Problem Set # 1
Selection is a statistical, probabilistic process, not a deterministic process. We’ll see several implications of that in this course, and your understanding of the previous sentence will deepen considerably. For this problem set we want to help you understand that, although selection sorts individuals (allowing only some of them to reproduce), selection’s effect can be seen at the level of the population average. We’re going to consider two (kinds of) examples of this process.
1. As discussed in Module 2, the beaks of the Galapagos finches changed during the 1976-1977 drought; they became deeper (stouter). A. Draw a simple frequency distribution (a bell curve) of finch beak depth before the drought. (Google “graph of frequency distribution” if you don’t know what one looks like.) B. On the same page, right below the curve you just drew, draw a frequency distribution of finch beak depth after the drought. C. What is different about the two distributions?
D. How did that change come about? What happened to finches with different beak depths during the drought that produced the change? E. Can you express that idea by describing selection as “editing” the distribution of beak depths? F. After the drought ends (and the rains return, the plants flower, and the seed bank is restocked), how do you think selection will act on the distribution of finch beaks?
2. Now let’s consider another example, one that was presented briefly in module 1. Remember that female song sparrows of average size tend to survive best. A. Draw a frequency distribution of body size in female song sparrows. (This graph is not the same as the graph in Module 1, but it is the evolutionary result of the process depicted in that graph, right? Make sure you understand the difference) B. Using your “editing” language, how is selection acting on the song sparrow size distribution you just drew? Be clear about how the action of selection in this case is...
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