Camouflage and Protective Coloration: A Model of Natural Selection
Edison High School
Type of entry:
Type of activity:
This activity helps to develop the student's concept of natural selection and differential survival among divergent phenotypes of a population. The activity helps to develop the concept that fitness is related to environmental conditions which can and do change within the activity.
Materials and Equipment: per team of two students:
1 page of newspaper want ads
1 sheet of butcher paper of the same size
Envelope with 100 each of butcher paper circles and want ads circles cut with a paper punch.
Place paper circles in envelopes until used and have students replace them when finished. Monitor students to be sure that circles are randomly scattered and not piled up. Students will often predict that each type of prey will have an advantage, but they usually do not predict that one or the other on its own "home court" has a better home court advantage. The plain circles on a plain background are protective coloration while printed on printed is camouflage. In camouflage, color is not relevant, only pattern and shades matter. Pattern breaks up the outline and will perform better than protective coloration whose shadow always reveals its outline.
It probably will be necessary to show your students how to calculate the percentage of survivors and to find the percentage for advantage. Seems that few students believe scientists need math!. The percentage of advantage is just the difference between the survival percentages of the two forms in each background environment. Since printed in a plain background has no survival advantage, its adaptive value is 0%. The same is true for plain in a printed background.
What is required of students:
Students should be familiar with natural selection. They should know that the term adaptive shift refers to a change in the gene frequencies in a population in response environmental change. Students must be familiar with expressing gene frequencies in decimal form such as 0.8 B and 0.2 b for the alleles B and b. Finally, students show know how to use the gene pool method of calculation the frequencies of the genotypes for the next generation. This is done by using algebraic multiplication on the expression:
Students will work in teams of two gathering class data on what provides better fitness in a given environment, camouflage or protective coloration. Protective coloration is an adaptation where color matching is used to blend into the background, in this lab it will be demonstrated by using plain butcher paper circles on plain butcher paper background. Camouflage on the other hand, can use two or more colors to create a matching pattern that visually breaks up the outline of the organism so it blends into its surroundings. Camouflage will be demonstrated by using punched circles from the newspaper on a want ads page. Students carry out three timed trials on each background, compute averages, and report data to the class. Allow for ten minutes each for pre-lab introduction, timed trials, data reporting, and discussion of the data. Students will carry out the lab extension on their own. One class period is adequate for the activity.
Camouflage and Protective Coloration:
A Model of Natural Selection
Natural selection operates on the principle of survival of the fittest. Fitness can be defined as the suitability of an organism to a given environment. One might ask if one set of features favorable in one environment might prove unfavorable in another environment. In this lab you will test the following hypothesis: If survival is related to specific characteristics in a given environment, then altering the environment will decrease the survival rate. Once you have formed a conclusion to this hypothesis and how it relates to the adaptations of camouflage and protective coloration, you will apply this information to the peppered moths of Manchester, England. You will predict the direction of an adaptive shift and the resulting gene frequencies..
In this experimental model, the features studied will be the adaptations of camouflage and protective coloration in two very different environments. You will act as the predator and your prey will be two types of paper circles, one with a plain color and the other with a printed pattern. Their environment will be either a sheet of plain colored paper (butcher paper) or a sheet of paper with a printed pattern (newspaper want-ads).
Work with a partner. One of you must always be the predator while the other will supervise the experiment so as not to introduce an uncontrolled variable. The experimenter's job is to spread out the prey (paper circles) randomly for each trial making sure they are not piled up and that they thoroughly cover the sheet of paper. Also, the experimenter will time each trial and record the data.
The predator's role here is to locate (count them, do not pick them up) as many prey of a particular color as possible in 10 second trials. With the predator standing with his back to the hunting area, the experimenter will give a signal, the predator then quickly turns around and begins to spot and count the plain circles until the experimenter tells him to stop (at 10 seconds). The predator calls out his total. On the next trial, the predator attempts to spot and count just the printed circles for 10 seconds. These trials are each repeated two more times and then the environmental background will be changed to a printed pattern. Again three trials will be made attempting to spot and count each type of circle in 10 seconds this time starting with the printed circles first.
The particular predator in this problem only hunts for prey just before sunrise and just after sunset, therefore, the light in the room will be very subdued.
After each trial, the experimenter should rearrange all paper circles on the paper to make sure the population is randomly distributed. When all data is recorded, compute your average and report the results to your teacher.
Before starting, make a prediction.
Which circle on which background will get the lowest count? Why?
# of plain circles
# of printed circles
# of printed circles
# of plain circles
Interpretations and Conclusions
In natural selection, selective pressure is the factor that reduces the frequency of a particular phenotype more than another phenotype. In this model the phenotypes are plain and printed. What was the selective pressure on this population?
Some circles went uncounted and thus escaped predation. Why do some escaped?
Is survival equal for each phenotype? Explain why, or why not.
From results of this experiment, what can you conclude about the relative nature of fitness with respect to the environment?
How Does Natural Selection Affect Gene Frequencies?
Consider the peppered moths of Manchester, England. The gene for dark is dominant over light. It is a historical fact that at one time light forms of the moth were most common. Over a period of a half of a century, the dark form became the most common. Biologists hypothesized that this was an adaptive shift in response to a changing environment. Increased burning of coal during the industrial revolution caused air pollution so bad that pale colored lichens which grew on the trunks of trees were killed. As a result, the normally light scaly color of the trees took on a very dark tone of color. The tree trunk color is important to the survival of moths. Moths are active at night and rest on tree trunks during the day. Birds are their primary enemies, mostly eating varieties which are easy to find. According to the hypothesis, the light form of this moth became rare as a result of air pollution changing their habitat.
To test this hypothesis, scientists raised moths in the laboratory and released them in both wooded areas that were and were not affected by air pollution. Equal numbers of dark and light forms of the moths were released with gene frequencies of .5B and .5b, dark and light respectively. After a period of time during which the moths would have reproduced, it was expected that dark moths would out number light moths by a ratio of 3:1. This is expected through the Hardy-Weinberg calculation (gene pool method) assuming that dark (B) is dominant over white (b). Moths were sampled at night using lights. The capture results from one location revealed that instead of a 3 : 1 ratio of dark to light moths expected, the ratio was closer to 1 : 1.
Which forest do you think these results came from, polluted or non polluted? Why?
If the gene frequencies in the surviving adults had shifted to .4B and .6b, predict the frequencies on the genotypes in the next generation.
Use the gene pool method: (.4B +.6b)(.4B + .6b) = _______BB ________Bb _______bb
Make the unlikely assumption that the next generation is exactly 1000 moths. Also assume that all white moths escape predation. How many of the black moths must fall prey to birds to keep the ratio of black to white 1 : 1? Show your work.
Explain what and why a change would occur in the gene frequencies of future generations of moths if black soot from air pollution began settling on the pale colored tree bark?
Explain why it is unlikely that the white moths would completely disappear even if the forests became heavily polluted with black soot?
Look at pictures of the peppered moth in your textbook. Which form illustrates protective coloration? Which form illustrates camouflage?
What can you conclude about the relative nature of fitness with respect to experimental and the real life situations involving the peppered moth?