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Speciation and the Threespine Stickleback

By Joan Sharp, Erin Barley, Kevin K.-W. Lam, Suraaj Aulakh, Allison Cornell, Kathleen A. Fitzpatrick

Speciation and the Threespine Stickleback



This case study teaches students about allopatric speciation through an investigation of the benthic and limnetic sticklebacks of Paxton Lake, which are among the youngest species on Earth, diverging from each other after the Pleistocene glaciers melted and the Gulf Islands formed. Researchers at the University of British Columbia have carried out a variety of fascinating studies on these hardy little fish. Results from this research (formatted as data sheets included in the teaching notes) are provided to students who design experiments and then compare actual data to investigate why benthic and limnetic sticklebacks seldom interbreed in Paxton Lake. Developed for a first-year biology course for majors organized around the general theme of evolution and the history of life on Earth, this case study is an updated version of another case in the collection, “Something’s Fishy in Paxton Lake” (Sharp, 2001). The current version is especially suited for a flipped classroom in which students prepare for class ahead of time with a reading assignment that also involves the viewing of a video by the case authors that introduces the mechanisms of allopatric speciation.


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  • Apply the mechanisms of allopatric speciation to a real life situation.
  • Explain how natural selection may act to favor divergent morphologies as incipient species adapt to different ecological roles.
  • Explain how natural selection acts to favor the evolution of reproductive isolating mechanisms.
  • Design experiments to test hypotheses about speciation.
  • Interpret data and understand how they may be used to support or reject hypotheses.
  • Discuss the status of benthic and limnetic stickleback fish as distinct species, with reference to different species concepts (biological, morphological, ecological).


Allopatric speciation; species; founder effect; genetic drift; mutation; natural selection; character displacement; dispersal; vicariance; stickleback; Gasterosteus aculeatus; lake zonation; causation; assortative mating; predation risk; foraging success


Subject Headings

Biology (General)
Evolutionary Biology
Wildlife Management


Undergraduate lower division












Discussion, Flipped, Interrupted



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