Assignments for non-majors
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 traits-characteristics 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 mean beak size, heritability of beak size, variation in beak size, fitness, and clutch size) and one environmental parameter (precipitation) of the system, and observe changes in the distributions of beak size and population numbers over time. The six different parameters that can be manipulated using EvolutionLab are defined in the lab itself. Be sure you review and understand each parameter before you begin the assignments.
For your ease in completing each assignment, the background text relevant to the experiment that you will perform is brown text, instructions for each assignment are indicated by plain text, and questions or activities that you will be asked to provide answers for are indicated by bold text.
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?
Data from the Beak Size view, and Population view are shown in tabular form in the Field Notes view. Click on the Field Notes view. A table showing each year of the experiment, mean beak size, and population will appear.
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 exported by clicking on the Export button. A separate window will now open. You can print your data directly from this window or save this data to a disk.
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?
d. Decrease beak size on both of the islands to an intermediate value. Decrease rainfall on one island to a value close to zero. On the other island, increase rainfall close to the maximum value. Run the experiment for 300 years.
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.
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.