Artificial Selection Experiment on Littorina Popgenica: Analysis and Experiments

Explanation of the Artificial Selection Experiment

Question 1  

You are working on a population of marine snail, Littorina popgenica that lives in the intertidal zone along the Pacific coast of North America.  You decide to start an artificial selection experiment in the lab to see if you can select for a decrease in the thickness of the shell.  Figure 1 shows the distribution of shell thickness in your snail population before you start the experiment, Figure 2 shows the results of your selection experiment over about 20 generations (it's a long experiment!).  The average shell thickness in your starting population is 3 mm.  Answer the following questions about this experiment.

[1a] After about generation 6, your snail population stops responding to selection for decreased shell thickness, which stabilizes at about 2.6 mm.  You do a regression analysis of midparent shell thickness on offspring shell thickness and find that there is no significant correlation between the shell thickness of parents and their offspring.  How do you explain this selection plateau given the results of the regression analysis?

[1b] At generation 10, you stop selecting for decreased shell thickness of your snails but maintain the population in culture.  After about 4 generations, the average thickness of the shells starts to increase again.  How do explain this increase in shell thickness given your answer to [1a]?

[1c]The green crab, Carcinus maenas does not occur in the area where the population from which you chose snails for your artificial selection experiment occurs.  However, this crab does live in the area of a snail population about 1000 km to the south. The mean shell thickness of this snail population is 4.5 mm.  Design an experiment(s) to determine if the difference in mean shell thickness between the two snail populations is an adaptation to crab predation.  Explain the specific objectives of each component of your research plan in relation to the underlying evolutionary principles you are trying to address, and describe the results you expect to get if your hypothesis about shell thickness is correct.

Question 2

You collect Littorina popgenica snails from the southern population in Question 1 to make some crosses and find that the proportion of eggs that develop into juvenile snails (this species has direct development) varies depending on the source of the parents.  In general, about 5% of the eggs in crosses between parents from the same site fail to develop.  However, up to 35% of the eggs from crosses between populations fail to develop.  Indicate the name for this phenomenon and suggest an explanation for it.

Question 3

Littorina popgenica occurs on the Pacific coast between Vancouver, BC and San Francisco, CA.  You sample 50 snails every 100 km along this transect, extract DNA and then PCR amplify and sequence 800 bp of the mitochondrial cytb gene from each individual.  You then generate a mismatch distribution using these sequences. The results are shown in Figure 3.  

[3a] Suggest an explanation for the distribution that you observe and support it using all the information you have been given about this snail species. 

[3b]  Based on the fossil record, you know that L. popgenica diverged from a closely related species, L. purpurata 10 million years ago. The average observed pairwise nucleotide divergence at cytb between these 2 species is 0.15.  Use these data to estimate λ, the rate of substitution per nucleotide site per year.  Show your calculations and round your answer to 2 significant digits.

[3c]You use the sequence data to calculate Tajima’s D for this population.  You find the value to be 3.56, which is significantly different from 0.  What do you conclude from this result?

Question 4  

As part of an aquaculture project, fisheries biologists are investigating inheritance of body length in rainbow trout (Oncorhynchus mykiss). Researchers determined individuals’ genotype at a locus encoding a growth hormone using PCR and sequencing, and then chose 50 individuals of each genotype and measured their body length. 

There are 2 alleles at the growth hormone locus, A and B. The frequency of the A allele in the trout population is 0.7. The fish population is panmictic. The genotype and length data for each fish in the sample are provided in the excel file:  A4-trout-data.xlsx

Answer the following questions using the data.  Show you calculations except for [4a]. Round all answers to 2 decimal places.