Modeling Natural Selection: The Peppered Moth Adapted from Global Change

Introduction: The case of the peppered moth (Biston betularia) provides a classic example of natural selection. Go through the 2nd and 4th links of the online tutorial to learn why. Pay close attention to the variation, survival pressure, and selection of the moths. Identify these elements from the tutorial in the space below.

Variation:

Pressure:

Selection:

Scientists have determined that body color in the peppered moth is controlled by a single gene. The allele for dark body color is dominant, which means that a moth possessing at least one such allele will have a dark body. To have a light body, the moth has to have both recessive alleles for light body color.

Record the possible genotypes for:

Dark moth: Light moth:

Dark moths were at a distinct disadvantage, however, due to their increased vulnerability to bird predation. Thus the frequency of the dark allele was very low (about .001%), maintained primarily by spontaneous mutations from light to dark alleles.

By 1819, the proportion of dark moths in the population had increased significantly. Researchers found that the light-colored lichens covering the trees were being killed by sulfur dioxide emissions from the new coal burning mills and factories built during the industrial revolution. Without the light background of the trees, the light moths were more visible to vision-oriented predators (birds). They were losing their selective advantage to the dark moths, which, against the trees’ dark bark background, were less visible to birds. In 1848, the dark moths comprised 1% of the population and by 1959 they represented ~90% of the population. So, in 100 years the frequency of dark moths increased by 1000 fold!

In this exercise, we will work with a model simulating the effects of differential predation pressures on a hypothetical peppered moth population. To do this, we will need to incorporate the genetics of moth body color into a population dynamics model. We are assuming that body color is the only trait that confers any significant selective advantages on peppered moths. Because genetics will be central to our investigation, it’s important to remember the following terms/concepts:

• Alleles • Genotype • Phenotype • Homozygous

• Heterozygous

We have three different genotypes represented in our model:

• AA MOTHIES: homozygous dominant moths that are dark in color • Aa MOTHLETS: heterozygous moths that are also dark in color • aa MOTHS: homozygous recessive moths that are light in color

Total initial moth population is: 250 AA + 500 Aa + 250 aa = 1000 moths Total initial moth frequency is: .25 AA + .5 Aa + .25 aa = 1.0

Investigating the Model:

Simulation 1: Phenotype Frequency Since pollution is the true driver of the change in genotype frequency in the peppered moth population, it is the variable that we are most interested in modifying. In this simulation, you will be comparing the phenotype frequencies (light and dark moths) under three conditions:

1. Set the pollution level to 0 (no pollution) and run the model. Record the final values for Red (frequency of light moths) and Blue (frequency of dark moths).

2. Set the pollution level to 0.5 and run the model. Record the final values for Red (frequency of light

moths) and Blue (frequency of dark moths).

3. Set the pollution level to 1 (maximum pollution) and run the model. Record the final values for Red (frequency of light moths) and Blue (frequency of dark moths).

*Frequencies are out of 1000 moths

Red Frequency (Light Moths)

Blue Frequency (Dark Moths)

Trial 1: Pollution = 0

Trial 2: Pollution = .5

Trial 3: Pollution = 1 Table 1

Analysis 1: How does increased pollution affect the frequencies of dark and light moths? Explain.

Simulation 2: Genotype Frequencies In this simulation, you will be comparing genotype frequencies (AA, Aa, and aa) as pollution levels are increased: AA: homozygous dominant moths that are dark in color (initial frequency = .25) Aa : heterozygous moths that are also dark in color (initial frequency = .50) aa : homozygous recessive moths that are light in color (initial frequency = .25)

1. Set the pollution level to 0 (no pollution) and run the model. Record the final frequency values for AA (blue), Aa (red), and aa (pink).

2. Set the pollution level to 0 .5 (moderate pollution) and run the model. Record the final frequency values

for AA (blue), Aa (red), and aa (pink).

3. Set the pollution level to 1 (maximum pollution) and run the model. Record the final frequency values for AA (blue), Aa (red), and aa (pink). You will use the data from this step in the next simulation.

AA (Blue) Frequency Aa (Red) Frequency aa (Pink) Frequency

Trial 1: Pollution = 0

Trial 2: Pollution = .5

Trial 3: Pollution = 1 Table 2

Analysis 2: How does increased pollution affect the frequency of each allele in the population? Simulation 3: Comparing genotype frequencies as the effects of pollution are reduced. At the beginning of the exercise, the moth population reflects the effects of maximum pollution in the environment. If the pollution level is thereafter reduced, how will the populations of light and dark colored moths change? Prediction:

1. Using the frequencies from Simulation 2, Trial 3 (when pollution was greatest), turn the dials to the corresponding number of moths with each genotype for our starting values. Remember: the total starting population is 1000, and each frequency represents the percentage of moths of that type. For example, a frequency of .35 would mean there are 350 moths of that genotype.

2. Gradually decrease pollution levels over three trials to see the effect of reduced pollution on genotype

frequencies. Record your selected pollution value and the frequencies of each genotype in the table below.

AA (Blue) Frequency Aa (Red) Frequency aa (Pink) Frequency

Trial 1: Pollution =

Trial 2: Pollution =

Trial 3: Pollution =

Analysis 3: Was your prediction supported? Why or why not? Try to explain what happened and why. Discussion:

1. Why does the heterozygous genotype play a significant role in maintaining genetic diversity in a population?

2. Why is this type of modeling program useful to evolutionary biologists in general?