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Chapter 23
The Evolution of Populations
Teaching Objectives
Population Genetics
1. Explain the statement “It is the population, not the individual,
that evolves.”
2. Explain how Mendel’s particulate hypothesis of inheritance
provided much-needed support for Darwin’s theory of
evolution by natural selection.
3. Distinguish between discrete and quantitative traits. Explain
how Mendel’s laws of inheritance apply to quantitative traits.
4. Explain what is meant by “the modern synthesis.”
5. Define the terms population, species, and gene pool.
6. Explain why meiosis and random fertilization alone will not
alter the frequency of alleles or genotypes in a population.
7. List the five conditions that must be met for a population to
remain in Hardy-Weinberg equilibrium.
8. Write the Hardy-Weinberg equation. Use the equation to
calculate allele frequencies when the frequency of
homozygous recessive individuals in a population is 25%.
Mutation and Sexual Recombination
9. Explain why the majority of point mutations are harmless.
10. Explain why mutation has little quantitative effect on allele
frequencies in a large population.
11. Describe the significance of transposons in the generation
of genetic variability.
12. Explain how sexual recombination generates genetic
variability.
Natural Selection, Genetic Drift, and Gene Flow
13. Explain the following statement: “Only natural selection
leads to the adaptation of organisms to their environment.”
14. Explain the role of population size in genetic drift.
15. Distinguish between the bottleneck effect and the founder
effect.
16. Describe how gene flow can act to reduce genetic
differences between adjacent populations.
Genetic Variation, the Substrate for Natural Selection
17. Explain how quantitative and discrete characters contribute
to variation within a population.
18. Distinguish between average heterozygosity and nucleotide
variability. Explain why average heterozygosity tends to be
greater than nucleotide variability.
19. Define a cline.
20. Define relative fitness.
a. Explain why relative fitness is zero for a healthy, longlived, sterile organism.
b. Explain why relative fitness could be high for a short-lived
organism.
21. Distinguish among directional, disruptive, and stabilizing
selection. Give an example of each mode of selection.
22. Explain how diploidy can protect a rare recessive allele from
elimination by natural selection.
23. Describe how heterozygote advantage and frequencydependent selection promote balanced polymorphism.
24. Define neutral variations. Explain why natural selection does
not act on these alleles.
25. Distinguish between intrasexual selection and intersexual
selection.
26. Explain how female preferences for showy male traits may
benefit the female.
27. Describe the disadvantages of sexual reproduction.
28. Explain how the genetic variation promoted by sex may be
advantageous to individuals on a generational time scale.
29. List four reasons why natural selection cannot produce
perfect organisms.
Student Misconceptions
1. Students often misunderstand the significance of individuals
and individual variation to the theory of evolution by natural
selection.
a. In everyday language, adaptation refers to changes in an
individual over its lifetime. Students may think that such
changes are or lead to evolutionary change.
b. Students may mistakenly think that evolutionary change
comes about as traits gradually change in all members of
2.
3.
4.
5.
a population, rather than realizing that individuals with
favorable heritable traits come to make up an increasing
proportion of the population.
Many students find it hard to understand the HardyWeinberg theorem and do not know how and when to use
the Hardy-Weinberg equations. They do not realize that the
Hardy-Weinberg theorem clarifies the factors that alter allele
frequency, and that it does not imply that allele frequencies
are static. These students do not appreciate that the HardyWeinberg equations are used with respect to a particular
gene.
Students can be confused about the role of chance in
evolution and natural selection. New alleles arise by chance
mutations, new combinations of alleles arise by the shuffling
of genes in sexual recombination, and chance events may
alter allele frequencies in small populations. Certainly
chance is important in evolutionary change. However,
students may think that evolution itself proceeds by an
accumulation of changes occurring by chance. Such
students completely misunderstand the role of natural
selection as the mechanism of adaptive evolution. Genetic
variation arises by chance. However, the action of natural
selection to favor variants that survive and reproduce with
relatively high success in their environment is not based on
chance.
Students may think that most selection is directional and not
realize that stabilizing selection is the norm. Organisms are
generally well adapted to their environments. Selection
primarily acts to remove deleterious mutations that alter the
phenotype in ways that reduce fitness.
Students may have great difficulty in understanding the
subtle arguments about the costs and benefits of sex.
Ensure that your students realize that the issue is not
whether sex promotes genetic variation—of course it does—
but rather what short-term advantage this variation confers
on individuals.
Further Reading
Anderson, D. L., K. M. Fisher, and G. J. Norman. 2002.
Development and evaluation of the conceptual inventory of
natural selection. Journal of Research in Science Teaching,
39(10), 952–978.
Bishop, B. A., and C. W. Anderson. 1990. Student conceptions
of natural selection and its role in evolution. Journal of
Research in Science Teaching, 27(5), 415–427.
Chapter Guide to Teaching Resources
Overview: The smallest unit of evolution
Concept 23.1 Population genetics provides a
foundation for studying evolution
Transparencies
Figure 23.3 One species, two populations
Figure 23.4 Mendelian inheritance preserves genetic variation
from one generation to the next
Figure 23.5 The Hardy-Weinberg theorem
Student Media Resources
Investigation: How can frequency of alleles be calculated?
Biology Labs On-Line: PopulationGeneticsLab
Activity: Causes of microevolution
Concept 23.2 Mutation and sexual recombination
produce the variation that makes evolution
possible
Concept 23.3 Natural selection, genetic drift, and
gene flow can alter a population’s genetic
composition
Transparencies
Figure 23.7 Genetic drift
Figure 23.8 The bottleneck effect
Student Media Resources
Biology Labs On-Line: EvolutionLab
Activity: Genetic variation from sexual recombination
Concept 23.4 Natural selection is the primary
mechanism of adaptive evolution
Transparencies
Figure 23.11 Does geographic variation in yarrow plants have a
genetic component?
Figure 23.12 Modes of selection
Figure 23.13 Mapping malaria and the sickle-cell allele
Figure 23.14 Using a virtual population to study the effects of
selection
Figure 23.16 The “reproductive handicap” of sex
For additional resources such as digital images and lecture
outlines, go to the Campbell Media Manager or the Instructor
Resources section of www.campbellbiology.com.
Key Terms
average heterozygosity
balanced polymorphism
balancing selection
bottleneck effect
cline
directional selection
disruptive selection
duplication
fitness
founder effect
frequency-dependent selection
gene flow
gene pool
genetic drift
genetic polymorphism
geographic variation
Hardy-Weinberg equilibrium
Hardy-Weinberg theorem
heterozygote advantage
intersexual selection
intrasexual selection
microevolution
modern synthesis
mutation
neutral variation
phenotypic polymorphism
population
population genetics
pseudogene
relative fitness
sexual dimorphism
sexual selection
stabilizing selection
Word Roots
inter- 5 between (intersexual selection: individuals of one sex
are choosy in selecting their mates from individuals of the
other sex; also called mate choice)
intra- 5 within (intrasexual selection: a direct competition among
individuals of one sex for mates of the opposite sex)
micro- 5 small (microevolution: a change in the gene pool of a
population over a succession of generations)
muta- 5 change (mutation: a change in the DNA of genes that
ultimately creates genetic diversity)
poly- 5 many; morph- 5 form (polymorphism: the coexistence
of two or more distinct forms of individuals in the same
population) Instructor’s Guide for Campbell/Reece Biology, Seventh
EditionChapter 23 The Evolution of Populations
Instructor’s Guide for
Campbell/Reece Biology, Seventh EditionChapter 23 The Evolution of
Populations
Instructor’s Guide for Campbell/Reece Biology, Seventh Edition