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Homework answers / question archive / 1)Natural selection and genetic drift can both result in the fixation of alleles

1)Natural selection and genetic drift can both result in the fixation of alleles


1)Natural selection and genetic drift can both result in the fixation of alleles.  How are the two forces of evolution different even though the population genetic consequences may be the same?  Under what conditions would you expect each to work most effectively and why?

  1. How do stabilizing and disruptive selection differ and how are each related to overdominance and underdominance respectively?  Give examples of traits in natural populations that are favored by stabilizing and disruptive natural selection.
  2. When natural selection is stabilizing, must heterozygotes always have the highest fitness?  Why or Why not?
  3. Explain how the balance between mutation and selection can actually lead to the maintenance of deleterious recessive or dominant mutant alleles.  Demonstrate your answers mathematically and graphically
  4. You know that in a finite population, the probability that a neutral allele will achieve fixation from this point in time on is simply its relative frequency in the population.  Summon your powers of deductive reasoning and tell us what is the probability that a brand new neutral mutant will be fixed in a population of N haploid individuals?
  5. In a multilocus system (with no dominance at each locus) all of the following genotypes are favored by stabilizing natural selection.  Which are examples of overdominance (i.e., heterozygote advantage) and which are not?  There are 10 loci and each + or – is the allele at the locus.
  6. Which of the three genotypes is most likely to increase in frequency under strong stabilizing selection when there is not heterozygote advantage?
  7. Distinguish zygotic from gametic gene flow and offer an example of a taxon in which each is likely to be the dominant form of gene flow.
  8. What are the specific genetic effects of inbreeding?  What are the specific genetic effects of genetic drift?  How do the two processes differ in their genetic consequences?  Which is more likely to cause deviations from Hardy-Weinberg equilibrium and why?
  9. What are the phenotypic effects of inbreeding and how do they arise?  Is inbreeding always bad for organisms in terms of mean fitness in a population?  Why or why not?
  10. Consider the following two populations.  Population A has about 10,000 breeding individuals while Population B has about 50 breeding individuals.  In principle, should the outcome of directional natural selection be the same in each population?  In practice, what is likely to happen?
  11. How strong must natural selection be, to overcome the effects of genetic drift in small populations?


  1. It can be theoretically shown that one migrant per generation can provide enough genetic contribution to keep populations from differentiating locally.  What are the genetic consequences of gene flow among populations?  What evolutionary forces are responsible for promoting and maintaining population genetic divergence?  How will gene flow interact with these forces to influence the level of population subdivision?
  2. What is the definition of a consanguineous mating?


  1. Many plant species are obligate selfers.  What would be an advantage of mating with yourself and how could you do so but still avoid inbreeding depression?
  2. What is effective population size (Ne) and why is it an important measure in population genetics models?


  1. The time to fixation of a neutral allele is directly proportional to which factor and why?
  2. What is a population bottleneck and how can it affect allele frequencies in a population?
  3. Why do founder events often lead to a decrease in the genetic diversity of a population?
  4. What types of traits are considered quantitative traits?  How is their inheritance controlled?


  1. Distinguish between additive genetic variance and dominance genetic variance.  Which of these contributes to evolutionary change and why?
  2. Many families of angiosperms include species whose growth form is a climbing vine.  In some families, the vines use tendrils to hold onto other plants.  In the Passifloracea the tendrils are modified stipules, while in the Bignoniaceae the tendrils are modified leaflets (part of a compound leaf) and in Ranunculaceae the tendrils are modified entire leaves.







  1. When bird watching you notice a male bird with exaggerated secondary sexual characteristics (i.e., really long tail feathers). 














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