BIOL 3020 Population Genetics

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?