photos:
- Pre-experiment... 1400 "worms" are distributed randomly over the front lawn
- Instructions to the class on how to collect the items.
- "You have 90 seconds... Ready, set, go"
- Back to the classroom to separate the collection and enter the data into the worksheet.
See one class's data below >>>
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04.16.08
THE WOOLY WORM HUNT is an exciting "game" where the students are the Predators, hunting the great (and desirable) wooly worm. The class is given 90
seconds to pick up as many colored wooly worms as possible from the 1400 worms scattered across the Prep's front lawn. The 14 varieties vary with colors like white, neon pink, yellow, peach, teal, green and
brown. Some varieties survive the onslaught of the dreaded "frosh-sophs" while others are hunted to near extinction. To test the hypothesis that there is preference for particular colors and that the wooly
worms are not picked up randomly the class uses Chi Square analysis. This illustrates with greater than 99.5% certainty that earth tones are selected for and bright colors are selected against. The students are then
asked to suppose that these are live, reproducing organisms and to predict what upcoming generations of wooly worms would look like. A really fun and interesting way to teach about Natural Selection
Wooly Worm Lab • The Chi Square Test:
If the wool pieces are collected randomly, then the number of each color collected should be nearly equal. Thus, a null hypothesis may be proposed that states
that there will be no significant difference in the number of each color of yarn collected. If this null hypothesis is not supported by the data, then selection of some colors over others must occur. You use the chi
square test to test this null hypothesis by comparing the number of each color of yarn expected to be collected against the number that is actually collected. The chi square value calculated from the formula is a
measure of the variation from the expected values. The closer the expected and observed values, the smaller the chi square value will be and the more likely that the data is the result of random choice. Once you get
your chi square value, you can use this number to estimate the probability that the null hypothesis is acceptable, i.e. that the wool was collected randomly without color preference. If the null hypothesis is
unacceptable then the selection of some colors over others must have occurred. To get the probability look on the chi square distribution chart in the row with the correct degrees of freedom (# of colors - 1). Look for
the nearest value and look at the top row for the probability. If the probability is to the left of the center vertical line then the null hypothesis is acceptable. If to the right then the hypothesis is not acceptable
and natural selection probably took place. Be sure to include a discussion of this in your paper.
- Questions
:
- 1. Was the original null hypothesis acceptable?
- 2. What is the probability for accepting or rejecting the null hypothesis?
- 3. Which worms were collected more than would be expected by the random model?
- 4. Why were more of these worms collected?
- 5. Which colored worms were collected less than would be expected by the random model?
- 6. Why were fewer of these worms collected?
- 7. What other factors contributed to the number of worms collected besides their color?
- 8. What would happen to the population of wooly worms if they could reproduce?
- 9. What would happen if we repeated this experiment for many generations of these worms?
- 10. What would be an adaptation (besides color change) that would allow the brightly colored ones to survive?
Formula for chi square: Wooly Worm Lab
chi square = [(observed - expected)2] / expected
degrees of freedom (df) = number of colors - 1
photos: RCM/SHP
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