AbstrAct

One of the key aspects of natural selection is competition, yet the concept of competition is not necessarily emphasized in explanations of natural selection. Because of this, we developed an activity for our class that focuses on competi-tion and provides an example of the effects of competition on natural selec-tion. This hands-on activity models the field study completed by David Lack on Darwin’s finches. By using this historical study, we also give students an exam-ple of the nature-of-science concept of multiple methods. Although this activity was created for a college introductory biology course, it is appropriate for high school. We also provide an additional objective for upper-level ecology and evo-lution courses.

Key Words: Competition; Darwin’s finches; David Lack; evolution; multiple methods; natural selection.

The Next Generation Science Standards (NGSS Lead States, 2013a, b) identify natural selection as a disciplinary core idea in biology (HS-LS4.B), yet studies have repeatedly shown that less than 10% of students, even biology majors, begin college with a conceptual under-standing of natural selection (e.g., Nehm & Reilly, 2007). Therefore, student comprehen-sion of basic concepts associated with natural selection should be a major objective for col-lege introductory biology courses. In order to meet this objective, Nehm and Reilly (2007) combined active-learning elements, such as working in small groups, having group and class discussions, and nature of science (NOS), in a college introductory biology course. They found that students in this course significantly increased their conceptual understanding of natural selection compared to students taking a lecture-based course. We used these active-learning elements in creating an activity for our class that is intended to promote comprehension of one of the key aspects of natural selection: competition. Since this activity models an influential study done by David Lack on Darwin’s finches, we will

briefly explain Darwin’s finches and Lack’s study before discussing the activity.

Darwin’s Finches & David Lack’s StudyJ JJ

In 1835, before David Lack’s famous study on Darwin’s finches, Charles Darwin stopped at the Galápagos Islands while surveying the South American coast. These islands are about 500 miles from South America, just below the equator, and are composed of 10 main islands (Darwin, 1897). Darwin suggested that these islands formed from volcanic activity and were never attached to the mainland. He also commented on how the wildlife on the Galápagos Islands was morphologically similar to that on the mainland, yet the habitat was very different. Moreover, he discovered that the finches seemed very similar in beak shape, body structure, and color, yet had a wide range in beak size. Unfortunately, because of time restrictions, Darwin was unable to collect a comprehensive amount of data or

specimens. In remembrance of Darwin’s foun-dational work, the finches on the Galápagos Islands are referred to as “Darwin’s finches” (Lack, 1947).

Lack (1947) and others traveled to the Galápagos Islands in 1938 to study the char-acteristics of the finches and infer hypo theses regarding finch distribution between the islands and possible selection forces. During Lack’s study, the birds were captured, and beak length and depth, diet, and location were recorded. These measurements were taken from birds on several different islands; on some islands, as many as 200 were captured, creating a very large sample size. Therefore, although one

cannot be absolutely certain of their distribution patterns, with the amount of data taken, reliable inferences were made.

Regarding beak depth, Lack made three key observations: (1) there is variation within similar species occupying the same area, (2) there is variation among allopatric populations of the same

312 The american biology Teacher volume 76, no. 5, may 2014

Student comprehension of

basic concepts associated

with natural selection

should be a major

objective for college

introductory biology

courses.

The American Biology Teacher, vol. 76, no. 5, pages 312–317. iSSn 0002-7685, electronic iSSn 1938-4211. ©2014 by national association of biology Teachers. all rights reserved. request permission to photocopy or reproduce article content at the university of california Press’s rights and Permissions Web site at www.ucpressjournals.com/reprintinfo.asp. Doi: 10.1525/abt.2014.76.5.4

A n d r e A M . - K . B i e r e M A , d Av i d W. r u d g e

A R T I C L E Using David Lack’s Observations of

Finch Beak Size to Teach Natural Selection & the Nature of Science

species, and (3) there is a similarity between allopatric populations of different species. Most notably, Lack (1947) observed these pat-terns in three finch species, Geospiza fuliginosa (small ground finch), G. fortis (medium ground finch), and G. magnirostris (large ground finch). On the island Daphne, G. fortis is present without G. fuliginosa, and vice versa on the island Crossman; but G. magnirostris is not present on either of those islands. All three species are present on the island James.

