Lecture5_HWE_2009 - Exam Thursday 9/17/09 Evolutionary...

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Evolutionary Processes: Natural selection, cont. Gene flow S. Hedtke BIO 370 9/10/09 Exam Thursday 9/17/09 Will cover all material through today: Evolution, variation, mutation, natural selection, HWE, gene flow No Tuesday or Friday discussion sections next week Sample questions posted on the blackboard site Exam Thursday 9/17/09 Review sessions: Exam Thursday 9/17/09 Exam will include: A few multiple choice Definitions Short essay Grading will be curved IF NECESSARY (mean below 80) Natural Selection can be complicated by: • Changing environment (“fluctuating selection”) • Frequency-dependent selection • Multiple traits (pleiotropy) Fluctuating Selection Under fluctuating selection, the environment changes over time. Different phenotypes/genotypes are favored as the environment changes. This means that the fitness function (which tells you what phenotypes have the highest fitness) changes over time!
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Drought big bill HUGE rains small bill Fluctuating selection 1977 – drought favored big bills 1978 – rains favored small bills Frequency Dependent Selection: • The relative fitness of individuals depends on the phenotype/genotypes of others in the population • As the frequency of one particular phenotype/genotype increases, the advantage of that phenotype over the other individuals in the population decreases. • The fitness function isn’t constant over time, but not because the environment changes… because the population itself is changing. Tall out-competes short Competitive ability depends on frequency of types Examples of frequency dependent selection: • Host-parasite relationships (virulence/resistance) • Predator searching • Prey mimicking a poisonous species
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Pleiotropy Pleiotropy : when a single gene affects multiple traits Gene A Trait A Trait B Pleiotropy Selection favoring Trait A affects Trait B Gene A Trait A Trait B Pleiotropy Selection could favor one allele for trait A and a different allele for trait B! Gene A Trait A Trait B Who wins?? Pleiotropy Trade-offs or constraints in evolution may be common, because pleiotropy means that two traits don ! t necessarily evolve independently. Gene A Trait A Trait B Who wins?? Natural Selection…. There may not be a “perfect” optimum for a trait! Fluctuating selection and frequency- dependent selection means different alleles might be favored at different times Pleiotropy ! s effect on fitness is to constrain the ability of some traits to evolve, because one allele could increase fitness of one trait while decreasing fitness of another trait. Population Genetics
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Lecture5_HWE_2009 - Exam Thursday 9/17/09 Evolutionary...

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