Lesson on Natural Selection and Homeostasis in the Mussels Mytilus edulis... mussel Mytilus edulis.jpg


The 2016 AP Biology exam opened with a fantastic free response question that asked students to incorporate multiple biological concepts. Students were asked to understand:

I loved this AP Biology exam question because it incorporates the Big Ideas of biology, ecology, genetics, homeostasis, and evolution. The question from the College Board can be found HERE. This question is based on a research paper from Koehn, Newell, and Immermann (1980) called Maintenance of an aminopeptidase allele frequency cline by natural selection. I created a simulation to help my students work through the paper and teach the principles of the Hardy-Weinberg equilibrium. HERE is the simulation.

Here is how I structured my lesson:

Day 1: Assigned the Koehn et. al. paper with the following questions. HERE is a link to the article. I allowed 2 days to complete the reading questions.

  1. What is the function of the Lap gene in bivalves?
  2. Distinguish between Lap94 and Lap96 alleles.
  3. Explain the significance of each of the following:
  1. Figure 1:
  2. Table 1
  1. On page 5387 the authors state “...the frequency of Lap94 in immigrants was not significantly correlated with either environmental salinity or total enzyme activity measured in adults.” Why is this evidence important in defending their claim that the aminopeptidase allele frequency cline is being maintained by natural selection.
  2. Why are the mussels with the Lap94 allele found more frequently in one sample site more than others? Explain and justify their selective advantage.

Day 3: Using the simulation we reviewed the following problems. 

LAP Alleles, Mussels, and Natural Selection

  1. Run the simulation under initial conditions. LAP94 is the dominant allele. What is the allele frequency of the LAP94 allele at time 0? Time 50?

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  1. Adjusting the salinity to 29 ppt, then run the simulation.
  1. What is the allele frequency of the LAP94 allele at time 0? Time 50?

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  1. Examine the “Phenotypes” tab on the graph. Did the population evolve? Support your claim by calculating the Chi-square test for the population at time 0 versus time 50.

  1. Experiment with the simulation.
  1. What is the optimal salinity for individuals with the LAP94 allele? How do you know?

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  1. What is the optimal salinity for individuals with that are homozygous LAP96/LAP96? How do you know?

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  1. Explain why individuals with the LAP94 allele increase over time in waters with higher salinity (36 ppt)?

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  1. What happens to mussels in water with a salinity of 45 ppt? What happens to mussels in pure water?

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  1. Draw a blue mussel cell would behave in each of the four examples. Justify your drawing.

LAP94/LAP94 mussel cell in a salinity of 23 ppt.

Justification:

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LAP96/LAP96 mussel cell in a salinity of 23 ppt.

Justification:

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LAP94/LAP94 mussel cell in a salinity of 36 ppt.

Justification:

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LAP96/LAP96 mussel cell in a salinity of 36 ppt.

Justification:

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Student Example

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My Example on Whiteboard at the end of the class period.

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Day 4: As a review students completed the College Board 2016 AP Biology question. This was an independent assessment.

  1. What is the niche of Mytilus edulis?

  1. What is aminopeptidase?

  1. What is the function of the Lap gene in bivalves?

  1. Distinguish between Lap94 and Lap96 alleles.

  1. Explain the significance of each of the following:
  1. Figure 1:

  1. Table 1

  1. On page 5387 the authors state “...the frequency of Lap94 in immigrants was not significantly correlated with either environmental salinity or total enzyme activity measured in adults.” Why is this evidence important in defending their claim that the aminopeptidase allele frequency cline is being maintained by natural selection.

  1. Why are the mussels with the Lap94 allele found more frequently in one sample site more than others? Explain and justify their selective advantage.