Genetics Natural Selection I

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p.reilly1227
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Genetics Natural Selection I
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2011-03-22 17:10:41
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Genetics Natural Selection
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Genetics Natural Selection I
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  1. Natural selection was proposed by? (2)
    • Darwin
    • Alfred Russel Wallace
  2. Def Natural Selection
    • Differential reproduction and survival of genetically determined forms
    • higher fitness genes are selected for
  3. Adaptation
    Feature of organism that improves ability to survive and reproduce within natural habitat
  4. Natural selection important points
    • not synonymous with evolution
    • operates through the "agents" in the enviornment (disease, parasitism, predation, competition)
    • selection operates to better adapt pop to their enviornment
    • Phenotype is the target of natural selection
    • Other genetic forces do not improve adaptiveness, only NS improves overall fitness of pop
  5. Adaptive vs Neutral variation
    • Adaptive -
    • Neutral -
  6. Fitness
    • A concept related to NS
    • Fitness - repoductive contribution of a genotype to future generations
  7. Selection coefficient and what does it mean?
    • (s) the intensity of selection = 1-w
    • (w) = relative fitness
    • if w = 1 then s = 0 and you are perfectly fit for env, selection is not working for or against you
  8. Components of Fitness
    multidimensional = overall fitness is made of different components
  9. Fitness Components (3)
    • viability (zygotic) selection =
    • Sexual - some genotypes mate better than others usually when competition between sexes is involved
    • less common phenotypes have more chance of mating because different
    • Fertility - some genotypes are more fertile than others
  10. How to calculate fitness value - convert absolute fitness to relative fitness value (w)
    divide each absolute frequency value by the largest absolute genotype frequency
  11. Points about fitness values(3) + note about trend
    • Enviornment specific
    • Rarely constant
    • Difficult to measure

    As high fitness geneotype becomes more prevelent/high frequency, fitness declines
  12. Howt is HW model modified to account for Natural Selection Model? How do you calculate frequencies?
    • zygotes form by random mating according to (p+q)2 over time (t)
    • selection occurs before zygotes are adults
    • t+1: ratios of genotype after selection are p2wAA etc
    • convert ratios to frequencies (average pop fitness = Wbar) = w(p+q)2
    • Frequency = w(p+q)2/(wbar)
  13. How do you calculate q'?
    q' = q(pwAa + qwaa)/(Wbar)
  14. Types of Selection
    • select for dominant
    • select for recessive
    • For heterozygote/overdominance/Heterozygote advantage/selection against homozygose
    • partial dominance selects for dominant, and partialy hetero - recessive selected against
    • underdominant - rapidly one or the other allele will be fixed depending on which way it is pushing (heterozygote is selected against)
  15. Things to know about selection against a recessive
    • steepness of curve is determined by s
    • q --> 0 asymptotically: recessive alleles hidden in heterozygote
  16. Things to know about selection for Dominant
    • Steepness of curve determined by s
    • Curves do not approach q --> asymptotically
    • Dominant lethal will be removed from population in 1 generation
    • System has trivial equilibria
  17. things to know about selection for heterozygote
    • qeq determined by selection coefficients
    • stable equilibrium
    • mechanism for maintaining both alleles in the population
    • polymorphism and genetic load are arguments agianst any advantage to selecting for heterozygotes
  18. Most famous example of overdominance is ________?
    Sickle-cell anemia
  19. Things to know about Partial dominance
    Equilibrium is reached faster than in full dominance
  20. Underdominance
    • selecting against heterozygote
    • three possible equilibrium:
    • pop becomes fixed for A1
    • pop becomes fixed for A2
    • both alleles in pop - unstable
    • not a good way to maintain polymorphism in a population
  21. Fitness increase over time
    • Average pop fitness will increase over time as less fit genotypes become more rare unless;
    • there is no variation
    • the fene pool is at equilibrium
  22. Mutation selectin model
    • Model considers the joint effects of selectionand mutation.
    • The model:
    • 1. Dominant allele A present in frequency p.
    • 2. Recessive allele a present in frequency q.
    • 3. Selection against aa (waa= 1 - saa.(4. Forward mutation of A to a at rate µ
  23. Mutation-Selection model
    Mutation and selection operating in opposite directions

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