Student Exploration: Rainfall and Bird Beaks - Answer Key (2024)

Student Exploration: Rainfall and Bird Beaks

Vocabulary:adaptation, beak depth, directional selection, drought, evolution, natural selection, range, stabilizing selection

Prior Knowledge Questions(Do these BEFORE using the Gizmo.)

During the voyage of the HMSBeagle(1831–1836), the young Charles Darwin collected several species of finches from the Galápagos Islands. Two of Darwin’s finches are shown below.

Which species do you think is best adapted to a diet of small, delicate seeds? Explain why you think so.

Which species do you think is best adapted to a diet of large, tough-to-crack seeds? Explain.

Gizmo Warm-up

Darwin’s finches are one of many types of animals on the Galápagos Islands that have uniqueadaptations, or traits that help an organism survive in its environment. TheRainfall and Bird BeaksGizmo™ allows you to explore how rainfall influences the range of beak shapes found in a single finch species.

Thebeak depthof a finch is the distance from the top of the beak to the bottom, as shown.

What is the current average beak depth in the Gizmo?

Select the HISTOGRAM tab. Do all the finches have the same beak depth?

ClickPlay() and let the simulation play for five years with average rainfall (25 cm/yr). Select the GRAPH tab and view theFinches vs timeandBeak depth vs timegraphs.

How does the finch population change?

Does the beak depth change significantly?

Introduction:The Galápagos Islands are very dry, with an average rainfall on some islands of only 12 centimeters per year. The amount of rainfall has a large impact on the abundance and types of seeds that are available to be eaten by finches. In the process ofnatural selection, only the finches that are best adapted to the available seed types survive and have offspring.

Question: How is the finch population affected by a period of average rainfall?

Observe: With theRainsliders set to 25 cm, clickPlay, and thenPause() after one year has passed. Select the TABLE tab and look at theMonthandFinchescolumns.

How did the finch population change over the course of one year? ______________

The finches have their young during the rainy season. Based on the table, which part of the year do you think is the rainy season?

Analyze: ClickReset, and choose the HISTOGRAM. The bars represent the numbers of finches that have different beak depths. Therangeof beak depths is equal to the difference between the largest and smallest beaks.

What is the average beak depth of the current finch population?

What is the range in beak depths in the population?

Do most of the finches have beak depths near the lower extreme, the middle, or the higher extreme of the range?

Experiment: ClickPlay, and observe the histogram as the simulation plays for five years.

What is the average beak depth now?

What is the current range of beak depths?

Based on what you have seen, are finches with very small, medium, or very large beaks most likely to survive in times of normal rainfall? Justify your answer.

Introduction:In years of extremedrought, Galápagos plants don’t produce new seeds. The small, delicate seeds get eaten up quickly, leaving behind only the largest, toughest seeds.

Question: How does drought affect the finch population and average beak depth?

Form hypothesis: What type of beak do you think will be best for finding food in a drought?

Predict: Select the HISTOGRAM tab. On the left side below, sketch the current histogram and list the average beak depth and range of beak depths. On the right side, sketch what you think the histogram will look like after five years of drought. Explain your prediction.

Initial beak depths Beak depths after 5 years (predicted)

Initial number of finches: ___________ Explanation: ________________________

Initial average depth: _________ ___________________________________

Initial range of beak depths: _________ ___________________________________

Experiment: Use the sliders to set theRainto 5 centimeters for each of the five years in the simulation. ClickPlay, and wait for five years. Observe the beak of the finch.

How does the beak depth change over time? _______________________________

What is the final average beak depth? ____________________________________

What is the final range of beak depths? ___________________________________

(Activity B continued on next page)
Activity B (continued from previous page)

Describe: Compare the final histogram to the initial histogram. How have the finches been affected by drought? Describe at least two changes that you notice.

Analyze: Was the increase in the average beak depth caused by an increase in large-beaked finches or a decline in small-beaked finches? Explain your answer.

Draw conclusions: What do you think caused the changes in the finch population and average beak size during the drought?

Interpret:Directional selectionoccurs when individuals at one end of a range are more likely to survive than intermediate individuals or individuals at the opposite end of the range.Stabilizing selectionoccurs when intermediate individuals are the most likely to survive.

Is directional selection, stabilizing selection, or both operating in this example? Explain.

Think and discuss:Evolutionis the process by which populations of organisms can change over time. How is directional selection related to evolution?

Introduction:In years of abundant rainfall, there can be 50 centimeters or more of rain. In these years, plants produce an enormous number and variety of seeds.

Question: How does plentiful rainfall affect the finch population and average beak depth?

Form hypothesis: What beak shape do you think will be best for finding food in a period of abundant rainfall? __________________________________________________________

Predict: Select the HISTOGRAM tab. On the left side, sketch the current histogram and list the average beak depth and range of beak depths. On the right side, sketch what you think the histogram will look like after five years of abundant rain. Explain your prediction.