CASE TEACHING NOTES
The three-spined stickleback (Gasterosteus aculeatus) is a fascinating little fish, widely distributed throughout the Northern Hemisphere. It is found in coastal marine, estuarine, and freshwater environments. A typical stickleback is 5 cm long from nose tip to tail, with bony armor and spines for protection from predators.
In the spring, each male stickleback builds a nest in shallow water. The nest is a small cavity lined with algae with an entrance and an exit. The male develops bright mating colors, with blue back and sides and a bright red throat, and vigorously defends his territory and nest from invaders. Female sticklebacks also frequent the nesting area, carrying eggs within their distended abdomens. A male recognizing a female by her swollen shape may court her, displaying a zigzag dance or nipping at her fins. If the female is receptive, she displays her abdomen and swims after the male to his nest entrance. She enters the nest and he taps on her side with his snout, prompting her to release her eggs and swim away. The male then follows the female into the nest and fertilizes the eggs. He may continue to court new females, and sometimes several will spawn in his nest. After a time, the male devotes himself to caring for the eggs. He defends his nest actively, and fans the developing eggs with his pectoral fins until the tiny fry hatch. Once the young sticklebacks have left the nest, the male may build a new nest and start the cycle again.
Upon completion of this case, students should be able to:
In southwestern British Columbia, six lakes on three islands in the Strait of Georgia of British Columbia contain two forms of three-spined stickleback (see Figure 1). The benthics are stout bodied, wide mouthed, poorly armored, and have few, short gill rakers. These fish forage on benthos in the shallow margins of the lake. The limnetics are slender bodied, slim mouthed, well armored, and have many long gill rakers. They feed on zooplankton in the open waters of the lake (McPhail 1993a). The differences between the two forms can be seen in Figure 2.
In the lab, F1, F2, and backcross hybrids can be easily formed and are viable, fully fertile, and morphologically intermediate between benthic and limnetic fish. A small lake on Texada Island in the Straight of Georgia was stocked with F1 hybrid fish in 1981. The hybrids have become well established and persist in this lake 20 years later (McPhail 1993a).
In the six lakes containing benthic-limnetic pairs, the benthic and limnetic forms differ slightly but stably in allele frequencies (McPhail 1984). The level of hybridization is low but persistent. One to two percent of wild fish show intermediate morphology and represent possible hybrids. Despite a small amount of gene flow between the populations, the benthics and the limnetics retain their genetic and morphological integrity in each lake. This suggests that they represent separate gene pools, and it has been argued that they represent separate species (McPhail 1993a).
Males of both forms build nests in the shallow margins of the lake, and benthic and limnetic males may be neighbors. However, benthic males select sites with heavy vegetation while limnetic males select exposed sites. Benthic and limnetic pairs show well developed positive assortative mating. In laboratory and field tests, both male and female benthics and limnetics choose only conspecific mates, and benthics and limnetics differ in their courtship behaviors (Ridgeway and McPhail 1984).
The divergence of the benthic-limnetic pairs in these six British Columbia lakes is very recent. Glaciers retreated from coastal areas of British Columbia 13,000 years ago. As the ice melted, the land rebounded and formed a number of small Gulf Islands, located between Vancouver Island and the British Columbia mainland. As the islands rose, lakes formed and gradually became brackish and then fresh water. Marine sticklebacks trapped in these lakes became adapted to a fresh water existence.
The origin of the benthic-limnetic pairs is controversial. The similarity of the mitochondrial DNA between the benthic-limnetic pairs in each lake (<0.5% sequence divergence) suggests that each pair arose independently by sympatric speciation, following a single invasion of each lake by ancestral marine sticklebacks (Schluter 1996).
However, McPhail (1992) found that the limnetic forms show greater genetic similarity to marine sticklebacks than do the benthic forms (based on microsatellite DNA and allozyme analysis), and he suggests a double invasion hypothesis. According to this model, sticklebacks colonized newly formed lakes on the Gulf Islands postglacially, following the isolation of the lakes 13,000 years ago. Directional selection favored morphological changes that allowed the sticklebacks to exploit benthic resources in the lakes. Then, 11,000 years ago, a rise in sea level reflooded the lakes and allowed a second invasion of marine sticklebacks. With the benthic resources already exploited by the first invaders, the second set of colonists survived as planktivores and are the ancestors of the modern limnetics (McPhail 1993a). The extent and timing of the second sea level rise is still controversial, and appears to have been small and regionally variable. There is little evidence for a second sea level rise flooding the three islands that contain the benthic-limnetic stickleback pairs (Clague 2001). However, even a slight increase in sea level may have allowed marine stickebacks to reinvade the lakes, and an allopatric origin of the benthic-limnetic pairs seems most likely.
Whatever their origins, the benthic-limnetic pairs of sticklebacks appear to be in the process of speciating. Since their recent divergence, reproductive isolation and resource partitioning between the two forms are well developed, but neither process is fully complete. There are no intrinsic sterility or developmental barriers to interbreeding, but the rate of hybrid formation is very low.
The separation (and incipient speciation) of the benthics and limnetics is maintained by strong positive assortative mating and strong selection against hybrids within the lakes. Hybrids are less likely to be selected as mates than conspecific forms (Hatfield and Schluter 1996; Vamosi and Schluter 1999). The intermediate morphology of the hybrids makes them less effective foragers than either parental form, unable to efficiently exploit limnetic or benthic resources (Schluter 1993; Schluter 1995; Schluter 1996). The benthic-limnetic pairs of sticklebacks in six small British Columbia lakes offer a fascinating and well-studied look at the process of speciation.
