by Michael Palladino, Monmouth University
Degree of Difficulty
|1||Easy Assignment for Non-majors: 1, 6|
|3||Easy Assignment for Majors: 1, 4|
|5||Average Difficulty: 2, 5|
|7||Difficult for Freshmen, OK for Upper-Division Students: 7|
|10||Difficult for Upper Division Students: 3|
For your ease of use, the background text relevant to the experiment to be performed and instructions for each assignment are indicated by plain text. Questions or activities that students will be asked to provide answers for are indicated by bold text.
The finches on Darwin and Wallace Islands feed on seeds produced by plants growing on these islands. There are three categories of seeds: soft seeds, produced by plants that do well under wet conditions; seeds that are intermediate in hardness, produced by plants that do best under moderate precipitation; and hard seeds, produced by plants that dominate in drought conditions. EvolutionLab is based on a model for the evolution of quantitative traitscharacteristics of an individual that are controlled by large numbers of genes. These traits are studied by looking at the statistical distribution of the trait in populations and investigating how the distribution changes from one generation to the next. For the finches in EvolutionLab, the depth of the beak is the quantitative trait. You will investigate how this trait changes under different biological and environmental conditions.
You can manipulate various biological parameters (initial beak size, heritability of beak size, variation in beak size, clutch size, and population size) and two environmental parameters (precipitation, and island size) of the system, then observe changes in the distributions of beak size and population numbers over time.
Getting to Know EvolutionLab: The Influence of Precipitation on Beak Size and Population Number
Notice that default values on both islands are the same. Click on the Change Inputs button at the left of screen to begin an experiment. A view of initial beak size will now appear.
In the Change Input view you can change the biological and environmental parameters in EvolutionLab to design an experiment. This first experiment is designed to study the influence of beak size on finch population numbers. For finches, deep beaks are strong beaks, ideally suited for cracking hard seeds, and shallow beaks are better suited for cracking soft seeds.
Develop a hypothesis to predict how changes in beak size will affect population numbers for these finches. Test your hypothesis as follows:
Look at the plots of average beak size over time. What do you observe? Do you notice any trends in beak size? Click on the Population button and look at the plots of population numbers over time. What changes do you see? Do the two islands differ? Does the data support or refute your hypothesis?
Click on the Histograms button. These are plots of surviving birds and total birds plotted against beak size. Click and drag the slider to advance the years of the plot and to see how beak size on each island may have changed over time. Note how the distributions of beak size change over time.
What happened to beak size on Darwin Island compared to Wallace Island over time? Is this what you expected? Why or why not?
Click on the Input Summary button to see a table of your input values for this experiment. Data from each of the views that you just looked at can be saved in a virtual notebook by clicking on the Export Data button. A new window will appear. You can type comments on your results in this window. To print your lab notes, click on the Export Notes button. A new window will appear. You can now save your notes to disk and/or print a copy of your lab notes using the print feature of your browser software.
Click the New Experiment button, click the Change Inputs buttons then click the Precipitation button. Recall the relationship between precipitation and seed growth. There are three categories of seeds: soft seeds, produced by plants that do well under wet conditions; seeds that are intermediate in hardness, produced by plants that do best under moderate precipitation; and hard seeds, produced by plants that dominate in drought conditions.
Develop a hypothesis to consider how a decrease in precipitation on Darwin Island might affect beak size and develop a hypothesis to explain how a decrease in precipitation might influence population numbers for these finches over time. Test your hypotheses as follows:
Did you notice any trends in the distributions of beak size? What did you observe? Did you notice any trends in population number? What did you observe? Explain your answers.
Run another experiment for 200 years by clicking on the Revise Experiment button. Use the popup menu at the lower left corner of the screen to select a value of 200 years, then click the Run Experiment button. Repeat this experiment for 300 years.
What changes did you observe in beak size and population numbers? Do these results confirm or refute your hypothesis? If necessary, reformulate your hypothesis and test this hypothesis.
Did you notice any differences between precipitation, changes in beak size, and population numbers for the finches on Wallace Island compared with those on Darwin Island? Explain your answers.
Click the New Experiment button, return to the Change Inputs view then increase the precipitation on Darwin Island fourfold while leaving precipitation on Wallace Island at the default value. Run this experiment for 300 years to test your hypothesis.
What did you observe? After you have observed the data for this experiment, rerun this experiment. Look at the output results in the Beak Size and Population views. Do you notice any differences in this rerun compared with the previous run? Are the general trends observed in this run the same as the previous run? Explain your answers. Run and rerun each experiment for 100, 200, and 300 years. Perform another experiment to test your hypothesis by increasing precipitation on Wallace Island to 50 cm/year and increasing beak size to 28 mm. Run an experiment for 300 years and describe your results. Do these results support your hypothesis?
Were the effects on each island the same or different? What did you observe? Were these the results you expected? Explain your answers to justify what is happening to finches on each island. Be sure to provide explanations for any differences in beak size and population numbers that you observed when comparing finches on both islands.
