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Selection on a Case by Case Basis


Adam R. Irvine
Biological Sciences
University of Denver
Eva S. Horna-Lowell
Biological Sciences
University of Denver
Mathew J. Driscoll
Geography and Natural Sciences
University of Denver
Emily D. Broder
Biological Sciences
University of Denver


This "clicker case" gives students an opportunity to apply their understanding of three different types of selection (directional, stabilizing, and disruptive) to a variety of model systems. The case describes each type of selection in detail, presents a paradigm case of each, and then uses clicker questions to test whether students can identify the mode of selection in other systems (answers included in the teaching notes).  Nine different studies are presented so that students gain a broad understanding of selection and how environmental factors can impact species differently. The case concludes with an optional discussion activity (also discussed in the teaching notes) to deepen understanding. Although designed for a high school classroom, it could easily be used in a lower-level college course as an engaging introduction or review of concepts. The case could be used at the beginning of an evolution unit, but it would be helpful if the students were familiar with natural selection before completing the activity.


  • Understand the three ways selection can act on traits: directional, stabilizing, and disruptive.
  • Understand how both trait distributions and fitness curves shift in response to those selective forces.
  • Be familiar with examples of each type of selection and be able to identify the applicable type of selection.
  • Improve basic graphing skills including understanding the x- and y-axis, histograms, and distribution curves.


Natural selection; evolution; selection curves; selection; types of selection; directional; stabilizing; disruptive;

Topical Areas


Educational Level

High school, Undergraduate lower division


PDF, PowerPoint

Type / Methods

Clicker, Discussion, Journal Article, Mini-Case



Subject Headings

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

Date Posted


Teaching Notes

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Teaching notes are intended to help teachers select and adopt a case. They typically include a summary of the case, teaching objectives, information about the intended audience, details about how the case may be taught, and a list of references and resources.