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Chapter 22
Descent with Modification: A
Darwinian View of Life
Teaching Objectives
The Historical Context for Evolutionary Theory
1. Explain the mechanism for evolutionary change proposed by
Charles Darwin in On the Origin of Species.
2. Define evolution and adaptation.
3. Compare and contrast Aristotle’s scala naturae to Carolus
Linnaeus’ classification scheme.
4. Describe the theories of catastrophism, gradualism, and
uniformitarianism.
5. Explain the mechanism for evolutionary change proposed by
Jean-Baptiste de Lamarck. Explain why modern biology has
rejected Lamarck’s theories.
The Darwinian Revolution
6. Describe how Darwin’s observations on the voyage of the
HMS Beagle led him to formulate and support his theory of
evolution.
7. Explain how the principle of gradualism and Charles Lyell’s
theory of uniformitarianism influenced Darwin’s ideas about
evolution.
8. Explain what Darwin meant by “descent with modification.”
9. Explain what evidence convinced Darwin that species
change over time.
10. Explain how Linnaeus’ classification scheme fit Darwin’s
theory of evolution by natural selection.
11. Describe the three inferences Darwin made from his
observations that led him to propose natural selection as a
mechanism for evolutionary change.
12. Explain how an essay by the Rev. Thomas Malthus
influenced Charles Darwin.
13. Distinguish between artificial selection and natural selection.
14. Explain why an individual organism cannot evolve.
15. Describe the experiments that supported Reznick and
Endler’s hypothesis that differences in life-history traits
between guppy populations are due to selective pressure
based on predation.
16. Explain how the existence of homologous and vestigial
structures can be explained by Darwin’s theory of natural
selection.
17. Explain how evidence from biogeography supports the
theory of evolution by natural selection.
18. Explain the problem with the statement that Darwinism is
“just a theory.” Distinguish between the scientific and
colloquial use of the word theory.
Student Misconceptions
1. Many first-year students misunderstand the vitally important
theory of evolution by natural selection. One problem is that
many of the biological terms associated with evolution have
familiar, everyday meanings that are different from their
strict biological definitions. The following terms may be
problematic:
 Fitness. When students think of fitness, they usually think
of an organism’s general health, vigor, strength, or
intelligence. As a result, they may find it hard to
appreciate that any trait that increases an organism’s
relative reproductive success increases its fitness.
 Adaptation. In everyday use, adaptation refers to an
individual changing over its lifetime in response to the
environment. Students may confuse the colloquial and
scientific meanings of this term and arrive at the mistaken
notion that changes (“adaptations”) over individual
lifetimes accumulate to bring about evolutionary change
in populations.
 Theory. Students may tell you, “Evolution is only a theory,
not a fact.” In common usage, the term theory means a
tentative explanation. In a scientific context, a theory is a
useful, comprehensive, and well-supported explanation
for a wide range of observations.
 Evolution. In its strict biological meaning, evolution is
defined as a change in allele frequencies in a population
over time. By this definition, no one can challenge the
“fact” of evolution. Of course, evolution is also used in its
broader sense of macroevolutionary change. Students
may confuse these two meanings.
2. Many students think that evolution results from a purposeful
striving for progress and complexity that results in the
appearance of more advanced, “higher” life forms. Adaptive
processes are misunderstood as purposive and goal-driven.
3. Many students do not appreciate that two separate and
distinct processes are necessary for new traits to arise and
flourish in populations. New traits appear because of
random mutation and sexual recombination. A novel trait
that increases the fitness of individuals in their environment
will persist in the population and increase in frequency over
time. Many students think that a single process leads to the
appearance and survival of new adaptive traits. These
students think of this process in Lamarckian rather than
Darwinian terms. If carefully questioned, they will express
Lamarckian ideas of need, use and disuse, and inheritance
of acquired characteristics. Such students think that
individual organisms develop traits that allow them to survive
and reproduce in their environment, change over their
lifetimes in response to environmental pressure, and pass
on acquired changes to their offspring.
4. Although students will state that mutations are rare and
random events, careful questioning will show that some
students think of mutations as adaptive responses to
environmental conditions. In discussion, such students will
provide explanations that suggest that mutations are in
some sense intentional: that an organism mutates in order
to adapt to its environment.
5. Few students are able to generalize their understanding of
homology beyond familiar examples such as vertebrate
forelimbs. To test this, ask students to give plant examples
of homologous structures. For a student who understands
the concept of homology, this is very easy. Students who
have learned the text examples, but not the concept, will be
unable to come up with suitable answers.
6. Heritable variation in a population is an essential condition
for evolutionary change. Many students do not fully
understand this and do not realize that variation is important
to evolution. They discount variation within populations and
think of populations as consisting of equivalent or identical
individuals.