Moreover, Lack (1947) hypothesized that “all the main differ-ences between the species may be regarded as adaptations to dif-ferences in diet” (p. 72). Lack acknowledged that species with the same niche could not coexist, a principle later called the “competitive exclusion principle” (Hardin, 1960). Therefore, he further concluded that when one species first immigrated to an island where another species used the same food resource, the two species gradually cre-ated a “subdivision of the food supply and habitats, and then … an increased restriction in ecology and specialization in structure of each form” (Lack, 1947, p. 148). For these conclusions, the main traits that Lack focused on were beak measurements, because beak size limits the types of food they can eat (Snodgrass, 1902). Although differences in traits among the same three species had been noted previously, this was the first time that the cause of variation among these species was explained as interspecific competition that occurred in the past as a result of having a similar niche (i.e., eating similar foods). Lack’s work (and that of many others) has subsequently been supported by natural experiments (e.g., Grant & Grant, 2006).

Having morphological differences between sympatric popu-lations of similar species yet overlap in characteristics when the similar species are allopatric is termed “character displacement.” Furthermore, biologists infer that character displacement is likely due to interspecific competition that occurred in the past. This hypo-thesis is now referred to as “ghost of competition past.” Although

Lack (1947) did not use these terms specifically, he did provide a great deal of evidence for them during his field observational stud-ies on Darwin’s finches. Please see Pfennig and Pfennig (2010) for a review of character displacement.

Laboratory InvestigationJ JJ

Student Profile

We created this activity for our college-level introductory bio-logy course, but it may also be appropriate for high school biology students. We also suggest modifications for advanced evolution or ecology courses at the end of the article.

Learning ObjectivesThis activity meets the Next Generation Science Standards (NGSS Lead States, 2013a, b), and there are several Cross-cutting Concepts and Science and Engineering Practices that are met via this activ-ity (see Table 1). Moreover, because one of the four main factors of natural selection is “competition for limited resources” (HS-LS4-2, p. 91), one of the objectives is for students to be able to explain the importance of competition by using a historical example. In order to meet this objective, we assumed that students should already have a basic understanding of evolution, natural selection, and ecology. Therefore, in our introductory biology course, students completed this activity toward the end of the evolution unit, which was after the ecology unit.

The other main objective, an NOS concept included in the Next Generation Science Standards, is for students to be able to identify that “scientific investigations use a variety of methods” (NGSS Lead States, 2013: Appendix H, p. 4). For instance, science can be explored in the field, not just in a laboratory, and science can occur via observations.

The american biology Teacher naTural SelecTion 313

Table 1. An explanation of how Cross-Cutting Concepts and Science and Engineering Practices in the Next Generation Science Standards are met by this activity.

Cross-Cutting Concept Activity Component

Patterns: Observed patterns of forms and events guide organization and classification, and they prompt questions about relationships and the factors that influence them.

After collecting data and creating histograms, students identify patterns. Later, students provide possible explanations for these patterns.

Cause and effect: Mechanism and explanation. Events have causes, sometimes simple, sometimes multifaceted. A major activity of science is investigating and explaining causal relationships and the mechanisms by which they are mediated. Such mechanisms can then be tested across given contexts and used to predict and explain events in new contexts.

After identifying the patterns in the data (i.e., the “event”), students hypothesize possible causes of these patterns. A class discussion also occurs, which closes with the current scientific explanation for the cause of the observed patterns.

Science & Engineering Practices Activity Component

Analyzing and interpreting data After students collect data, they analyze the data by looking for patterns.

Constructing explanations After students have examined the patterns, they need to identify possible explanations for why the patterns are occurring.

Engaging in argument from evidence After students have formulated their explanations, they must use their evidence (i.e., the patterns from the data) to justify their explanation to their group and to the class.

Experiments are not always necessary and are sometimes even impractical. To illustrate these points, we chose to model a histori-cal example. Before making comparisons between experiments and other methods of science, we advise that students learn what a true experiment entails (e.g., having a control and an experimental vari-able) in a previous lesson, as we have found that some students inter-change the terms “science” and “experiment.”