This case is used in the final week of a General Biology course organized around the general theme of evolutionary mechanisms and the history of life on earth. By this point in the course, students have covered the origin of life, organismal diversity, genetics, ecology, natural selection, and behavior.
Students require familiarity with a number of concepts before encountering this case. These concepts include the biological species concept, basic genetic terminology, zonation in fresh water lakes, and the mechanisms of speciation.
The case can be completed within a 90-minute time block. It starts with a brief (5- to 10-minute) introduction by the instructor. Three-spined stickleback morphology and life cycle are described, with illustrations of the male in mating coloration with his nest. A brief video clip of the courtship and mating of the sticklebacks can be shown.
The case is then presented, and the students work in groups of three or four to design the initial lab experiment and field research. Groups are given 20 minutes to complete this task. The instructor and teaching assistants move from group to group, prompting students to consider the biological species concept in designing these experiments and encouraging them to think carefully about what data they need. As students design their lab experiments and plan their field data collection, they are supplied with appropriate data.
Virtually all the lab experiments proposed involve crossing benthics with limnetics and rearing the hybrid offspring. Various data are available on such crosses. Data sheet 1 shows the anatomical characteristics of F2 hybrid fish. Data sheet 6 gives the percent survival of hybrid, benthic, and limnetic fertilized eggs to hatching. Data sheet 10 demonstrates the long-term viability of hybrid sticklebacks.
Students usually request field data on the numbers of hybrid fish in the lake (data sheet 3). They may also ask about observations of courtship and mating between benthics and limnetics (data sheet 11).
Once students have planned their lab experiments and field data collection and have received the appropriate data, a brief general discussion follows. Ask the students questions such as "What is happening in Paxton Lake?" "Are benthics and limnetics separate species?" Students may refer to the biological species concept to suggest that speciation has not occurred; the two forms can mate with each other and form viable, reproductive hybrids. However, benthics and limnetics do not mate with each other (or interbreed very seldom) and there are very few hybrid fish in the lake (1-2%).
The students are then asked to design a lab or field experiment to answer a further question: "Why do these two groups of sticklebacks not interbreed in Paxton Lake since they are capable of interbreeding with each other?" Students again spend 20 minutes in their small groups designing a lab or field experiment to address this question. The instructor and teaching assistants move from group to group, prompting students to consider what may prevent the formation or reduce the fitness of hybrid sticklebacks. As students describe their experiments, they are supplied with the appropriate data.
Most experiments that students design fall into one of two categories: experiments about mating differences and preferences (data sheets 4, 5, 7, and 9) and experiments testing the ability of hybrids to feed and grow (data sheets 2 and 8). It is important that results be available in both categories for the final discussion, so you may wish to steer some groups if necessary. Usually, it's not a problem. Groups will likely design both types of experiment within your class.
Note: Groups may anticipate the second question and design experiments about mating preferences and hybrid success when investigating the initial question. If so, give them the data they request and suggest that they present their results during the second discussion. They can design another experiment during the second small group task, and can present the results for both experiments during the final discussion.
A general discussion completes this case, and no formal writeup or presentation is required. Each group presents their "research findings" and discusses their significance. Students usually conclude that the benthic-limnetic pair of Paxton Lake is in the process of speciation. Hybrids are at a selective disadvantage in the lake, foraging less efficiently and growing more slowly than either benthics or limnetics. As a result, selection has favored benthics or limnetics mating with "their own kind." Benthics and limnetics court differently, and each will select a conspecific mate when offered a choice. Hybrids are also less likely to be selected as mates.
Instructors wishing to complete this case in a single 60-minute period can give the students a single task to complete in small groups. The final questions in the case can be changed to the following:
Don has hired you as a summer student to investigate the sticklebacks of Paxton Lake. What data could you collect from Paxton Lake to help you decide if these two groups are indeed separate species? You know that sticklebacks are easy to keep and rear in the lab. What lab experiment could you carry out to find out whether the groups are capable of interbreeding, and whether their offspring survive? Plan one lab and one field experiment to address these questions.
A single discussion of all experiments would complete the case. It might be necessary to nudge students towards some experiments, such as those investigating assortative mating or hybrid feeding. If necessary, the instructor could present missing data during the discussion.
It is advisable to hand out a complete set of data sheets to each student at the conclusion of the case. As a minimum, each student should receive four sheets. Two sheets detail the viability (data sheets 1, 6, or 10) and scarcity (data sheets 3 or 11) of hybrid fish. One sheet demonstrates assortative mating (data sheets 4, 5, 7, or 9), and one sheet demonstrates the reduced foraging success of the hybrids (data sheets 2 or 8).
This case will make speciation more immediate to biology students, and demonstrates that species continue to arise.
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Acknowledgements: This case study was developed with support from The Pew Charitable Trusts and the National Science Foundation as part of the Case Studies in Science Workshop held at the University at Buffalo, State University of New York, on June 12-16, 2000.
Image Credit: "Paxton Lake Sticklebacks" by Elizabeth Carefoot, Learning and Instructional Development Centre, Simon Fraser University. Used with permission.
Date Posted: 11/02/01 nas
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