Download Genetics 2024, Winter 2004

Population Genetics of the Peppered Moth: A Natural Selection Simulation
(Adapted from J. Higginbotham and Carolina Biological Supply Co.)
The response of the peppered moth Biston betularia to industrial pollution in England is a well-known
example of natural selection acting on a wild population. The moth has two color morphs, a dark
melanic form and a light mottled form. Coloration is determined by a single gene where the allele for
the dark melanic form (M) is dominant to the allele for the light mottled form (m). During the mid
1800s, the light mottled form, called typica, dominated the English countryside. With the industrial
revolution came both pollution from burning coal and an increase in the frequency of the dark
melanic morph, called carbonaria, especially in the heavily industrialized areas. Field experiments
later showed that the melanic morph is better camouflaged than the light morph on the darkened
tree trunks found in polluted areas. Conversely, the light morph is more cryptic on the lichencovered tree trunks found elsewhere. The birds that prey on the moths preferentially take the
melanic morphs in unpolluted areas and the light morphs in polluted areas. This has caused
measurable changes in the frequencies of the alleles controlling for melanism and light mottling in
certain areas, a classic example of microevolution.
Population genetics is the application of genetic principles to entire populations of organisms. A
population is defined as a group of organisms belonging to the same species that live within a defined
geographical area. We can study the genetic composition of a population by tracking the frequencies
of particular alleles of interest. In this lab, you will simulate changes in a population of peppered
moths as a result of natural selection by bird predation in three different environments. Begin by
acquiring 20 peppered moths of each genotype. The light moths are homozygous recessive (mm), the
dark ones are homozygous dominant (MM), and the half light/half dark ones are heterozygous (Mm.)
The heterozygous moths are colored half and half in this simulation so that you can distinguish them
from the dominant homozygotes. In nature, only a trained eye can distinguish the heterozygotes
from the dominant homozygotes. Acquire 3 habitat boxes, corresponding to low, moderate, and high
pollution environments.
This simulation will begin with allele frequencies of 0.5 for each allele (half M and half m). That is, for
Generation 1:
the frequency of M: f(M) = p = 0.5, and
the frequency of m: f(m) = q = 0.5.
Further, we will assume that the population is at Hardy-Weinberg equilibrium. The Hardy-Weinberg
principle can be used to determine allele frequencies from generation to generation in a population
provided certain conditions are maintained. If at Hardy-Weinberg equilibrium, allele frequencies can
be determined by the following formula:
p2 + 2pq + q2 = 1
Given starting allele frequencies of 0.5, the genotype frequencies of our simulation at Generation 1
are:
the frequency of MM: f(MM) = p2 = (0.5)2 = 0.25
the frequency of Mm: f(Mm) = 2pq = 2x(0.5)x(0.5) = 0.5, and
the frequency of mm: f(mm) = q2 = (0.5)2 = 0.25
Genetics
1
Lab
The initial population size of our simulation will be 20 moths, so the following moths will compose
Generation 1:
20x(0.25) = 5 dominant homozygotes (MM)
20x(0.5) = 10 heterozygotes (Mm)
20x(0.25) = 5 recessive homozygotes (mm)
1. Place these moths into the high pollution population habitat box. To save time, each group
member can be responsible for data gathering of one environment.
2. Gently shuffle the box to distribute the moths at random.
3. Select against the poorly camouflaged moths by removing the ones that do not match their
backgrounds. Heterozygotes should be considered to match the dark background. If a moth
overlaps two or more blocks, use the position of the head to determine which block the moth is
in. This completes selection on the first generation.
4. Record all of your results in the appropriate spaces on your data sheet.
5. To determine the genetic composition of the next generation (Generation 2), calculate the allele
frequencies for the survivors.
6. Plug these numbers back into the Hardy-Weinberg equation to determine the genotype
frequencies for Generation 2 (we are assuming Hardy-Weinberg equilibrium).
7. Record these values on the data sheet under Generation 2.
8. Assume that the population grows back to 20 each year before selection, which simulates the
carrying capacity of the habitat. Determine how many moths out of 20 will be of each genotype
in Generation 2, rounding to the nearest whole number. Record these numbers on the data
sheet.
9. Place Generation 2 in the habitat box and repeat steps 2 – 4.
10. To determine the composition of the next generation (Generation 3), calculate the allele
frequencies p and q for the survivors.
11. Now repeat steps 6 – 8 to complete Generation 3.
12. Repeat the process to get Generations 4, 5, and 6, recording your results in the data sheet.
13. Repeat the simulation for each of the two other habitat types or share the information among
group members. As before, record your data and graph the results.
14. For your lab report, graph results for both genotype frequencies and allele frequencies in three
different environments over six generations. Use histograms (bar graphs) for the genotype
frequencies and separate line graphs for the allele frequencies. Use different colors or patterns
for different environments. Make sure graphs are accurately and logically labeled.
15. Write a paragraph of conclusions discussing the effects of pollution level on the evolution of
color in these moths, starting with a couple sentences about your hypothesis. Draw directly
and heavily from your graphs.
After you complete this simulation, you should be able to solve problems that require calculation of
allele frequencies and genotype frequencies.
To be successful for your lab quiz, please read this most updated story about the peppered moth:
http://www.nature.com/news/2011/110414/full/news.2011.238.html
Genetics
2
Lab
Population of 20 in the high/moderate/low (circle one) pollution environment
Allele frequency
Genotype frequency
Individuals
Generation
f(M)=p
f(m)=q
f(MM)=p2
f(Mm)=2pq
f(mm)=q2
Beginning of 1
0.5
0.5
0.25
0.5
0.25
Dark
(MM)
Mixed
(Mm)
Light
(mm)
5
10
5
(20x0.25)
(20x0.5)
(20x0.25)
End of 1
Beginning of 2
End of 2
Beginning of 3
End of 3
Beginning of 4
End of 4
Beginning of 5
End of 5
Beginning of 6
End of 6
Genetics
3
Lab