selection Stabilizing Selection In stabilizing selection, a narrow range of phenotypes, often those that are intermediate and usually more common, are favored over extreme variants. Extremes tend to decrease from generation to generation. Stabilizing selection is more likely when the environment remains constant. Some organisms have been stable for eons, such as the Gingko tree and coelocanth. Horseshoe crabs, found off the southeast coast of the United States have been unchanged for thousands of years.
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Unformatted text preview: Directional Selection • In directional selection the population consistently shifts toward a phenotype that is favored at the expense of others. This differs from stabilizing selection where extremes are reduced at both ends of the variation continuum. • Directional selection is typically a response to a changing environmental condition or to a new environment. • Some examples of directional selection include: • Peppered moth mentioned earlier is a dimorphic population where the shift has been to the dark form in response to pollution • Resistance to pesticides is common in many organisms • Most induced selection or artificial selection is directional. We narrow the population for some characteristic....
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