Exercise 1.1

The Peppered Moth as an Example of Evolution by Natural Selection

(This exercise is based on Grant, B. S, D. F. Owen, and C. A Clarke. 1996. Parallel rise and fall of melanic peppered moths in America and Britain. Journal of Heredity 87: 351—357

(Note: The reference above links directly to the article on the journal’s website. To access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)


One of the early examples supporting Darwin’s evolution by natural selection involved changes in the frequencies of color morphs in Biston betularia, the peppered moth. During the nineteenth century, the dark (melanic) form of this moth went from being very rare to very common in the United Kingdom. This change in frequency coincided with the Industrial Revolution and and corresponding increase in air pollution. Presumably, as trees became darker due to soot, the darker form of the moth was less visible to predators (such as birds) than the lighter form. Studies in the middle twentieth century showed that the color difference was due primarily to a single gene, with the melanic allele being dominant to the non-melanic allele.

During the latter part of the twentieth century, clean air laws were passed in both the United Kingdom and the United States. This led to reductions in air pollution. Given that the increase of the melanic form coincided with the increase in air pollution making it less visible to predators, one might expect that a decrease in air pollution would lead to a decrease in the frequency of the melanic form.

Bruce Grant and his colleagues examined changes in the frequency of the melanic form of the moth over time in both the United States and the United Kingdom.

Figure 1 Frequency of melanic moths in Caldy Common, U.K. (solid circles) and in Michigan, U.S. (solid diamonds) between 1959 and 1995. Note that the moths at Caldy Common were sampled annually, but there was a large gap in the Michigan samples. (The open circles denote the theoretical expectations based on a constant selection coefficient of s = 0.153 against the dominant allele.)


Question 1. What pattern was observed in the frequency of the melanic moths at Caldy Common?


Question 2. During which year was the frequency of the melanic moths approximately 50% at Caldy Common?


Question 3. Compare the changes in frequency of melanic moths at Caldy Common and in Michigan. Are there any differences?


Question 4. The open circles in the figure represent expected values given continual selection against the melanic form at a rate sufficient to explain the decline in frequency of the melanic moths at Caldy Common. How do the expected and observed changes in frequency compare?


Question 5. What are some possible explanations for the discrepancy in Question 4?

Figure 2 The concentration of sulfur dioxide in the atmosphere in Michigan from 1964 to 1994. The regression line shows the average trend over time.


Question 6. What pattern is observed regarding sulfur dioxide concentrations?


Question 7. Based on the regression line, approximately when were sulfur dioxide concentrations half of what they were in 1964?

Figure 3 Concentration of particulates in the atmosphere in Michigan from 1964 to 1994. The line is a regression line showing the average trend over time.


Question 8. What pattern is observed regarding particulate concentrations? Is the pattern similar to that observed with sulfur dioxide?


Question 9. Does finding parallel changes in the frequency of melanic forms in both Europe and North America increase or decrease support for the claim that the changes in these frequencies are due to natural selection arising from cleaner air?


Question 10. If the differences in color of the moths were due entirely to environmental and not genetic factors, would the observed changes still be due to evolution by natural selection? Explain.