Natural Selectio2 - curve create their own smaller curves...

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Natural Selection Directional selection In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In the case of such selection, the mean of the population graph shifts. Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks. Figure%: The effect of directional selection on trait distribution Disruptive Selection In disruptive selection, selection pressures act against individuals in the middle of the trait distribution. The result is a bimodal, or two-peaked, curve in which the two extremes of the
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Unformatted text preview: curve create their own smaller curves. For example, imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants. Such a population, in which multiple distinct forms or morphs exist is said to be polymorphic. Figure%: The effect of disruptive selection on trait distribution...
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This note was uploaded on 01/27/2012 for the course BIOLOGY BSC1005 taught by Professor Rodriguez during the Winter '09 term at Broward College.

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Natural Selectio2 - curve create their own smaller curves...

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