11.2 Natural Selection in Populations
11.2 Natural Selection in Populations
11.2 Natural Selection in Populations
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<strong>11.2</strong> <strong>Natural</strong> <strong>Selection</strong> <strong>in</strong> <strong>Populations</strong><br />
KEY CONCEPT<br />
<strong>Populations</strong>, not <strong>in</strong>dividuals, evolve.
Individuals Individuals DON’T survive are evolve… selected or don’t survive… but<br />
Individuals only <strong>Populations</strong> populations reproduce evolve evolve! or don’t…<br />
AP Biology<br />
2007-2008
<strong>11.2</strong> <strong>Natural</strong> <strong>Selection</strong> <strong>in</strong> <strong>Populations</strong><br />
<strong>Natural</strong> selection acts on distributions of traits.<br />
• A normal distribution graphs as a bell-shaped curve.<br />
– highest frequency near<br />
mean value<br />
– frequencies decrease<br />
toward each extreme<br />
value<br />
• Traits not undergo<strong>in</strong>g<br />
natural selection have a<br />
normal distribution.
3 Types of <strong>Natural</strong> <strong>Selection</strong><br />
• Changes <strong>in</strong> the average trait of a population<br />
DIRECTIONAL<br />
SELECTION<br />
STABILIZING<br />
SELECTION<br />
DISRUPTIVE<br />
SELECTION<br />
AP Biology<br />
giraffe neck<br />
horse size human birth weight rock pocket mice
<strong>11.2</strong> <strong>Natural</strong> <strong>Selection</strong> <strong>in</strong> <strong>Populations</strong><br />
– Directional selection favors phenotypes at<br />
one extreme.
<strong>11.2</strong> <strong>Natural</strong> <strong>Selection</strong> <strong>in</strong> <strong>Populations</strong><br />
– Stabiliz<strong>in</strong>g selection favors<br />
the <strong>in</strong>termediate phenotype.
<strong>11.2</strong> <strong>Natural</strong> <strong>Selection</strong> <strong>in</strong> <strong>Populations</strong><br />
– Disruptive selection favors<br />
both extreme phenotypes.
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