WOOLY WORM LAB
By Betty Ann Wonderly This is an adaptation of a lab published by Ken House in American Biology Teacher Volume 48, #4, April 1986. http://www.accessexcellence.org/AE/ATG/data/released/0310-BettyAnnWonderly/
Introduction:
In this lab you will study natural selection. During the exercise you will represent a predatory bird who feeds on wooly worms. The wooly worms are pieces of colored yarn that have been randomly distributed over an area on the school grounds. Some of the "wooly worms" will blend into the habitat while others will be easy to find. The colored yarn will be tallied and a Chi-square test will be used to determine if the wool pieces are collected randomly or by a selection process. It is hoped that this lab activity will add to your understanding of natural selection.
PROCEDURE AND QUESTIONS:
1. During a 3 minute "feeding" period you will collect as many wooly worms as possible in the collection area.
2. Return to the lab and list the various colors of "worms" in column A.
3. Tally the number of each kind of worm you "ate" and record the numbers in Table 1 under Column B (Observed Number)
4. Total this column of figures and then divide by the number of kinds of worms available. This will give the average number of worms of each kind that you would expect to find if you collected them randomly. This value should be entered in the C column as Expected number.
5. If the wool pieces were collected randomly, then the number of each color collected should be nearly equal. If the collection is random, then the differences between observed numbers and expected numbers could be attributed to chance. We can propose a Null Hypothesis that states that there will be no significant difference in the numbers of each color yarn collected. If the null hypothesis is not supported by the data, then selection of some colors over others must have occurred.
6. You will use the Chi-square test to examine the differences between the number of worms expected and the actual number you collected. The Chi-square test will tell you if the differences between what you collected and what was expected are too large to be attributed to chance alone. That is, does the variance from the expected fall within statistical limits and still support the null hypothesis? The Chi-square test cannot prove or disprove a hypothesis, but it can provide you with a statement of probability concerning the original null hypothesis.
7. Using a calculator, determine the differences between observed and expected values. Enter this number in Column D. Square this difference and enter in Column E. Divide this squared difference by the expected value from column C. Enter this value in column E. Total up all the values in column F to get the Chi-square value.
8. After the Chi-square value is determined, refer to a Chi Square Distribution Table. This table tells how much of a variance can be tolerated before the original null hypothesis can be accepted or rejected? Most biologist agree that Chi-square values above the 0.05, or 5% level of probability , would tend to support the null hypothesis by indicating that the numbers of each color yarn observed does not vary significantly from the expected. However, values at or below 0.05 level of probability suggest that the numbers that you observed are not likely to result from chance factors alone. Therefore, such observed numbers suggest that certain yarn colors are being selected over others. Thus, the original null hypothesis must be rejected.
9. Determine the level of probability (p) for your Chi-square value. The degrees of freedom used are always one less than the number of events or colors observed. If you used 11 colors then use 10 degrees of freedom to determine the level of probability.
10. Answer the following questions:
o What was your original null hypothesis.
o What was your Chi-square value for this experiment?
o How many degrees of freedom were there for this experiment?
o What was the probability that your null hypothesis was acceptable?
o Did you accept or reject your null hypothesis?
o What does this mean?
o Which color worm had the best chance of being eaten?
o Which color worm had the best chance of survival?
o What will happen to the gene frequencies for the various colors of worms?
o What determines which genes will be an advantage and which ones will be a disadvantage?
o How does this experiment illustrate Darwin's theory of Natural Selection?
Table 1 Chi-Square Calculations
X2 = Σ (Obs-Exp)2 / Exp (reminder df = # phenotypes -1)
Color / A (observed) / B (expected) / C (O-E) / D (O-E)2 / (O-E)2/ E1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Grand Total-→ / X2 = Σ →
The X2 (Chi-Square) Distribution Chart
Probability Values
df 0.05 0.025 0.01 0.001
1 3.841 5.024 6.635 10.828
2 5.991 7.378 9.210 13.816
3 7.815 9.348 11.345 16.266
4 9.488 11.143 13.277 18.467
5 11.070 12.833 15.086 20.515
6 12.592 14.449 16.812 22.458
7 14.067 16.013 18.475 24.322
8 15.507 17.535 20.090 26.125
9 16.919 19.023 21.666 27.877
10 18.307 20.483 23.209 29.588