MaterialsFinch packets (described below)•

Instructions on measuring beak depth (Figure 1)•

Calipers or rulers•

Worksheet (Figure 2)•

Optional: bird netting (or garden netting)•

PreparationWe created this activity so that our students can experience collect-ing data on bird specimens that consist of laminated, colored photo-graphs. Only two finch species are used in the activity: G. fuliginosa and G. fortis. Pictures of these finches can be found by doing a simple online image search. We varied the sizes of the photographs slightly in order to better visualize the variation in beak depth between individuals, as well as the size variations between populations (for examples of beak depth measurements, see Table 2). Geospiza fuligi-nosa has a larger average beak depth on Crossman than on James, and G. fortis has a larger average beak depth on James than on Daphne. We used 12 birds of one species on each island (James had 24 birds total, because both species are on that island). Once the finch images were completed, we separated them by island to make “finch packets” for the students. To make the activity more realistic, we recommend actually setting up bird netting in the classroom and hanging the bird photographs from the netting by paperclips. All in all, although finding, modifying, printing, laminating, and cutting out the photo-graphs can be time consuming, the laminated photographs can be repeatedly used.

Procedure OverviewIntroduction (~5 minutes)•

Collecting data (~10 minutes)•

Creating histograms (~10 minutes)•

Answering worksheet questions (~20 minutes)•

Class discussion and activity wrap-up (~25 minutes)•

ProcedureOur introductory discussion follows this outline:

Review the location and importance of the Galápagos Islands.•

Review Darwin’s observations on the voyage and that several scientists •have since studied “Darwin’s finches.”

Describe Lack’s field study, how he collected finch specimens and •made measurements, including beak depth, but do not reveal his results.

Explain the purpose of the activity: to model Lack’s field study.•

Define “beak depth” (but describe how to measure it only after • students have received materials).

Name the finch species and the islands on which they are found.•

314 The american biology Teacher volume 76, no. 5, may 2014

Table 2. Example measurements of beak depth (mm) for Geospiza fortis and G. fuliginosa from the three islands, Crossman, Daphne, and James. A sample size of 12 was used for each island–species combination.

IslandG. fuliginosa, Small

Ground FinchG. fortis, Medium

Ground Finch

Cros

sman

13.1

13.4

13.5

13.6

14.0

14.0

14.5

14.6

14.8

15.2

15.3

15.8 D

aphn

e

12.3

12.8

13.5

13.6

13.7

13.9

13.9

13.9

14.3

14.4

14.5

14.7

Jam

es

11.2 14.7

11.5 15.5

11.6 15.8

11.7 15.8

12.1 15.8

12.5 16.7

12.5 16.7

12.8 17.1

13.4 17.4

13.4 18.0

13.6 18.3

13.7 18.7

Next we divide the class into three “islands,” with twice as many students assigned to James because both species are on that island. We pass out (1) finch packets (see Preparation), (2) beak depth

measurement instructions (Figure 1), (3) calipers (possible alter-native: rulers), and (4) the student handout (Figure 2). We explain how to use the calipers to measure beak depth and remind students to fill in the correct cell of the data table, since everyone is gathering data for a different island. After the explanation, students take turns measuring and recording beak depth in Part A of the handout until all birds are measured. While students are working on this activity, we walk from group to group to make sure that beak depth is being measured accurately. After initial assistance, our students are able to measure beak depth with ease.

After measuring all birds, students write their data on the board so that all have the data from the three islands. After recording all the data, the students create histograms (Figure 3). (Because histograms had not been used previously in the course, we had to explain how a histogram differed from a bar chart and how to create histograms, as many students were initially confused.)

Once the students finish the histograms, they answer the questions in the handout. Before any class discussion takes place, we have stu-dents answer all questions from both Part B, which covers biologi-

cal content, and Part C, which covers the NOS concept. (Few students asked us any questions.) After the students are done answering questions, there is a class discussion.

We discuss Part C first, and the handout serves as a general outline of the discussion. During this discussion, our students were quick to identify that there is no single scientific method. Moreover, they rapidly acknowledged that inves-tigations can occur outside of the laboratory and do not have to involve experiments. They explained that the method employed depends instead on the research question. However, the “myth of the scientific method” had been briefly introduced at the beginning of the course, when several other NOS tenants were defined (the activity took place several weeks later). Because of this, it was unclear whether students had already accepted that there was not a single sci-entific method. Without the prior experience, they may or may not have been so quick to dis-agree with the scientific method.

