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Something’s Fishy in Paxton Lake

Speciation in Sticklebacks


Joan Sharp
Department of Biological Sciences
Simon Fraser University


In this interrupted case study, students explore the mechanisms of speciation while working in groups to design a series of experiments to determine whether two populations of sticklebacks in Paxton Lake in British Columbia represent separate species or not. As the case progresses, 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 (a set of 11 data sheets for this purpose is included in the teaching notes). Due to the open-ended nature of the activity, there is no separate answer key for this case.  This 60–90 minute activity was designed for use in the final week of a general biology course organized around the general theme of evolutionary mechanisms and the history of life on earth. [For an updated, flipped version of this case study, see “Speciation and the Threespine Stickleback,” also in this case collection.]


  • Use the biological species concept to decide whether two populations represent separate species.
  • Apply the mechanisms of speciation to a real life case.
  • Explain how prezygotic and postzygotic reproductive isolating mechanisms reduce genetic exchange between incipient species.
  • Explain how natural selection may act to favor divergent morphologies, as incipient species adapt to different ecological roles, and favor the evolution of reproductive isolating mechanisms.
  • Design experiments to test hypotheses.
  • Interpret data and understand how they may be used to support or reject hypotheses.


speciation; stickleback; biological species concept; reproductive isolation; Gasterosteus aculeatus; benthic; limnetic; gene flow; hybrid; British Columbia

Topical Areas


Educational Level

Undergraduate lower division, Undergraduate upper division



Type / Methods




Subject Headings

Biology (General)  |   Evolutionary Biology  |  

Date Posted


Teaching Notes

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Robin Pals-Rylaarsdam
Trinity Christian College
Palos Heights, IL 60463

Yesterday [05/08/2006] I was teaching my Genetics course and finishing up our work on molecular evolution. At the end of the class period, one of the seniors, a student who’s going to graduate next Saturday and go to physical therapy school, wanted to ask what he thought might be an “irrelevant” question. He wanted to know how the material we were dealing with related to the stickleback speciation case—a case that he and his classmates had encountered in the fall of their freshman year! He said it had always bugged him that speciation was a messier event than “Bingo! A new species is here!,” and he wanted to know how/if the sequence alignments and phylogenetic relationships we were looking at could give us insight into speciation.

I would never have imagined that this student (or most any student) would have remembered and cared about a case study for so long. It’s a great testimonial to the effectiveness of that case, and to the effectiveness of getting out of lecture mode frequently and regularly.