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Selection and the Blond Beach Mouse



Author:

Joan Sharp
Department of Biological Sciences
Simon Fraser University
jsharp@sfu.ca

Abstract:

This "clicker" case study explores ultimate and proximate explanations for cryptic coloration in animals through the work of Dr. Hopi Hoekstra of Harvard University, who studies Gulf and Atlantic Coast beach populations of oldfield deer mice that have evolved blond fur coloration. An ultimate question addresses the adaptive value of blond coloration, exploring how cryptic coloration increases the evolutionary fitness of a beach mouse. Proximate questions address the mechanisms that produce blond coloration, such as the genetic and developmental mechanisms that alter mouse coat color. Students work in small groups to plan an experiment to assess the design of a published experiment and analyze the results. They learn about roles of mutant alleles of two pigment genes (Mc1r and Agouti) in producing blond coloration in several subspecies of beach mice, as well as woolly mammoths. Finally, students weigh evidence from genomic analysis to select between two contrasting hypotheses about the origin of the blond Mc1r allele. The case is presented in PowerPoint format and includes both a detailed and simplified slide set, allowing instructors to select the most appropriate version for their class. Several recommended videos make it possible to teach the case in a "flipped classroom" setting.

Objectives:
  • Identify mutation as a random process changing an organism's DNA.
  • Identify natural selection as a nonrandom process that favors traits that are adaptive in a specific environment.
  • Explain that ultimate questions about evolution by natural selection address the adaptive value of a trait in a specific environment.
  • Explain that proximate questions about evolution by natural selection may address the specific genes and mutations that cause a phenotypic trait.
  • Assess whether a mutation producing a phenotypic trait is advantageous, deleterious, or neutral, depending on the environment in which the organism lives.
  • Design a simple experiment.
  • Apply the concepts of evolutionary fitness and adaptation to predict results of an experiment.
  • Interpret experimental data and use these data to support or reject hypotheses.
  • Explain how mutations in different genes may result in the same phenotype.
  • Interpret genomic analysis from related populations to assess where and when a mutant allele arose.
Keywords: Natural selection; adaptation; evolution; mutation; ultimate explanation; proximate explanation; evolutionary fitness; experimental design; melanin; coat coloration; crypsis; Mc1r; Agouti; convergence
Topical Area: N/A
Educational Level: Undergraduate lower division
Formats: PDF, PowerPoint
Type/Method: Clicker, Discussion, Flipped
Language: English
Subject Headings: Biology (General)   Evolutionary Biology  
Date Posted: 8/25/2015
Date Modified: N/A
Copyright: Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

Teaching Notes


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Answer Key


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Supplemental Materials


The supplemental materials listed below are appropriate for use with this case.

  Activity: Molecular Genetics of Color Mutations in Rock Pocket Mice hhmi/
Students transcribe and translate portions of the wild-type and mutant rock pocket mouse Mc1r genes. Students compare sequences to identify coat color mutation location and type.
  Activity: Biochemistry and Cell Signaling Pathway of the Mc1r Gene hhmi/
Students analyze partial DNA sequences of the Mc1r gene and identify the effects on the MC1R protein pathway responsible for rock pocket mouse coat color. Students explore cell signaling and its role in coat color.
  Activity: Color Variation over Time in Rock Pocket Mouse Populations hhmi/
Students collect and analyze data about for coat color in pocket mouse populations on different color substrates over time. The data reinforces concepts of variation and natural selection.
  Activity: Allele and Phenotype Frequencies in Rock Pocket Mouse Populations hhmi/
Students use real rock pocket mouse data collected by Dr. Michael Nachman and his colleagues to illustrate the Hardy-Weinberg principle.
  Activity: Natural Selection and Evolution of Rock Pocket Mouse Populations hhmi/
Students analyze amino acid data and draw conclusions about the evolution of coat color phenotypes in different rock pocket mouse populations. Students review key concepts and mechanisms of evolution, including mutation, gene flow (or migration), genetic drift, and natural selection.

Videos

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

  Animation: Pocket Mouse Predation hhmi/
This short animation shows the different visibility of light and dark mice to predators in different environments. The dark morph is more vulnerable on light sandy desert, and the light morph on dark lava rock. Running time: 0:20 sec. Produced by HHMI BioInteractive.

  Animation: Pocket Mouse Evolution hhmi/
This short simulation shows the spread of a favorable mutation through a population. Even a small selective advantage can lead to a rapid evolution of a population. Running time: 1:05 min. Produced by HHMI BioInteractive.

  A Life in Science: Hopi Hoekstra
This video introduces Hopi Hoekstra, a Harvard researcher who studies the evolution of coat color. In the video, she explains how she decided on a career in biology and introduces her research organism, the oldfield deer mouse. Running time: 3:04 min. Produced by Thomas Lin and Tony Cenicola for The New York Times, 2013.

  The Making of the Fittest: Natural Selection and Adaptation hhmi/
This short award-winning film introduces students to the rock pocket mouse, Chaetodipus intermedius. In New Mexico’s Valley of Fire, rock pocket mice inhabit both desert habitats and recently formed lava flows. The film addresses how natural selection favors cryptic coloration for mice living both on sand and lava flows, due to predation by visual predators. Running time: 10:25 min. Produced by HHMI BioInteractive, 2012.

  Genetics of Color Adaptation
After students finish the case, they may wish to watch this optional, excellent video on the genetics of coat color, in which Dr. Hopi Hoekstra summarizes and expands on the research findings that are presented in the case study. Running time: 12:15 min. Produced by SeriousScience, 2014.




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