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148 Notes to Instructors
Notes to Instructors
Chapter 22 Descent with Modification: A Darwinian View of Life
What is the focus of these activities?
Campbell Biology, 9th edition, defines natural selection as “differential success in the
reproduction of different phenotypes resulting from the interaction of organisms with their
How can we change students’ misconceptions? It would be easy if all we had to do was
tell them what the current evidence indicates. However, the vast majority of students
don’t learn new concepts and ideas simply by hearing about them in lecture. To learn new
concepts and modify their existing ideas, students have to work with the evidence, test it,
and prove to themselves that their new understanding works and has value. For this to
happen, we need to address these questions:
• Where do student misconceptions and problems about evolution originate?
What are the particular activities designed to do?
Activity 22.1 How did Darwin view evolution via natural selection?
Activity 22.2 How do Darwin’s and Lamarck’s ideas about evolution differ?
Activity 22.3 How would you evaluate these explanations of Darwin’s ideas?
These activities are designed to help students understand the logic Darwin used to
What misconceptions or difficulties can these activities reveal?
Activity 22.1
Activity 22.2
Activity 22.3
It should come as no surprise that some students’ misconceptions come from our own
teaching and lectures, textbooks, and popular videos and movies dealing with evolution.
We also have to be aware that certain key words used in explaining evolution may have
very different meanings to our students. For example, the words adaptation, fitness,
primitive, and advanced have very different meanings in common use compared to their
meanings in the study of evolution. When I use the terms primitive and advanced, I make
sure to let my students know that primitive is what came first and advanced is what came
later; that is, in evolution, these are chronological terms, and no value is implied. For
example, the chairs you’re sitting in are advanced; the Louis the 14th chairs in museums
are primitive. Which are better? In some environments, the more primitive have the
advantage (are better), and in other situations, the more advanced have the advantage.
To uncover students’ misconceptions and alter them, we need to find out what each
individual student’s understanding is. We also have to recognize that simply presenting students
with the evidence one more time will not necessarily alter their existing understanding. For
students to change their ideas, we need to provide them with the basic concepts and with novel
examples, exercises, and problems that challenge their existing conceptions.
The activities for Chapter 22 reveal some common misconceptions:
• Many students have a Lamarkian view that organisms are modified by their
environment.
• Some think that evolution and natural selection lead directly to speciation.
Notes to Instructors 149
Given these misconceptions, when we teach the basics of evolution and natural selection,
we need to make sure that we include the following ideas:
1. Mutation is random. Almost all mutations that could have occurred in the past
2. Variability exists in populations, and only the variability that is heritable can be
acted upon by natural selection. This variability includes some genes that are
currently advantageous, some that are disadvantageous, and some that are neither;
that is, some genes are neutral. Because students often think that a gene can be only
one of these, I provide the following examples:
• Currently eye color in humans is a neutral mutation. It is neither advantageous
nor disadvantageous to survival and reproduction.
Environmental conditions can and did change over the course of life’s history on
Earth.This means that some genes that were advantageous could have become
disadvantageous, and vice versa. It also means that some genes that were previously
neutral could have become either disadvantageous or advantageous. Here are two
examples:
150 Notes to Instructors
• To use a real example, consider the frizzle chicken. Normal chickens sent to the
tropics to be used in agriculture don’t survive well in the heat. Frizzle chickens
do very well, however. Ask your students, “So, is the frizzle gene advantageous
or disadvantageous?”
3. It is not that advantageous genes are selected for, so much as that disadvantageous
genes are selected against. For example, if you have a mutation that prevents
4. Note that these activities can be used to probe student understanding. They require
students to recognize that they may need to modify their ideas and hypotheses based
on differences in the evidence that is available at any one time.
Answers
Activity 22.1 How did Darwin view evolution via natural
selection?
Darwin is remembered not because he was the first to propose that evolution occurs.
Many others had presented this idea before. Instead, he is remembered for defining the
Working alone or in groups of three or four, construct a concept map of Darwin’s view
of evolution via natural selection. Be sure to include definitions or descriptions of all
the terms in the list below. Keep in mind that there are many ways to construct a
concept map.
• Begin by writing each term on a separate sticky note or piece of paper.
Here is an example:
Activity 22.1 151
Terms
fact Darwin fit individuals
biogeography vertebrate limb structure fossil record
gradualism species population embryology
Use the understanding you gained from creating the concept map to answer the
questions.
1. In the 1860s, what types of evidence were available to indicate that evolution had
occurred on Earth?
Evidence was available from studies of taxonomy (classification of organisms based
2. How did knowledge of mechanisms of artificial selection (used in developing
various strains of domesticated animals and plants) help Darwin understand how
evolution could occur?
