CHAPTER 20: GENES WITHIN POPULATIONS
WHERE DOES IT ALL FIT IN?
Chapter 20 takes a larger look at genetics in its coverage of population genetics. Students should be
versed on the information in Chapters 16 and 18. Students many feel the information in Chapter 20
stands alone and is not related to the earlier genetics coverage. It should be reinforced that population
genetics forms part of the explanation linking gene function to the adaptations seen in populations of
organisms. The information in this chapter should be briefly reviewed before progressing with
discussions on evolution and natural selection.
SYNOPSIS
Variation in the genetic composition of living organisms is the driving force behind evolution as
a whole. Darwin and Wallace proposed that natural selection was the mechanism through which
it occurs. As a population evolves it becomes better adapted to exist in its local environment.
Their idea greatly contrasts with Lamarck who proposed a mechanism by which evolution
occurred via inherited characteristics acquired over an organism’s lifetime. Population genetics
explains the behavior of alleles within populations. Most alleles are highly polymorphic within a
population and provide for greater variation than that caused by mutations alone.
The Hardy-Weinberg equation was derived to explain why dominant alleles did not drive out
recessive alleles and eliminate genetic variation. In large populations exhibiting random mating,
the original proportions of a genotype remain constant over time. The genotypes of such
populations are in equilibrium. The Hardy-Weinberg equation is a binomial expansion of the
sum of the frequency of the recessive and dominant alleles. One can determine the frequency of
one allele given the frequency of the other, as the sum of the two alleles must equal one. The
number of heterozygote dominant individuals can also be calculated from such information.
Although the Hardy-Weinberg principle predicts consistency in allele frequencies over time,
actual frequencies change as a result of (1) mutation, (2) gene flow, (3) nonrandom mating, (4)
genetic drift, and (5) selection. Thus the value of using the equation is being able to examine a
population’s genotypes at a point in time and then follow that population to determine if the
genotypes are changing.
The goal of selection is to have the individuals that are best suited to an environment leave the
most progeny, something that biologists refer to as fitness. In addition, fitness is a combination
of mating successes, and survival. Elimination of undesirable traits is difficult because selection
does not operate on rare recessive alleles, unless they are deleterious, then they could have
selection pressure against them. Frequency-dependent selection can be negative or positive.
When a rare form of genotype exist in a population it might have an advantage not permitted to
the more common genotype, or is some cases the rare form is selected against, while the more
common form is maintained in the population.
Natural selection can also maintain variation in a population through the existence of successful
heterozygotes. An example of this principle is sickle-cell anemia and its association with malaria
in Central Africa. Individuals with the heterozygous condition are more likely to survive and
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reproduce than either homozygote, one which succumbs to sickle-cell anemia and the other to
malaria. Disruptive selection, directional selection, and stabilizing selection act to eliminate one
or both extremes or to eliminate the intermediate from an array of phenotypes. Although all five
forces stated above cause genetic variation, only selection produces evolutionary change since
only it depends on the nature of the environment.
LEARNING OUTCOMES
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Recognize that natural selection is the correct mechanism for explain evolution.
Understand the importance of and identify sources of genetic variation.
Know how to solve various problems associated with Hardy-Weinberg equilibrium.
List the five factors, and state an example of each, that affect Hardy-Weinberg equilibrium
and understand how each can produce evolutionary changes in a population experiencing any
one of the five.
Compare and contrast the founder effect and the bottleneck effect.
Understand the term fitness and all of the considerations that are involved with the concept of
fitness.
Distinguish between positive and negative frequency-dependent selection.
Comprehend what is meant by “the heterozygote advantage,” and explain the example given
in the text.
Describe and be able to graph the examples of the three kinds of selection.
Appreciate the limitations of selection
COMMON STUDENT MISCONCEPTIONS
There is ample evidence in the educational literature that student misconceptions of information
will inhibit the learning of concepts related to the misinformation. The following concepts
covered in Chapter 20 are commonly the subject of student misconceptions. This information on
“bioliteracy” was collected from faculty and the science education literature.
