2011 Slide Set XI Spr11

2011 Slide Set XI Spr11 - I A Closer Look at Natural...

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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 modification that increases one’s ability to survive and reproduce in a particular environment Fitness - ability of an organism to survive and reproduce (and thus to contribute genes to the next generation) Relative fitness - “w”; an individual’s contribution of offspring to the next generation relative to others in the population; a way to measure adaptation. The most fit genotype in a pop’n. is assigned a w value of 1; therefore, relative fitness ranges from 0-1: w = 0 (totally unfit) w = 1 (most fit) Better-adapted phenotypes increase in frequency in a population due to natural selection because they have greater relative fitness.
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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?)
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There are 3 basic patterns of natural selection that describe the relationship between a trait and relative fitness: 1. Directional selection 2. Stabilizing selection 3. Disruptive selection
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1. Directional selection: when an extreme (“non-average”) phenotype has higher fitness (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
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Ex: Directional (“artificial”) selection on egg laying in domestic hens *you see an obvious steady trend in one direction Egg production 1933 125 eggs /yr 1968 245 eggs /yr
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Ex: Directional selection on size in pink salmon *A switch to net fishing meant more large salmon caught (= removed from gene pool) and inadvertent selection FOR small sizes (small salmon have higher fitness: survive & reproduce)
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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 fitness (result: the mean phenotype is unchanged but there is less variation among phenotypes )
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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 A fairly high proportion of deaths between birth & one month of age is caused by selection against non-optimal phenotypes (birth weight).
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more chicks = too difficult to feed Ex: Stabilizing selection on a “moderate” number of eggs in starlings (4-5 eggs is optimal) fewer eggs = lower fitness (fewer genes passed on)
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Fig. 23. 12 Effects of selection on phenotype distributions Disruptive selection: when both extreme (“non-average”)
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