Cutter_Lecture_11 - Howtointerpretyourmidtermscore 30 25...

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22 /30 Æ A range 17 21 /30 Æ B range 13 16 /30 Æ C range 10 12 /30 Æ D range 9 /30 Æ Fail How to interpret your midterm score 1 0 5 10 15 20 25 30 1234567891 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8
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2 Population Genetics of Natural Selection (3)
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Directional selection = selection that reduces polymorphism Results in fixation of alleles Balancing selection = any kind of selection that promotes polymorphism May result in persistence of multiple alleles Forms of Selection 3 Heterozygote advantage (overdominance) Antagonistic selection Varying selection Temporal Spatial Frequency dependent selection
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AKA: underdominance Heterozygote genotype has LOWEST fitness Allelic polymorphism will NOT be maintained But genotype with max fitness might not get fixed! Heterozygote disadvantage 4 Adaptive valley Adaptive peaks
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In real, finite populations… BOTH drift and selection affect dynamics of loci that contribute to fitness differences Changes in allele frequency are affected by: Effective population size (N e ) Strength of selection (s) Relative size of s and N e determine which force dominates If s >> 1/(4N e ), then SELECTION is dominant evolutionary force N e >> 1/(4s) Changes in allele frequency will be mostly deterministic If s << 1/(4N e ), then DRIFT is dominant evolutionary force N e << 1/(4s) … “nearly neutral” Changes in allele frequency will be mostly stochastic Selection and Drift 5
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1. Exact equilibrium frequencies predicted by genotype fitness values may not be attainable Frequencies will drift among values close to the equilibrium 2. Selection is less effective in small populations Greater influence of chance in small pops. A weakly advantageous allele is less likely to fix in a small population A weakly deleterious allele is more likely to fix in small populations 3 Implications of Selection Drift Trade off 6
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3. Population bottlenecks temporarily increase the role of drift Such drift may counteract selection E.g. allow deleterious alleles to reach higher frequencies
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Cutter_Lecture_11 - Howtointerpretyourmidtermscore 30 25...

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