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Beak of the Finch Argument

Relatively few researchers have been able to witness evolutionary change in their lifetimes; among them are Peter and Rosemary Grant. The short film The Beak of the Finch focuses on the Grants’ 40-year study of the finches of the Galapagos Islands. In 1973, the Grants began observing and studying finches on several islands in the Galapagos archipelago. They wanted to understand how species change over time and, in particular, how changes in the environment can influence a species’ physical characteristics.

As a part of their work, the Grants intensively studied the population of the medium ground finches (Geospiza fortis) on the island of Daphne Major. Every year, the Grants measured the wing length, body mass, and beak size of hundreds of individual medium ground finches. They focused on these characteristics because they vary widely among individual birds within the same species. Small changes in these structures can be important for survival in different environments.

In early 1977 a drought began on Daphne Major. The drought lasted for 18 months and caused the type and abundance of food available to the finches to change rapidly. Medium ground finches prefer to eat the small, soft seeds of the bushy plant chamaesyce (Chamaesyceamplexicaulis), but the supply of chamaesyce seeds was extremely limited as a result of the drought. As the drought progressed and the hungry finches quickly at the small, soft chmaesyce seeds, one of the only remaining food sources for the medium ground finch became the seeds of a plant called caltrop (Tribuluscistoides). Caltrop seeds are much larger and harder than those of the chamaesyce and are covered with pointy spines. More than 80% of the 1,200 medium ground finches on the island did not survive the drought of 1977.

The question the Grants were attempting to answer was: What trends were evident in the survivors vs non-survivors, and what caused these trends? The Grants were interested in determining whether there were any differences between the finches that survived the drought and the finches that did not – in particular, whether any physical characteristics were key to survival. They used statistical tests to compare the data they collected for wing length, body mass and beak depth.

Table 1 shows body measurements from 100 medium ground finches living on Daphne Major in 1976. Fifty of those birds did not survive the 1977 drought (nonsurvivors) and 50 did (survivors). These data are also provided in an Excel spreadsheet; use either the data in Table 1 or in the Excel spreadsheet to construct several graphs – you will need to determine which graphs will be useful data to support your claim.

Select one of the following claims that addresses the second part of the Scientific Question.

  • Claim 1: The environment caused the next generation of finches to have changes in their body structures so they could use the available resources on the island.
  • Claim 2: The birds that survived the drought were able to pass down certain body structures to their offspring that allowed them to survive those environmental conditions.

Science Practices for Assessment:

  • Can apply mathematical routines to quantities that describe natural phenomena [SP 2.2]
  • Justify the selection of the kind of data needed to answer a particular scientific question [SP 4.1]
  • Evaluate sources of data to answer a particular scientific question [SP 4.4]
  • Includes an analysis / manipulation of raw data that supports the claim [ARG 2.2]

Table 1. Morphological measurements (wing length, body mass, tarsus length, and beak size) taken from a subsample of 100 medium ground finches (Geospizafortis) before the drought began on the island of Daphne Major in 1977. Half of the birds in the sample (n = 50) did not survive the drought (Nonsurvivors) and half (n = 50) did (Survivors).

In the data table below, the sample variance has been calculated for you. The variance:

  • Measures how far a set of numbers are spread out in relation to each other
  • Of zero indicates that all values are identical.
  • That is small indicates the data points tend to be close to the mean and to each other
  • That is high indicates that the data points are very spread out around the mean and from each other

You can use the variance to calculate the standard deviation. This is used to compare how far the data deviates from each measurement in the sample to the mean (average) of the sample. This is also used as an estimate of the standard deviation in a larger population. The standard deviation:

  • That is lower indicates that the data points tend to be very close to the mean or expected value
  • That is higher indicates that the data points tend to be very spread out over a large range of values

These can also be used to determine the standard error of the mean – sometimes referred to as “confidence intervals.” Very rarely (as in this example) do you have all of the data – but rather, a sample. By calculating the SEM/confidence interval, you can obtain an “interval estimate” for the mean (average) of the population. This provides a range of values within which the mean of the entire population is likely to be found.

The graph below models how you would see the SEM/95% CI on a graph using error bars.

The means of both of these bars are different, and the error bars do not overlap. This would suggest that there is a statistically significant different between the two mean fin heights. You would need to do additional statistical tests to confirm this, but this is a starting point.

Using the above information, calculate the missing statistical data using Table 1 for the chart below. The formulas are below Table 2.

Select the data that best answers the question and supports the claim you have chosen. Graph the data. You may need to select multiple data sets and create multiple graphs to support your claim.

Table 2: Descriptive statistics for morphological measurements taken from 100 medium ground finches (Geospizafortis).

Descriptive Statistics / Nonsurvivors / Survivors
Body
Mass (g) / Wing Length (mm) / Tarsus Length (mm) / Beak Depth (mm) / Body
Mass (g) / Wing Length (mm) / Tarsus Length (mm) / Beak Depth (mm)
Mean
Variance (s2) / 1.842 / 5.181 / 0.701 / 0.775 / 3.087 / 5.448 / 0.735 / 0.709
Standard Deviation
95% Confidence Interval
  • Standard Deviation – take the square root of the variance
  • 95% CI = 2(s) / n where s = standard deviation and n = number of samples

Once your group has developed evidence that supports your claim, use the whiteboard to share and justify your claim. Your whiteboard should include all of the information indicated in the figure below. Note: you need to have all of the sections – how you organize may be different than the diagram depicted.

Things to consider as you construct your argument:

  • How did you analyze or interpret your data? Why did you decide to do it that way?
  • How do you know that your analysis of the data is free from errors?
  • Why does your evidence support your claim?
  • Why did you decide to use that evidence? Why is your evidence important?
  • How does your justification of the evidence fit with accepted scientific ideas?

What is your Argument?

After completing the whiteboard session in class, you will write an argument in order to persuade another biologist that your claim is acceptable. As you write, remember to do the following:

  • State the claim you are trying to support
  • Include genuine evidence (data + analysis of the data + interpretation of the analysis)
  • Provide justification of your evidence that explain why the evidence is relevant and why it provides support for your claim.
  • Organize your argument in a way that enhances readability (i.e. a logical sequence)
  • Use a broad range of words, including vocabulary you have learned in this activity or in class
  • Correct grammar, punctuation and spelling.

You will be writing your summary in Moodle. Be sure you have all the necessary information that you will need to do this recorded in your lab notebook.

Adapted from HHMI Biointeractive