After wrapping up the discussion on Part C, we move to Part B, addressing each question one at a time. First, students discuss why it is impor-tant to study beak depth. With the hint provided in the worksheet (What are beaks used for?), our students were able to discuss how the beaks determine what the birds can eat. To ensure that they saw the patterns in the data, we asked them to describe patterns between different species and different populations of the same species. Then we asked them to hypothesize explanations for the patterns observed. Most of our students concluded that the differences found among populations living on the same island were due to competing for limited food resources, but they needed further direction in identifying how the competition influenced the evolution of these

The american biology Teacher naTural SelecTion 315

Figure 1. Calipers are extended to fit around the beak to measure beak depth.

Figure 2. Student handout. Table 1 illustrates the data table provided for Part A (1), and Figure 3 illustrates the charts provided for Part A (2).

populations. The discussion commenced with how Lack’s study, which the students modeled, provided evidence for the importance of competition to natural selection.

Summative AssessmentJ JJ

Both the biological concepts and the NOS concept were covered in the summative assessment, in order to stress the importance of both

objectives. Summative assessment can include both general definitions and application to the histori-cal episode. If students are allowed to practice, novel examples can also be provided to students to let them predict what could happen in certain situations. For example, Anderson et al. (2002) provided a tested conceptual inventory of multiple-choice questions on natural selection that utilizes three case studies, including Darwin’s finches.

Activity ModificationsJ JJ

The following are possible modifications of the activity, most of which will increase the level of inquiry.

Students write a web-based research report •on both finch species before starting the activity. The report may cover (1) morphology, (2) life history, and (3) biogeography. Possible resources:

http://bioquest.org/birdd/ #

http://eol.org/pages/36192/details #

Before beginning the activity, discuss the impor-•tance of measuring beak depth. This leads into a discussion about possible hypotheses to test.

Continue the activity by instructing students •to create an experiment that could provide additional evidence for Lack’s conclusions. The students’ designs can then be compared to Lack’s methods.

After completing the activity, students select •an article that provides evidence for character displacement (either on their own or predeter-mined by the instructor) and summarize, cri-tique, and compare the article to Lack’s study.

For Advanced Ecology or Evolution Courses: •During the discussion, introduce the compet-itive exclusion principle, character displacement, and ghost of competition past (as described above in Darwin’s Finches & David Lack’s Study).

ConclusionJ JJ

All in all, we believe that this activity is an excellent addition to a biology course. Our students were able to complete the activity with little confusion and comprehend the information, especially the NOS concept. The activity gives students a chance to collect data, while giving instructors the ease of

relying on materials that can be used repeatedly. Moreover, because of the nature of the activity and the explicit questions, students learn more about the possible ways that science can be explored.

AcknowledgmentsJ JJ

We thank our laboratory coordinator for her assistance in creating the materials for this activity. We also acknowledge the reviewers

Figure 3. Histograms of beak depth using data from Table 1 for (A) Geospiza fuliginosa on Island Crossman, (B) G. fortis on Island Daphne, (C) G. fuliginosa on Island James, and (D) G. fortis on Island James.

316 The american biology Teacher volume 76, no. 5, may 2014

for their extremely helpful and constructive feedback; one, in par-ticular, provided several of the modifications that are listed in this article.

Referencesanderson, D.l., Fisher, K.m. & norman, g.J. (2002). Development and evaluation

of the conceptual inventory of natural selection. Journal of Research in Science Teaching, 39, 952–978.

Darwin, c. (1897). Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the command of Captain Fitz Roy, R. N., 2nd Ed. new york, ny: D. appleton.

grant, P.r. & grant, b.r. (2006). evolution of character displacement in Darwin’s finches. Science, 313, 224–226.

hardin, g. (1960). The competitive exclusion principle. Science, 131, 1292–1297.

lack, D. (1947). Darwin’s Finches. london, uK: cambridge university Press.

nehm, r.h. & reilly, l. (2007). biology majors’ knowledge and misconceptions of natural selection. BioScience, 57, 263–272.

ngSS lead States. (2013a). Next Generation Science Standards: For States, By States. achieve, inc. on behalf of the twenty-six states and partners that collaborated on the ngSS. Washington, Dc: national academies Press.

ngSS lead States. (2013b). Topical arrangement of the Next Generation Science Standards. achieve, inc. on behalf of the twenty-six states and partners that collaborated on the ngSS. [online.] available at http://www.nextgenscience.org/search-standards.

Pfennig, D.W. & Pfennig, K.S. (2010). character displacement and the origins of diversity. American Naturalist, 176, S26–S44.