7. Many students do not appreciate that natural selection acts
on populations consisting of variant individuals. They think
of natural selection as a process that acts on and gradually
changes species as a whole. Such students do not realize
that evolutionary change comes about as the proportion of
individuals in the population displaying a particular trait
increases from generation to generation. Instead, they think
that the trait changes gradually in all members of the
population. For example, if directional selection favors
increased antibiotic resistance in a population of bacteria,
these students will think that bacteria become more
resistant, rather than recognizing that more bacteria become
resistant.
8. How can instructors address the many misconceptions that
students may have about evolution by natural selection?
 Historical review can be very fruitful. Many of your
students’ mistaken notions match early evolutionary
theories that are now discredited. Provide examples of
Lamarckian explanations that may seem superficially
reasonable to your students and discuss why these
explanations are in error.
 Provide opportunities for your students to discuss their
ideas about evolution. Foster these discussions by asking
challenging questions that require students to reason and
to apply evolutionary principles. As students elaborate
and explain their ideas, challenge and correct incorrect
views.
 Be careful in your use of language. Avoid figurative
language that inappropriately attributes agency to
individuals or the environment, recognizing that these
explanations mislead students.
 Require your students to read and discuss Jonathan
Weiner’s The Beak of the Finch. Students who read this
excellent book will gain a much deeper understanding of
evolution by natural selection.
Further Reading
Lord, T., and S. Marino. 1993. How university students view the
theory of evolution. Journal of College Science Teaching,
22(6), 353–357.
Moore, R. 1997. The persuasive Mr. Darwin. Bioscience, 47(2), 107–115.
Weiner, J. 1994. The Beak of the Finch: A Story of Evolution in Our Time. New
York: Random House.
Chapter Guide to Teaching Resources
Overview: Darwin introduces a revolutionary theory
Concept 22.1 The Darwinian revolution challenged traditional views of a
young Earth inhabited by unchanging species
Transparency
Figure 22.2 The historical context of Darwin’s life and ideas
Instructor and Student Media Resource
Video: Grand Canyon
Concept 22.2 In The Origin of Species, Darwin proposed
change through natural selection
that species
Transparencies
Figure 22.5 The voyage of HMS Beagle
Figure 22.7 Descent with modification
Figure 22.10 Artificial selection
Instructor and Student Media Resources
Activity: Darwin and the Galápagos Islands
Video: Galápagos Islands overview
Video: Galápagos marine iguana
Video: Galápagos sea lion
Video: Galápagos tortoise
Video: Soaring hawk
Activity: The voyage of the Beagle: Darwin’s trip around the
world
Biology Labs On-Line: EvolutionLab
Video: Snake ritual wrestling
Video: Albatross courtship ritual
Video: Blue-footed boobies’ courtship ritual
Video: Sea horses
Concept 22.3 Darwin’s theory explains a wide range of observations
Transparencies
Figure 22.12 Can predation pressure select for size and age at
maturity in guppies?
Figure 22.13 Evolution of drug resistance in HIV
Figure 22.14 Mammalian forelimbs: homologous structures
Figure 22.15 Anatomical similarities in vertebrate embryos
Figure 22.16 Comparison of a protein found in diverse
vertebrates
Figure 22.17 Different geographic regions, different mammalian
“brands”
Figure 22.18 A transitional fossil linking past and present
Student Media Resources
Investigation: How do environmental changes affect a
population?
Investigation: What are the patterns of antibiotic resistance?
Activity: Reconstructing forelimbs
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
artificial selection
biogeography
catastrophism
descent with modification
endemic
evolution
evolutionary adaptation
fossil
gradualism
homologous structures
homology
natural selection
paleontology
sedimentary rock
taxonomy
uniformitarianism
vestigial organ
Word Roots
bio- 5 life; geo- the Earth (biogeography: the study of the past
and present distribution of species)
end- 5 within (endemic: a type of species that is found only in
one region and nowhere else in the world.)
homo- 5 like, resembling (homology: similarity in characteristics
resulting from a shared ancestry)
paleo- 5 ancient (paleontology: the scientific study of fossils)
taxo- 5 arrange (taxonomy: the branch of biology concerned
with naming and classifying the diverse forms of life)
vestigi- 5 trace (vestigial organs: structures of marginal, if any,
importance to an organism; they are historical remnants of
structures that had important functions in
ancestors) Instructor’s Guide for Campbell/Reece Biology, Seventh
EditionChapter 22 Descent with Modification: A Darwinian View of
Life
Instructor’s Guide for Campbell/Reece Biology, Seventh EditionChapter
22 Descent with Modification: A Darwinian View of Life
Campbell/Reece Biology, Seventh Edition
Instructor’s Guide for