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Blood Suckers! A Case Study on Evolution and Speciation


Troy R. Nash
Department of Biology
Mercer University


This directed case study in PowerPoint format focuses on the London Underground Mosquito, Culex molestus, and its potential relationship to the common mosquito, Culex pipiens, in order to explore the topics of evolution, reproductive isolation, and speciation. As the story unfolds, the case mirrors the process of science. The students receive some initial data and observations collected by researchers Byrne and Nichols in London. Based on these observations, the students then form a hypothesis and design an experiment. Finally, they receive more data collected by Becker et al. and draw conclusions. At the end of the case, the students are able to reflect on the importance of the iterative nature of scientific investigation.  In addition to using clicker questions and small group discussions, this case is also "flipped"; before coming to class, students prepare by watching several short videos that teach the basics of speciation and mechanisms of evolution, including a video created by the author on cladograms and two species concepts. Originally written for an introductory biology class, the case could be used in a majors, non-majors, or mixed-majors setting.


  • Compare and contrast the different mechanisms of speciation (allopatric, sympatric, parapatric, and peripatric).
  • Explain how the founder effect alters the genetic diversity of populations and species.
  • Define reproductive isolation (pre-zygotic and post-zygotic) and apply these definitions to specific examples.
  • Interpret a cladogram showing genetic relationships amongst organisms.
  • Explain the importance of the iterative nature of scientific inquiry.


Evolution; founder effect; speciation; allopatric; sympatric; parapatric; peripatric; reproductive isolation; cladistics; cladogram; mosquito; experimental design; scientific method; London

Topical Areas

Scientific method, Scientific argumentation

Educational Level

High school, Undergraduate lower division


PDF, PowerPoint

Type / Methods

Clicker, Directed, Flipped, Interrupted



Subject Headings

Biology (General)  |   Ecology  |   Evolutionary Biology  |   Science (General)  |  

Date Posted


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The following video(s) are recommended for use in association with this case study.

  • The Five Fingers of Evolution
    This video provides a brief introduction to the definition of evolution and the five mechanisms by which it can occur (genetic drift, sexual selection, mutation, immigration/emigration, and natural selection). Depending on when you are using this case during the semester, this video may not be necessary. Running time: 5:23 min. Produced by Paul Anderson and Alan Foreman for TedEd, 2012.
  • Speciation: Of Ligers and Men
    This video introduces the Biological Species Concept and explains how reproductive isolation (both pre-zygotic and post-zygotic) can occur. The video differentiates between allopatric and sympatric speciation. Natural selection and artificial selection are also mentioned. Running time: 10:25 min. Produced by Hank Green, Blake de Pastino, Nick Jenkins, Jesslyn Shields, Brandon Jackson, Michael Aranda, Peter Winkler, and Amber Bushnell, 2012.
  • Evolution and Speciation
    This video reinforces the concept of evolution and how it is commonly illustrated using cladograms. It also compares and contrasts two different species concepts. Running time: 9:12 min. Created by Troy Nash for the National Center for Case Study Teaching in Science, 2015.