NATURAL SELECTION OF PAPER BUGS
The scenario: Paper Bugs are little creatures that live in a curious habitat: they live atop fabric of varying patterns in biology laboratories. Just prior to their breeding season, they are terrorized by their major predator: Tweezer Birds. Tweezer Birds descend on the Paper Bugs, typically eating about half of the population of bugs before flying off to parts unknown for the rest of the year. The surviving Paper Bugs reproduce immediately after the departure of the birds and the population returns to approximately the same size it was before the arrival of the birds. Old Paper Bugs die after reproducing, so when the Tweezer Birds return the following year, they are returning to a completely new generation of bugs.
Some more details: There are three color morphs of Paper Bugs: red, white and blue. Populations of Paper Bugs live on different fabric habitats. The color of Paper Bugs is determined by a single genetic locus and there are two possible alleles for that locus. The B allele codes for red color and the b allele codes for white color. The Paper Bug color trait shows incomplete dominance (also know as co-dominance). Bugs that are homozygous for the B allele (genotype BB) are red, bugs homozygous for the b allele (genotype bb) are white, and heterozygotes (genotype Bb) are blue. Paper bugs breed randomly with respect to color and the Hardy-Weinberg equation accurately predicts the genotypic and phenotypic composition of the new generation after each breeding season.
The activity: Paper bugs are, in reality, small paper disks cut out of construction paper. The professor has given each group a population of Paper Bugs in which the frequency of each allele is 0.5. Students should check that each group indeed has a total population of 99 Paper Bugs, 33 of which are red, 33 of which are white and 33 of which are blue. Fold each disk in half to facilitate its being grasped with tweezers.
Students will play the role of the Tweezer Birds. Two students in each group will use the tweezers to represent the bird’s beak and a plastic dish to represent the bird’s stomach. Place the starting population of bugs in a bag, shake it and sprinkle the bugs onto the habitat. Feeding by Tweezer Birds is a competitive affair. After placing the bugs in the habitat, each student will turn their back to the habitat. On the word “go,” each student will turn around and try to be the first to put 20 bugs into his or her plastic dish.
After each round of competition the Tweezer Birds will have removed 40 Paper Bugs from the habitat. The allele frequencies for the two alleles can then be calculated. Every red bug contributes two B alleles to the total allele count and every blue bug contributes one B allele. The allele frequency for B is obtained by counting up the total number of B alleles in the surviving population and dividing by the total number of alleles in the survivors (round off the answers to two digits). Students should calculate the frequency of the b allele in the same manner. They can check their arithmetic by checking to see that the two allele frequencies sum to 1. Students should record the allele frequencies after each round of selection so that they can be graphed out later.
The next step is to use the Hardy-Weinberg equation (p2 + 2pq + q2 = 1) to generate the phenotypic composition of the next generation. Each student should make the calculations independently (using the attached table) to assure understanding of the mathematics involved. Take the frequency of B as p and the frequency of b as q. The frequency of red bugs in the next generation is p2, the frequency of blue bugs is 2pq, and the frequency of white bugs is q2. Calculate the actual number of bugs of each color by multiplying the frequencies of the three phenotypes by the total number of bugs in the next generation (round up to whole numbers). Bring the population back to the same size, 99 total Paper Bugs, before the predation event. Repeat the process for 10 generations, or until the professor indicates that it is time to stop.
A graph of p vs. round of selection will reveal if evolution by natural selection has occurred. Each group will transfer their graph to a transparency for class discussion.
After discussion of the first round of results, students will develop their own hypotheses and test them. There are additional colors of construction paper available, if needed.
Acknowledgements:
This exercise was modified from one developed by Douglas B. Woodmansee (Wilmington College), who was inspired by Stebbins, R. C., and B. Allen. 1994. Simulating evolution. In Investigating Evolutionary Biology in the Laboratory, National Association of Biology Teachers, Reston, VA. Additional advice was provided by Douglas J. Burks to DBW. FIRST SIMULATION:
Description of Habitat: ______
Proportion of each Paper Bugs Before Predation Paper Bugs After Predation genotype predicted from Hardy-Weinberg Generation Red Blue White Red Blue White p q Red Blue White p q (p2) (2pq) (q2) (BB) (Bb) (bb) (BB) (Bb) (bb) 1 33 33 33 0.5 0.5 2 3 4 5 6 7 8 9 10 Sketch graph of results below:
) B
f o
y
c 1.0 n e u q e r f
e l e l l a (
0.75 p
0.5
0.25
0.0
1 2 3 4 5 6 7 8 9 10
Number of Generations SECOND SIMULATION:
Description of hypothesis tested/experimental design:
Proportion of each Paper Bugs Before Predation Paper Bugs After Predation genotype predicted from Hardy-Weinberg Generation Red Blue White Red Blue White p q Red Blue White p q (p2) (2pq) (q2) 1 2 3 4 5 6 7 8 9 10 Sketch graph of results below:
) B
f o
y
c 1.0 n e u q e r f
e l e l l a (
0.75 p
0.5
0.25
0.0
1 2 3 4 5 6 7 8 9 10
Number of Generations