Problem Set 1 – According to the evolutionary tree

BI 310

Evolution

Problem Set 1

Answer each of the following questions (10 points each; 100 points total).

1. According to the evolutionary tree (phylogeny of mammals) below, which is more closely related to rodents: shrews and moles, or primates? Explain how the tree shows this.

2. In the tree above, identify a monophyletic group to which the aardvark belongs. Who else is a member? Identify a larger clade that it also belongs to.

3. Assuming the four living species of fish in the figure below evolved from their common ancestor (shown in figure) without convergence or reversal, reconstruct their evolutionary relationships and label the transitions. Which evolved first: stripes, or spiky back fins? How do you know?

4. Consider a population containing the following genotypes: Aa, Aa, AA, aA, aa, Aa, aa, aA, aa, Aa. What is the frequency of genotype aa? Allele A? Allele a? Can you tell which genotype is most advantageous? Can you tell whether Aa resembles AA or aa? Why or why not?

5. How do chromosome inversions happen? What consequences do they have for the evolution of populations?

6. What phenomenon is being described in this example: A beetle species is introduced to an island covered with dark basaltic rock. On this dark background, dark beetles, TT or Tt, are much more resistant to predation than are light-colored beetles, tt. The dark beetles have a large selective advantage. Both alleles are relatively common in the group of beetles released on the new island.

Once, you’ve identified the phenomenon, describe how it is likely to affect allele frequencies in succeeding generations.

7. In humans, the COL1A1 locus codes for a certain collagen protein found in bone. The normal allele at this locus is denoted with S. A recessive allele s is associated with reduced bond mineral density and increased risk of fractures in both Ss and ss women. A study of 1,778 women showed that 1,194 were SS, 526 were Ss, and 58 were ss (Uitterlinden et al., 1998). Are these two alleles in Hardy-Weinberg equilibrium in this population? How do you know? What information would you need to determine whether the alleles will be in Hardy-Weinberg equilibrium in the next generation?

8. Consider a new population that has just arrived at a remote island. Is the population likely to be large or small? Will founder effects, genetic drift, and additional waves of migration from the mainland play a relatively large or small role in the evolution of the new island population (compared to a similar population on an island close to the mainland)? Do your answers here help explain why unusual endemic species are more common on remote islands than on islands close to the mainland? Why or why not?

9. Inbreeding can reduce offspring fitness by exposing deleterious recessive alleles. However, some animal breeders practice generations of careful inbreeding within a family, or “line breeding,” and surprisingly many of the line-bred animals, from champion dogs to prize cows, have normal health and fertility. How can it be possible to continue inbreeding for many generations without experiencing inbreeding depression due to recessive alleles? (Hint: Consider some of the differences between animal breeders and natural selection in the wild.) Generally, if a small population continues to inbreed for many generations, what will happen to the frequency of the deleterious recessive alleles over time?

10. Imagine a population of pea plants that is in linkage equilibrium for two linked loci, flower color (P=Purple, p=pink) and pollen shape (L=long, l=round). What sort of selection event would create linkage disequilibrium? (for instance, will selection at just lone locus, like all red-flowered plants die) create linage disequilibrium? How about selection at two loci? How about selection on certain combination of genotypes at two loci?)

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Explanation & Answer

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