…………..…..A model of natural selection

California Science Content Standards:

Grade Seven: Focus on Life Science
Evolution
3. biological evolution accounts for the diversity of species developed through gradual processes over many generations. As a basis for understanding this concept: Students know
3a. Both genetic variation and environmental factors are causes of evolution and diversity of organisms.
3b. The reasoning used by Charles Darwin reaching his conclusion that natural selection is the mechanism of evolution.

Life Science/ Biology 9-12
Evolution
7. The frequency of an allele in a gene pool of a population depends on many factors
and may be stable or unstable over time. As a basis for understanding this concept: Students know
7a Why natural selection acts on the phenotype rather than the genotype of an organism.
7d Variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions.
7e *The conditions for Hardy-Weinberg equilibrium in a population, and why these conditions are not met in nature.
7f *How to solve the Hardy-Weinberg equation to determine the predicted frequency of genotypes in a population, given the frequency of phenotypes.
8. Evolution is the result of genetic changes that occur in constantly changing environments
8a How natural selection determines the differential survival of groups of organisms.
8b A great diversity of species increases the chance that at least some organisms survive large changes in the environment.
Investigation and Experimentation

 

Purpose: to use a model of a hypothetical population to explain how environmental factors select certain characteristics that help organisms in a population survive, reproduce and pass genes on to future generations, resulting in changes in gene frequencies.

 

Materials: white moths (see link above)
black moths (see link above)
classified ad pages in the newspaper
scissors
large sheets of white butcher paper or poster board.
plastic or paper cups

 

Procedure: In teams, have the class cut out 100 white moths, 100 black moths, and 100 moths out of the classified ads.
Have one member of each team place their moths face up on the white background on the floor. Be sure to have the same number of each kind of moth (phenotypes: white, black, speckled (newsprint).
Darken the room so that the visibility so that the white moths on the white field are not easily visible.
Have 4 volunteers come to field to be the parent birds. Ask each to choose a classmate to be a baby bird. Each baby bird holds a cup, which represents their stomach. Station each baby bird at least 3 feet from the edge of the field.
At a signal from you, each baby bird chirps for food and the parent bird picks up one moth at a time, goes to the baby bird, and drops it in the cup. You might decide that each parent bird’s feeding behavior includes a loud squawk and flap of their wings before picking off the moth. It is important that each parent bird picks up only one moth at a time and drops it in the cup before returning to the field.
Have the parent birds hunt for 15 –20 seconds. At the end of each trial, count the number of white moths that were eaten, number of black moths eaten and the number of speckled moths eaten. Write the new population data in the data chart (figure out frequencies later). Do this for at least 3 generations. Assume no emigration and immigration. In a real situation, there is emigration and immigration and surviving moths would reproduce and pass their genes on to their offspring.

After 3 generations, pick up the surviving moths. Change the field to newsprint, by covering the field exactly with newspaper (classified ads) . Place the surviving moths back down on the field. Continue the hunt for 3 more generation.

 

Invisible Moths Data Chart

Color of Tree Bark	Generation	Number of White Moths	Number of Dark Moths	Number of Speckled Moths (newsprint)	Total Population	% of white Moths (q2)	q	p
light	0	100	100	100	300	.33	.57	.43
	1
	2
	3
newsprint	4
	5
	6

 

 

 

Discussion Questions
1. In the beginning of the activity, what was the percentage of black moths? Of speckled moths?
2. When the bark was light, what happened to the frequency of the light allele (q) with each succeeding generation?
What happened to the frequency of the black allele (p)?
3. If the tree bark remains light, would the frequency of white stabilize after several generations?
4. Which genotypes have the light allele (q)?
5. What was the selecting factor for the moths that survived when the tree bark was light?
6. What does the newsprint background represent in the real peppered moth population?
7. What happened to the frequency of q with each generation after the newsprint was used (trees darkened with soot)?
Why did this happen?
8. When the trees darkened with soot, which moths were more likely to survive, reproduce and pass their genes on to the next generation?
9. How does this model differ from the real population of peppered moths in industrial England?
10. What predictions for the moth population can you make in the future? See Moth Mutants.; Discover, Nov90, Vol. 11 Issue 11, p20, 2/3p, 1c
11. Do you think there are examples of frequency change of phenotypes today? Cite some possible examples by posing questions
“Does the use of head louse shampoo change the frequency of lice that are resistant to the shampoo?”

 

Conclusions:
Does the data support the impact of the environment on genetic variation in a population? Why or why not?

Sources of Error:
What are the possible variables that may not have been controlled in the activity?

 

Performance Task: Write a 3 paragraph essay that addresses the following:
1. Explain in your own words, the reasoning behind the concept of natural selection.
2. Use the peppered moth model to give examples of how the environment may favor one genetic variation over another.
3. What are some of the possible implications of current practices of humans for the future? Be