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Natural Selection

Natural Selection - iv Why don’t we expect to see an...

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Bio201, Fall 2007 Lectures 4 & 5. Natural Selection I. Measuring fitness a. What are the components of Darwinian fitness? b. How do we calculate absolute fitness and relative fitness? c. What is a selection coefficient? II. What can we learn by modeling the evolutionary dynamics of traits under selection? a. Selection against a recessive trait i. At what values of p and q will a population experiencing selection reach equilibrium? ii. In this case, does selection increase or decrease genetic variation? iii. Is evolution fastest when selection is strong or weak? iv. Is evolution fastest when allele frequencies are rare, intermediate, or common? b. Example: evolution at the CCR5 locus in humans i. Is the CCR5- 32 allele dominant or recessive? ii. In this case, is selection acting for or against the recessive allele? iii. In a population where individuals have a 25% chance of becoming infected with HIV, why is the fitness of susceptible individuals 0.75?
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Unformatted text preview: iv. Why don’t we expect to see an increase in the frequency of the CCR5-∆ 32 in Europe? in Africa? III. Forms of Selection that maintain polymorphism a. Why is it a problem that Natural Selection often acts to reduce genetic variation? b. What is heterozygote advantage? i. What is an example? ii. Is it possible to get heterozygote advantage in cases where AA and Aa genotypes have the same phenotype? iii. Why are both alleles maintained in the population when heterozygotes have an advantage? c. What is frequency dependent selection? i. Advantage when rare. 1. How does ‘search image predation’ give an advantage to rare phenotypes? IV. Does selection always lead to adaptive improvement? a. What evidence do we have that selection for ‘sexy’ traits in males often reduces their survival?...
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