Name ______Period ______

Evolution and Adaptation

Wooly Worms Simulating Natural Selection

Wooly worms are simply pieces of yarn distributed in a random manner over adesignated area of campus. You are going to simulate the feeding of predators that preferthe wooly worms in their diet. You and a partner will feed on (collect) as many of thesedelectable organisms as possible in a timed session. The collected worms will be countedand recorded and then we will use a Chi-Square test to determine if the worms werecollected in a random process or by some sort of selection process.

Since worms of certain colors are best suited to survive in their environment,certain variations are more favorable to the individual and species than others. Thesefavorable variations are termed adaptations. Adaptations increase an organism’s chancesof survival and subsequent ability to reproduce and pass on its traits (favorable genes) toits offspring. An example of adaptation is cryptic coloration, whereby an organismblends into its environment so well that it is difficult to detect. Cryptic coloration canhelp animals escape predators or capture unsuspecting prey. This idea aligns with theconcept of natural fitness in natural selection.

As with many other worms, the wooly worms in this activity represent insectlarvae in a natural habitat. They will complete their metamorphoses into adult insects, solong as they survive. Predation places a selection pressure on certain colors of woolyworms; those who exhibit favorable adaptations are positively selected for. Those whomay contrast with their surroundings are easy prey for the predatory birds, and are said tobe selected against. The gene frequencies for wooly worm coloration will change–suchchanges illustrate the dynamics of natural selection. Drastic and sudden changes in theenvironment may lead to extinction, but this is not common in nature.

Random Numbers and Selection:

The different colors of yarn distributed randomly on campus represent thedifferent color varieties of wooly worms. If these yarn pieces are collected randomly, thenumber of worms of each color should be nearly equal. If, however, the data does notsupport this hypothesis, then selection of certain colors must have occurred. A nullhypothesis is proposed for these circumstances, something along the lines that the colorof the wooly worms will have no effect on numbers of each color collected. If you canreasonably show that this is not the case through a statistical process, then naturalselection must have occurred.

You will use the Chi-square test to examine the differences between the numberof worms expected and the actual number you collected. The Chi-square test will tell youif the differences between what you collected and what was expected are too large to beattributed to chance alone. That is, does the variance from the expected fall withinstatistical limits and still support the null hypothesis? The Chi-square test cannot prove ordisprove a hypothesis, but it can provide you with a statement of probability concerningthe original null hypothesis.

Procedure:

  1. In a 1 minute period, predators walk around and pick up as many worms as theycan find. React like a predator–when you sight a worm, focus in on it and pickthat worm up.
  2. At the end of the time period, groups come back and count the number of wormspicked up by color. The recorder of the group will note the group results.
  3. After recording, the predators go back and randomly spread the worms for thenext group and then return to the classroom to analyze the data.
  4. Tally the number of each kind of worm you "ate" and record these numbers on theclass data table.
  5. Obtain data from each of the other groups and record it in the class data table.
  6. Record the totals from the class data set in Column A for each color (Observed Number).
  7. Divide the class total value for all colors (total sum) by the number of differentworm colors available. This will give the average number of worms of each typethat you would expect to “eat” if they were collected randomly. Record this valuein Column B (expected number).
  8. Calculate and complete columns C through E.
  9. Total up all the values in column E to get the Chi-squared value.
  10. 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 nullhypothesis can be accepted or rejected. Most biologist agree that Chi-squarevalues above the 0.05, or 5% level of probability , would tend to support the nullhypothesis by indicating that the numbers of each color yarn observed does notvary significantly from the expected. However, values at or below 0.05 level ofprobability suggest that the numbers that you observed are not likely to resultfrom chance factors alone. Therefore, such observed numbers suggest that certainyarn colors are being selected over others. Thus, the original null hypothesis mustbe rejected.
  11. Determine the level of probability (p) for your Chi-square value. The degrees offreedom used are always one less than the number of events or colors observed. Ifyou used 11 colors then use 10 degrees of freedom to determine the level ofprobability.

Individual Group Data Table:

Color / # Observed / Color / # Observed
White / Green
Brown/Black / Blue-Green
Red / Blue
Maroon / Purple
Yellow / Camo Green

Class Data Table:

Group # /Initials / 1 / 2 / 3 / 4 / 5 / 6 / 7 / Class Totals By Color
Color
White
Brown/Black
Red
Maroon
Yellow
Green
Blue-Green
Blue
Purple
Camo Green
Total

Answer the following questions (on a separate piece of paper):

  1. What was your original null hypothesis?
  2. What was your Chi-square value for this experiment?
  3. How many degrees of freedom were there for this experiment?
  4. What was the probability that your null hypothesis was acceptable?
  5. Did you accept or reject your null hypothesis?
  6. What does this mean?
  7. Which color worm had the best chance of being eaten?
  8. Which color worm had the best chance of survival?
  9. What will happen to the gene frequencies for the various colors of worms?
  10. What determines which genes will be an advantage and which ones will be adisadvantage?
  11. How does this experiment illustrate Darwin's theory of Natural Selection?