NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

CASE TEACHING NOTES for “The Evolution of Human Skin Color” by Annie Prud’homme-Généreux, Life Sciences, Quest University, Canada

INTRODUCTION / BACKGROUND
While the concept of evolution by natural selection is very simple, it is often misunderstood by students. This is partly due to preconceptions they have as well as a lack of understanding or emphasis on the idea that reproductive success (and not survival) is what matters to evolution.
One way to ensure that students grasp this concept is to confront them with situations that require them to examine each factor’s effect on survival and reproduction.
For many years, the evolution of human skin color was presumed to be driven by the need to protect the body against ultraviolet (UV) light. Skin cancer was viewed as the driving force removing individuals lacking the proper amount of skin pigmentation (i.e., skin cancer affected survival). More recently, it has been appreciated that skin cancer is a disease that affects individuals in later life and therefore could not exert much selective pressure on evolution (i.e., it does not affect reproductive success).
This case study asks students to evaluate four factors that could affect the evolution of skin pigmentation in humans. In doing so, students acquire an understanding of the process of natural selection. Students also consider whether a combination of the factors identified can explain the distribution of skin colors around the world.
This case study was designed for a first-year introductory biology course for non-majors. To prepare for the case, students were required to read a chapter in their textbook on evolution by natural selection. Students had no prior exposure to Mendelian genetics, but some knowledge of biochemistry (central dogma of gene expression). This case could also be used to introduce students to the concepts (i.e., prior to any reading). If this option is selected, the instructor should extend the time allotted for class discussion.

• Predict patterns that would confirm their hypothesis.
• Interpret data and compare to predicted outcomes.
• Reformulate hypotheses when in conflict with existing data. • Identify factors that can exert evolutionary pressures
(discriminate between factors that affect reproduction and those that affect survival).
• Apply the concepts of natural selection to a real situation.
• Evaluate whether sexual selection is affecting a human trait. • Propose public policies and practices based on assimilated information.

CLASSROOM MANAGEMENT
This case is an interrupted case study, that is, information and data in the case is progressively disclosed to students.
The students work on each portion of the case in groups.
The case takes approximately two hours to complete
(longer if it is used as an introduction to students with no prior background in evolution).
Students arrive to class having read a chapter in their first-year biology textbook that provides an overview of evolution by natural selection. Class begins with a brief quiz to ensure that each student has taken responsibility for his/her learning. Students then break into groups of
3 to 5 students. I distribute the printed materials for the first part of the case and give students a preset amount of time for reading and group discussion (the time allotted for each section varies—see below for details). This is followed by class discussion to combine input and ensure everyone is on the right track. We then proceed with the next section, reiterating these steps.
For the class discussions, it would be helpful to have access to an overhead projector or white or black boards with pens.

Objectives
Upon completion of this case study, students should be able to:
• Formulate testable hypotheses given preliminary data.

Part I – Skin Cancer (15 min)
This first section provides a brief introduction to skin pigmentation and explores the causes of skin cancer.

Case Teaching Notes for