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Students believe that all genes program for visible traits
Students are unfamiliar with the exact nature of regulatory gene in determining traits
Students believe that only the observable phenotype is subject to selection
Students do not fully understand the role of genetic drift
Students believe that acquired traits are inherited
Student believe evolution is driven to make “better” organisms
Students do not take into account mutation in determining population genetics
Students do not take into account migration in determining population genetics
Students believe selection only kills off weaker individuals
Students believe “fitness” is an absolute set of characteristics
INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE
It will be necessary to review manipulation of fractions and other basic algebraic skills so that
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students can work with and understand the mathematics associated with the Hardy-Weinberg
equation. Prepare many practice problems if you intend to test heavily on that material. This isn’t
as necessary if your intent is to understand the basic concepts of Hardy-Weinberg without
relying too heavily on the mathematics. Use perfect squares (4, 16, 36, 49,…) and the examples
will come out really nice. Students will catch on to the concept faster. Discuss the rationale
behind nonrandom mating.
Certain physical traits are more “appealing” (i.e., long tails in many tropical species of birds) as
are certain behaviors. Red bellies in the male stickleback fish are another example. One could
extrapolate on this idea in terms of human appearance, body style, and dress and how it has
varied over time and with society and social status.
Genetic drift is perhaps the most difficult factor affecting equilibrium to understand. Discuss the
rapid radiation of life forms in newly opened territories and that it occurred after mass
extinctions as evidenced by the fossil record. Pass some of those fossil types around or provide
images for students to see.
HIGHER LEVEL ASSESSMENT
Higher level assessment measures a student’s ability to use terms and concepts learned from the
lecture and the textbook. A complete understanding of biology content provides students with the
tools to synthesize new hypotheses and knowledge using the facts they have learned. The
following table provides examples of assessing a student’s ability to apply, analyze, synthesize,
and evaluate information from Chapter 20.
Application
Analysis
Synthesis
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Have students calculate allelic frequencies using the Hardy-Weinberg
equation.
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Have students provide examples that refute Lamarck’s view of population
genetics.
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Ask students to explain why certain alleles are uncommon in a population
of organisms.
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Have students explain how meiosis is related to gene distribution in a
population of organisms.
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Ask students to assess the role of genetic drift in explaining allelic
variations in human populations from different regions of the world.
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Ask students to hypothesize about the environmental conditions necessary
for the population dynamics of an organism to obey the Hardy-Weinberg
equation.
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Ask students how changes in mating preferences of a certain group of
animals can affect the population genetics of the organism.
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Evaluation
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Ask students to explain what must be done in agriculture to insure little or
know genetic drift in the population of crops or livestock.
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Ask students describe how the population dynamics of an organism would
be affected if its inheritance followed Lamarck’s principles.
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Ask students to evaluate the effects on the human population of increased
mutation rates due to increased exposure to radioactivity associated with
building materials using in homes and other structures.
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Ask students to evaluate the accuracy and credibility of the population
genetics principles used by supporters of an ideology called human
eugenics.
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Ask student to evaluate the pros and cons of monoculture on the
population dynamics on crop plants.
VISUAL RESOURCES
Bring a copy of “Just So Stories” by Rudyard Kipling to demonstrate the idea of acquired
characteristics. Show a close up of an actual red blood cell in its normal shape and one that is in
the sickle shape. This really causes the students to appreciate the difference. Photographs or
actual samples of gels showing various polymorphisms are helpful to explain the concept. Photos
or drawings of various morphological polymorphisms are additionally valuable.
IN-CLASS CONCEPTUAL DEMONSTRATIONS
A. Virtual Hardy-Weinberg Demonstration
Introduction
Animated models are effective tools for visualizing mathmatical concepts such as the
Hardy-Weinberg equation. This demonstration provides a simple to following lecture
supplement for depicting population dynamics.
Materials
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Computer with live access to Internet
LCD projector attached to computer
Web browser with bookmark for Oklahoma State University Hardy-Weinberg Page at
http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf
o Set animation so manual play by pressing the right mouse control and turning off
the “Loop” and “Play” functions of the Flash animation.
o
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Procedure & Inquiry
1. Review the principles of the Hardy-Weinberg equation.
2. Tell students they will be viewing an animation showing how different variables affect
the distribution of genes in a population
3. Start animation by pressing the right mouse control and setting animation to “Rewind”
and then “Play”.
4. Stop the animation by pressing the right mouse control and setting animation so “Play” is
off.
