Mechanisms of Evol_1_web

Mechanisms of Evol_1_web - A few points about...

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A few points about variation - before we discuss: Mechanisms of Selection
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Recall: heritable variation raw material upon which Natural Selection operates. Not all phenotypic variation in a pop. is heritable (i.e. env. – induced) most: genes environment Phenotypes Variation Within Populations
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Variation in a pop is maintained through: Mutations - result in new alleles arising Sexual reprod. - provides new combinations of alleles Diploidy – can shield recessive allele from selection Selection – via heterozygote advantage, freq. – dependant selection, temporal or spatial variation in selection (i.e. environmental heterogeneity)
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Success of morph inversely proportional to frequency in population Frequency-dependent Selection Scale-eating cichlid (Lake Tanganyika) Fig. 23.10 5 th Edition
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1. Directional selection - favors individuals at one extreme of the distribution for a trait - the ave. value (size, color) of the trait increases or decreases over time. Three Ways For Selection to Affect A Trait
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Size of trait small large see Fig. 23.12
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Time (generations) small large size of trait see Fig. 23.12
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2. Stabilizing selection - favors individuals with an average value of a trait - selects against individuals with extreme values - the average value of the trait remains the same, but the variability declines
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Time (generations) small large size of trait see Fig. 23.12
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3. Disruptive selection - favors individuals at both ends of a distribution - selects against individuals with traits of average value
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Time (generations) small large size of trait see Fig. 23.12
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Consider Galapagos Finches
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Start with bird with bill of intermediate size
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Bills are tools for cracking seeds
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hard nuts small seeds
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hard nuts small seeds Size of bill small large
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hard nuts small seeds Size of bill small large
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hard nuts small seeds Size of bill small large
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directional stabilizing disruptive see Fig. 23.12
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