Snodgrass, r.e. (1902). The relation of the food to the size and shape of the bill in the galapagos genus Geospiza. Auk, 19, 367–381.

anDrea marie-Kryger bierema is a research Doctoral associate in Science education at Western michigan university, 1903 W. michigan ave., Kalamazoo, mi 49008-5444; e-mail: andrea.m.kryger@wmich.edu. DaviD WySS ruDge is an associate Professor of biological Sciences and mallinson institute for Science education at Western michigan university; e-mail: david.rudge@wmich.edu.

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The american biology Teacher naTural SelecTion 317

We encourage our readers, biologists with teaching interests, and biology educators in general, to write for The American Biology Teacher. This peer-reviewed journal includes articles for teachers at every level with a focus on high school and post-secondary biology instruction.

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Feature Article (up to 4000 words) are those of general interest to readers of ABT. Consider the following examples of content that falls into the feature article category:

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We encourage our readers, biologists with teaching interests, and biology educators in general, to write for The American Biology Teacher. This peer-reviewed journal includes articles for teachers at every level with a focus on high school and post-secondary biology instruction.

The general categories of articles are:

Feature Article (up to 4000 words) are those of general interest to readers of ABT. Consider the following examples of content that falls into the feature article category:

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e. Other timely and relevant and interesting content like discussions of the role of the Next Generation Science Standards in biology teaching, considerations of the history of biology with implications for the classroom, considerations of the continuum of biology instruction from K-12 to post-secondary teaching environments, contributions that consider the likely/ideal future of sci-ence and biology instruction.

Research on Learning (up to 4000 words) includes reports of original research on innovative teaching strategies, learn-ing methods, or curriculum comparisons. Studies should be based on sound research questions, hypotheses, discussion of an appropriate design and procedures, data and analysis, discussion on study limitations, and recommendations for improved learning.

Inquiry and Investigations (up to 3000 words) is the sec-tion of ABT that features discussion of innovative and engag-ing laboratory and field-based strategies. Strategies in this section should be original, focused at a particular grade/age level of student, with all necessary instructions, materials list, worksheets and assessment tools, practical, related to either a particular program such as AP and/or linked to standards like NGSS. The most appropriate contributions in this catego-ry are laboratory experiences that engage students in inquiry.

Tips, Tricks and Techniques (up to 1500 words but may be much shorter) replaces the How-To-Do-It and Quick Fix articles. This section features a range of suggestions useful for teachers including laboratory, field and classroom activities, motivational strategies to assist students in learning specific concept, modifications of traditional activities, new ways to prepare some aspect of laboratory instruction, etc.

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• Acceptable file formats are TIFF, BMP, and EPS.

• Set to one-column (3.5” wide) or two-column size (7” wide).

If you have any questions, contact Mark Penrose at managingeditor@nabt.org.

Submissions of cover photographs from NABT members are strongly encouraged. Covers are selected based on the quality of the image, originality, overall composition, and overall interest to life science educators. ABT has high standards for cover image require-ments and it is important for potential photographers to under-stand that the size of the cover image generally precludes images taken with cell phones, point-and-shoot camera and even some older model digital SLR cameras.

Please follow the requirements listed below.

1. E-mail possible cover images for review to Assistant Editor, Kathleen Westrich at kmwestrich@yahoo.com.

2. Choose images with a vertical subject orientation and a good story to tell.

3. Avoid cropping the subject too tightly. It is best to provide an area of background around the subject.

4. Include a brief description of the image, details of the shot (i.e., circumstances, time of day, location, type of camera, camera set-tings, etc.), and biographical information in your e-mail message.

5. Include your name, home and e-mail address, and phone num-bers where you can be reached.

6. Please ensure that the image meets the minimum standards for publication listed below and has not been editors or enhanced in any way. The digital file must meet the minimum resolution of 300 pixels per inch (PPI)—preferred is 400 PPI— and at a size of 8.5 x 11.25”. We accept TIFF or JPEG images only.

7. For exceptional images, the editors will also accept sharp, clear, color 35 mm slides. Submit only the original; duplicates will not be accepted. Be sure to clearly label your slides with your name and contact information in ink. Contact Assistant Editor Kathy Westrich beforehand to discuss the possibility of submit-ting a 35mm slide or other non-digital format for consideration as an ABT cover.

Requirements forSubmitting Cover Photographs forThe American Biology Teacher

Guidelines for Authors & Photographerscontinuation