Rainfall and Bird Beaks

Text Box: When Darwin sailed around the world aboard the Beagle from 1831 to 1836, he found many exotic creatures. But one of his most important discoveries was a group of small, plain-looking finches on the Galapagos Islands. Observing the variety of beak shapes and lifestyles of these birds, Darwin realized that he was seeing evolution in action. He theorized that a small group of finches had colonized the islands thousands of years before. As they struggled to survive, the finches adapted to their new environment, specialized their feeding habits, and were in the process of evolving into several new species. Since 1973, the husband-and-wife team of Peter and Rosemary Grant has patiently tagged, measured, and analyzed the same population of finches that Darwin observed in the Galapagos. One of their most stunning discoveries has been how rapidly the average beak size of the finches changes during periods of drought or heavy rain. For these finches, variations as small as a fraction of a millimeter could mean the difference between survival and death.  

 

 

 

 

 

 

 

 

                                                         

TABLE:

 

Number of Finches

Average Beak Depth

Initial Conditions

 

 

5 inches (average)

 

 

1 inch

 

 

10 inches

 

 

Effects of Drought

How does drought affect an isolated population of finches? To find out, you will study the number of finches and the average beak depth throughout a five-year period.

 

1.      First, take a few moments to familiarize yourself with the gizmo. In the Gizmo™, observe the Rain sliders on the SIMULATION pane, and the information given on the DESCRIPTION pane.

 

a)      Fill out the chart above as instructed while you complete the simulation.

 

b)      Fill out the Initial Conditions section in the table above.

 

c)      Select the BAR CHART tab.

 

1.      Do all of the finches have the same beak depth?            

 

2.      What is the range of Beak Depth (mm) in this population of finches?

 

3.      What is the mode for Beak Depth (mm) in this population of finches?

 

2.      Select the DESCRIPTION tab. Be sure that all the Rain sliders are set to 5 inches per year. Click Play and let the simulation run through the end of year 5 (it will then stop automatically).

 

a)      Fill in the table for 5 inches (average) by looking at the “Current Conditions” information.

 

b)      Click on the GRAPH tab and then choose Beak depth vs. time in the drop-down menu on the graph.

 

1.      Did the average beak depth change dramatically during the five years of average rainfall?  

 

2.      How can you tell, by looking at the line graph?                                                                                                                    

3.      Before we run another simulation, let’s form a hypothesis.  In this gizmo, we are investigating how the amount of rain affects the average beak size found in a finch population.  So, complete the following to make a testable hypothesis:

 

Independent Variable:                                                                 Dependent Variable:                                      

                                                                                         

If                                                                   decreases, then                                                         

                  Independent Variable                                                  Dependent Variable

 

will                                                .

               How will it change?

 

4.      Click Reset. To examine the effects of drought, set the Rain sliders to 1 inch (2.5 cm) for each year. Select the DESCRIPTION tab and click Play.

 

a)      Fill in the table on the first page for 1 inch.

 

b)      Select the GRAPH tab. Select Finches vs. time. During the five years of drought, how does the number of finches change?

 

c)      Now select Beak depth vs. time. Did beak depth increase or decrease over time?

 

d)     Look at the BAR CHART.

 

1.            According to the simulation, are finches with smaller beaks or larger beaks more likely to survive?

 

2.            What size beak did the birds need to have in order survive (give specific numbers)?

 

3.            If a mutation occurs and a bird has the beak size of 15 mm, would this be considered a positive mutation, negative mutation, or would it have no effect on the bird in this environment?

 

 

e)      The finches described in this Gizmo are seedeaters. During years of drought, the smaller, softer seeds are quickly eaten up, leaving behind only the larger, tougher seeds that are harder to crack and more difficult to find under the baked soil. What kinds of beaks are more useful in these conditions?

Why?

 

 

Assuming that beak depth is a genetic trait, and that an individual finch cannot change the shape of its own beak, how did the change in average beak depth actually occur during the drought?

 

Effects of Excess and Variable Rainfall

In this activity, you will explore whether excess rainfall influences the number of finches or the average beak size. You will then experiment with variable rainfall.

 

1.      Before we run another simulation, let’s form a hypothesis.  In this gizmo, we are investigating how the amount of rain affects the average beak size found in a finch population.  So, complete the following to make a testable hypothesis:

 

If the amount of rain increases, then average beak size will                                         .

2.      Click Reset. Set the Rain sliders to 10 inches (25.4 cm) for each year. Select the DESCRIPTION tab and click Play.

 

a)      Fill in the table on the front page for 10 inches.

b)      Select the GRAPH tab and then select Beak depth vs. time. How did the average beak depth change during the five-year period of above-average rainfall?

 

 

 

c)      Look at the BAR CHART. When rainfall is above average in the Galapagos Islands, a higher proportion of seeds are small. How does this explain the trend in beak size you observed?

 

 

d)     Why would a deeper beak be a disadvantage in times of excess rainfall?

 

e)      Which beak sizes are no longer found in the population?

 

f)       If a mutation occurs and a bird has the beak size of 15 mm, would this be considered a positive

mutation, negative mutation, or would it have no effect on the bird in this environment?