Modes of Natural Selection
There are three primary ways by which natural selection can influence the variation of a trait in a population. The three modes of selection are directional, stabilizing, and diversifying. In directional selection, changing environmental conditions can favor individuals with phenotypes that are at opposite extremes of the distribution range for a given trait (for example, finches with very deep beaks or finches with very shallow beaks). Stabilizing selection, unlike directional selection, selects against individuals with extreme phenotypes and favors individuals with more intermediate or average values for a given trait. Disruptive selection favors individuals at both extremes of the distribution range for a trait while selecting against individuals with average values for the trait. The following assignment is designed to help you understand how environmental changes can result in different modes of natural selection.
Effect of Clutch Size
Clutch size is the number of eggs that a female bird lays in her nest. In the EvolutionLab simulation model, birds mate for life and live for one year, and each female produces only one clutch of eggs per year. For a mating pair to replace themselves, they must produce at least two offspring; this is why clutch size is set to a minimum of two eggs. The maximum clutch size of thirty eggs is unrealistic; however, most bird species produce more than one clutch in their lifetime, so a total of thirty or more offspring per mating pair is possible. You can use the sliders to change the mean clutch size for each island population. As you move the slider, the number of eggs in the nest will change to reflect the value that you have chosen.
Did you notice anything odd? (If not, try again until you do). Propose a hypothesis to explain this result. Leaving the rainfall and beak sizes alone, what parameters would you change to prevent this? What parameter would you change to increase the likelihood of this happening? What type of selection is happening during the first several years of this experiment? If these birds were capable of assortative mating, what might happen on one island?
Effect of Island Size
The size of the living area for any population can strongly influence population numbers for organisms that live within that environment. The maximum number of organisms from a given population that an environment can support is known as the carrying capacity of that environment. Island size is one factor that can determine the carrying capacity of finches on each island. For the purposes of this simulation, the islands are assumed to be roughly circular and island size is represented as the radius of the island in kilometers. The size of each island remains constant throughout the simulation unless you choose to change this parameter. Although changing the entire size of an island is not something that could easily be done in real life, habitat changes and reducing the living environment for a population are real changes that occur through processes such as land development, and pollution. The following assignment is designed to help you learn about the influence of island size on the carrying capacity of finches on Darwin and Wallace Islands.
Test your hypothesis as follows: Begin your experiment by leaving all other parameters at their default values. Select the Island Size input and use the sliders to increase the size of either Darwin or Wallace Island. As you move the slider, the island image will change to reflect the values you have chosen.
What effect did this change in island size have on finch population? What effect did this change in island size have on beak size? Are the results what you expected? Explain your answers. Perform a new experiment to learn about the effects of a decrease in island size on beak size and finch populations.
Test the effect of these parameters to influence population size and beak size by designing and running experiments to confirm or refute your answers.
Variance is a measure of how different the phenotype is from one bird to the next. If the variance of a trait is large, then there will be large differences in phenotypes among birds for a given trait. If, on the other hand, variance is low (near zero), then all of the birds will be very similar to one another. This parameter determines two values for the simulation. First, it determines the initial population variancevariance in beak size for the entire population. The population variance may change during the course of the simulation. The variance parameter also determines the sibling variancevariance in beak size among individuals with the same parents. The sibling variance is a measure of how much variability is inherent in a trait. If you look at the plots of offspring beak size versus the midparent value for the heritability parameter, the sibling variance represents the amount of dispersion of the points around the regression line. In EvolutionLab, the sibling variance remains constant throughout the simulation. You can use the sliders to change the variance for each island population. As you move the slider, the width of the bell-shaped probability distribution will change to reflect the value that you have chosen.
Begin by decreasing variance to a value close to zero. Run your experiment for 300 years.
Explain the results of this experiment. Perform a similar experiment for an increase in variance. Describe your results. Can you reverse any of the trends in population number by changing other variables such as precipitation?
What were the results of this experiment? Explain why the differences in variance on each island produced these results. What do these results tell you about the benefit of a population with a broad variance compared to a population with narrow variance? Which condition do you think is most desirable for maintaining a diverse population and for minimizing a populations risk of extinction?
Now that you have manipulated many of the biological and environmental factors in EvolutionLab, consider how these factors could lead to extinction of the finch population on either island.
Group Assignment: Influence of Heritability
Heritability is a measure of the genetic contribution to a phenotype of an organism. If the heritability of a trait is large (near one), then the progeny of a mating will be more similar to their parents than unrelated individuals. If heritability is low (near zero), then the environment will be more important in determining the phenotype and the progeny will vary around the average value for the population. One method for measuring heritability is to plot the beak depth of each bird versus the average beak depth of each birds parents (midparent value). The slope of the line produced by this plot represents heritability.
You can use the sliders to vary the heritability of beak size on each island population. Heritability remains constant at this value throughout the simulation. As you move the slider, the graph of offspring beak size versus the midparent value will change to illustrate the heritability value that you are selecting. The mean value for the plot is equal to the initial mean beak size that you have chosen. The amount of dispersion of the points around the plot is determined by your selection for variance in beak size.
Work together in a group of four students to perform the following assignment, which is designed to help you understand the influence of heritability on population numbers. Click on the Heritability button and read the description of heritability and heritability plots.