5. Run the animation series and have students explain the results of each factor that affects
the outcomes of gene distribution in a population.
6. Have the class answer questions related to the limitations of the Hardy-Weinberg
equation.
B. Chapter Relationship Concept Map
Introduction
This fun and fast way to build a concept map engages students in developing a scheme
for reviewing all the facts and concepts associated with DNA replication. It helps student select
relevant information needed to understand DNA replication. In addition, it helps them
incorporate concepts learned in other sections of the book that contribute to an understanding of
DNA replication. The simple click and drag animated concept mapping tool should be practiced
before using in class.
Materials
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Computer with live access to Internet
LCD projector attached to computer
Web browser with bookmark to Michigan State University C-Tool:
http://ctools.msu.edu/ctools/index.html
Chapter outline of book for Chapters 1 through 19 projected on overhead
Procedure & Inquiry
1. Tell students that you would like to do a quick assessment of how population genetics is
related to the information covered in the prior chapters..
2. Then go to the Michigan State University C-Tool and add the concept map term
“Population Genetics”.
3. Use the “Add” and “Concept Word” feature to place a term on the map background
related to a concept in one of the prior chapters.
4. Solicit a few more terms or concepts from prior chapters and then ask the class how the
concepts are connected to population genetics. Use the “Add” and “Linking Line” feature
to build a connecting line.
5. Then ask the students to justify the concept linking lines. Use the “Add” and “Linking
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Word” feature to place student comments on the map.
6. Continue the activity until you feel the students made a comprehensive map.
USEFUL INTERNET RESOURCES
1. Contemporary and relevant applications of population genetics information is important
for maintaining student interest in Chapter 20. The Plant and Soil Sciences eLibrary
provided by the University of Nebraska at Lincoln has a valuable website for
downloading lessons on contemporary genetics principles related to agriculture. It can be
used to supplement a lesson based on Chapter 20. The website search feature can be
found at http://plantandsoil.unl.edu/croptechnology2005/gen/.
2. San Diego State University provides a value website about the role of Chaos Theory in
explaining population dynamics. It is an interesting side topic to spur student inquiry into
population dynamics. The website can be found at
http://www.banyantree.org/jsale/chaos/index.html.
3. Faculty who use Mathcad software in their teaching can download software add-ons that
permit students to do mathematical calculations of genetic drift and the Hardy-Weinberg
equation. This site is available at http://www.stanford.edu/group/Urchin/ani-plus.htm.
4. Cases studies are an effective tool for reinforcing the concepts related to population
dynamics. The University of Buffalo provides a teaching case study called “As the Worm
Turns: Speciation and the Apple Maggot Fly”. It encourages student use population
genetics data to investigate evidence of speciation. The case study can be found at
http://www.sciencecases.org/maggot_fly/maggot_fly.pdf
LABORATORY IDEAS
This virtual laboratory session provided by the University of Connecticut permits
students to control the variables associated with the population genetics concept genetic drift. It
engages students in an inquiry activity that provides graphical displays of their hypotheses.
a. Students should be provided with the following materials to perform open-ended
experiments on plant development.
a. Computer with Internet access
b. Web browser linked to http://darwin.eeb.uconn.edu/simulations/drift.html.
b. Tell the students that they will be asked to predict population dynamics of two alleles by
adjusting the frequency of p.
c. Have them to interpret the graphs and evaluate the change of p for different sized
populations.
d. Ask the students to explain the graphical results obtained for a particular population and
to compare the frequency of p for different population sizes.
LEARNING THROUGH SERVICE
Service learning is a strategy of teaching, learning and reflective assessment that merges the
academic curriculum with meaningful community service. As a teaching methodology, it falls
under the category of experiential education. It is a way students can carry out volunteer projects
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in the community for public agencies, nonprofit agencies, civic groups, charitable organizations,
and governmental organizations. It encourages critical thinking and reinforces many of the
concepts learned in a course.
1. Have students design population dynamics flash cards for use in high school biology
classes.
2. Have students tutor high school students covering population genetics in a biology class.
3. Have students do a short PowerPoint presentation on the Hardy-Weinberg equation for
high school teachers.
4. Have students collect an up to date list of references on population genetics for the
college library or a biology department website.
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