Bio II Slide Set X

Bio II Slide Set X - Monday, June 13, 2011 I. A Closer Look...

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Monday, June 13, 2011
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I. A Closer Look at Natural Selection (“NS”) Natural selection - differential survival and reproduction of heritable phenotypes (i.e., 1 allele favored over another); leads to adaptation Adaptation - a heritable phenotypic modiFcation that increases one’s ability to survive and reproduce in a aprticular environment Fitness - ability of an organism to survive and reproduce (and thus to contribute genes to the next generation) Relative ftness - “w”; an individual’s contribution of o±±spring to the next generation relative to others in the population; a way to measure adaptation. The most Ft genotype in a pop’n. is assigned a w value of 1; therefore, relative Ftness ranges from 0-1: w = 0 (totally unFt) w = 1 (most Ft) Better-adapted phenotypes increase in ±requency in a population due to natural selection because they have greater relative ftness. Monday, June 13, 2011
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QUESTION OF INTEREST HERE: How does NS bring about adaptation? (how does NS change the frequency of a particular phenotype/genotype in a population?) Monday, June 13, 2011
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There are 3 basic patterns of natural selection that describe the relationship between a trait and relative Ftness: 1. Directional selection 2. Stabilizing selection 3. Disruptive selection Monday, June 13, 2011
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1. Directional selection: when an extreme (“non-average”) phenotype has higher ftness (result: the mean phenotype shifts right or left and the variance in phenotypes may or may not change ) Fig. 23. 12 EFFects oF selection on phenotype distributions center oF curve = mean oF trait width oF curve = variance oF trait Monday, June 13, 2011
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Ex: Directional (“artifcial”) selection on egg laying in domestic hens *you see an obvious steady trend in 1 direction Egg production 1933 125 eggs /yr 1968 245 eggs /yr Monday, June 13, 2011
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Ex: Directional selection on size in pink salmon *A switch to net fshing meant more large salmon caught (= removed From gene pool) and inadvertent selection ±OR small sizes (small salmon have higher Monday, June 13, 2011
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center of curve = mean width of curve = variance Fig. 23. 12 Effects of selection on phenotype distributions 2. Stabilizing selection: intermediate phenotypes have higher Ftness (result: the mean phenotype is unchanged but there is less variation among phenotypes Monday, June 13, 2011
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Ex: Stabilizing selection on human birth weight Fig. from Evolution , Monroe W. Strickberger Prior to many medical advances, too-large and too- small babies had higher mortality…. = you see stabilizing selection on an intermediate phenotype Monday, June 13, 2011
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more chicks = too difFcult to feed Ex: Stabilizing selection on a “moderate” number of eggs in starlings (4-5 eggs is optimal) fewer eggs = lower Ftness (fewer genes passed on) Monday, June 13, 2011
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Fig. 23. 12 Effects of selection on phenotype distributions Disruptive selection: when both extreme (“non-average”)
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This note was uploaded on 06/13/2011 for the course BIOLOGY bsc2011 taught by Professor Dr.spears during the Spring '10 term at FSU.

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Bio II Slide Set X - Monday, June 13, 2011 I. A Closer Look...

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