Charles Darwin accumulated a tremendous collection of facts to support the theory of evolution by natural selection. One of his difficulties in demonstrating the theory, however, was the lack of an example of evolution over a short period of time, which could be observed as it was taking place in nature. Although Darwin was unaware of it, remarkable examples of evolution, which might have helped to persuade people of his theory, were in the countryside of his native England. One such example is the evolution of the peppered moth Biston betularia.

The economic changes known as the industrial revolution began in the middle of the eighteenth century. Since then, tons of soot have been deposited on the country side around industrial areas. The soot discolored and generally darkened the surfaces of trees and rocks. In 1848, a dark-colored moth was first recorded. Today, in some areas, 90% or more of the-peppered moths are dark in color. More than 70 species of moth in England have undergone a change from light to dark. Similar observations have been made in other industrial nations, including the United States. 
 

Purpose:

To study how a change in coloration has come about and to relate this to species evolution. 
 

Data:
 

PART A:
 

An experiment was performed with Biston betularia in the following way. A large number of the moths were captured. The underside of each moth was marked with a small spot of paint for identification. Known numbers of marked moths were released in an unpolluted forest. Moths were subsequently recaptured and the marked ones were counted. Of 488 dark moths and 496 light moths released in an unpolluted forest, 34 dark moths and 62 light moths were recaptured.

1. Why was the spot of paint put on the underside of the moth rather then on the top?
2. What may have happened to the moths not recaptured?
3. How can the results of this experiment be used to illustrate natural selection? Describe a variation of this experiment that might be performed to provide even better evidence of natural selection.

PART B:

In still another experiment, equal numbers of light and dark moths were placed on a light-colored tree in an unpolluted forest. These moths were kept under careful observation. Birds were seen to seize moths from the tree and rapidly carry them away. At the end of the day, approximately twice as many light moths were left on the trees as dark moths. Then the reverse experiment was performed. Equal numbers of light and dark moths were placed on a dark-colored tree in a polluted forest. At the end of the day approximately twice as many dark-colored moths were left. 

Non-polluted Forest

Polluted Forest

1. What is the chief predator of the peppered moths?
2. Assuming that equal numbers of both types of moths are present , which type of moth would most likely be preyed on in each case? Why?
3. Does the experiment described above help support the conclusions you have drawn thus far? Explain. 
4. Which type of moth is more likely to survive in a polluted forest? In an unpolluted forest? 
5. Is it likely that any dark-colored moths existed before the revolution?
6. If any dark-colored moths did exist before the industrial revolution darkened the forests, what happened to them?

In an old forest in Scotland, far removed from industrial cities, lives a species of moth called Cleora repandata. Of about five hundred individuals observed, approximately fifty moths, or 10% of the total population, were dark colored while 90% were light colored. When these moths rest on the bark of pine trees during the daytime, the dark form is more conspicuous. Observation has shown that many moths move from one tree trunk to another during the day if they are disturbed by ants or the heat of the sun. In flight, the dark moth is visible for a distance of about twenty yards, but the light moth is visible for a distance of more than one hundred yards. Observers have reported seeing light moths captured in flight by birds.

1. Which body coloration is protective when the moths are resting? Explain.
2. Which body coloration is protective when the moths are in daytime flight? Explain.
3. If, among the offspring of the light colored moths not more than 0.0005% are dark, how can you explain the fact that 10% of the total population of moths in an unpolluted forest are dark colored?

PART D:

Determine the relaationship between the environmental changes and the color variation of the peppered moth by using research data to graph the effects of an environmental adaptation.

1. Table A contains data from a 10 year study of two varieties of the same species of peppered moth. The numbers represent moths captured in each of 10 consecutive years. The traps were located in the same area each year.

2. Using the data provided in Table A, construct a graph comparing the numbers of each variety of peppered moth.

Conclusion:

On the basis of your interpretation of the preceding experiments, try to answer the following fundamental questions:

1. How has the striking change in coloration come about? (Include an explanation of how the dark moth appeared and how the proportion of dark moths changed from 0.0005% to more than 90% in polluted forests.)
2. What underlying law of nature has produced this change? (Use Darwin's theory of evolution and apply it to what you have learned in this investigation.)
3. Is the change related to the mechanisms by which one species is thought to evolve normally from another? (Apply Darwin's ideas on the origins of new species.) 
4. Using the data on the graph, draw a conclusion concerning the population of peppered moths in the samples area of England.
5. Explain the reason for the increase in the dark-colored moths.
6. What effect would cleaning up the environment have on the moths?