Darwin knew that artificial selection could lead to dramatic changes in the
3. Based on his studies, Darwin made a number of observations; they are listed in the
chart. Complete the chart by answering how Darwin made the observations.
152 Activity 22.1
4. Based on these observations, Darwin made a number of inferences. Which of the
observation(s) in question 3 allowed Darwin to make each inference?
Inference Observations that led to the inference
a. Production of more individuals
than the environment can support
leads to a struggle for existence
such that only a fraction of the
offspring survive each generation.
Darwin combined his understanding of the first three
observations just noted to make this inference.
Activity 22.1 153
Observation How did Darwin make this observation? That is, what did
he read or observe that gave him this understanding?
a. All species populations
have the reproduction
Darwin read Malthus’s essay on the potential for human populations
to grow at a rate far beyond the capacity of their food supply and
5. Based on these observations and inferences, how did Darwin define fitness?
Campbell Biology, 9th edition, defines fitness as “the relative contribution an individual
6. How did Darwin define evolution?
Darwin preferred to talk about descent with modification, or the idea that all
7. What is the unit of natural selection—that is, what is selected? What is the unit of
evolution—that is, what evolves?
The individual organism is the unit that is selected because it is the individual organ-
8. In a population of mice, some individuals have brown fur and some have black fur. At
present, both phenotypes are equally fit. What could happen to change the relative fitness
of the two phenotypes in the population? For example, what could cause individuals with
brown fur to show reduced fitness relative to individuals with black fur?
There are different possible ways of answering this question. Here is one: If both
populations are equally fit at present, we can assume that neither is more or less
9. Assume you discover a new world on another planet that is full of organisms.
a. What characteristics would you look for to determine that these organisms arose
as a result of evolutionary processes?
Again, this question has many possible answers. One approach is to look for the
154 Activity 22.1
b. What characteristics would you look for to determine that these organisms did
not arise as the result of evolutionary processes?
You would look for evidence that indicated there was no genetic relationship
10. Why is it incorrect to say: Vertebrates evolved eyes in order to see?
Natural selection occurs in the present. Organisms that survive must have traits that
Activity 22.2 How do Darwin’s and Lamarck’s ideas about
evolution differ?
Early in the 1800s Lamarck proposed a theory of evolution. He suggested that traits
acquired during an organism’s life—for example, larger muscles—could be passed on to
its offspring. The idea of inheritance of acquired characteristics was popular for many
years. No such mechanism is implied in Darwin’s theory of evolution via natural
selection, however. After Darwin published his work, scientists conducted many
experiments to disprove the inheritance of acquired traits. By the middle of the 20th
century, enough data had accumulated to make even its most adamant supporters give up
the idea of inheritance of acquired characteristics.
Given your understanding of both Lamarck’s and Darwin’s ideas about evolution,
determine whether the statements on the next page are more Lamarckian or more
Darwinian. If the statement is Lamarckian, change it to make it Darwinian. Here are two
example statements and answers.
Examples
A. The widespread use of DDT in the mid-1900s put pressure on insect populations
to evolve resistance to DDT. As a result, large populations of insects today are
resistant to DDT.
Answer: This is a Lamarckian statement. DDT worked only against insects that had no
Activity 22.2 155
Suggested change: Wide-scale use of DDT in the mid-1900s selected against insects that
B. According to one theory, the dinosaurs became extinct because they couldn’t
evolve fast enough to deal with climatic changes that affected their food and water
supplies.
Answer: This is a quasi-Lamarckian statement. Organisms do not purposefully evolve.
Suggested change: According to one theory, the dinosaurs became extinct because their
physiological and behavioral characteristics were too specialized to allow them to survive
Statements
1. Many of the bacterial strains that infect humans today are resistant to a wide range
of antibiotics. These resistant strains were not so numerous or common prior to the
use of antibiotics. These strains must have appeared or evolved in response to the
use of the antibiotics.
Answer: This is a quasi-Lamarckian statement. Although the strains evolved in
response to the use of antibiotics (the antibiotics killed off the strains that did not have
2. Life arose in the aquatic environment and later invaded land. Once animals came
onto land, they had to evolve effective methods of support against gravity and
locomotion in order to survive.
156 Activity 22.2
Answer: This is a Lamarckian statement. If the animals were not already able to
3. A given phenotypic trait—for example, height, speed, tooth structure—(and
therefore the genes that determine it) may have positive survival or selective value,
negative survival or selective value, or neutral (neither positive nor negative) survival
or selective value. Which of these it has depends on the environmental conditions the
organism encounters.
Answer: This statement is Darwinian. Each of the variants we see in phenotype has a
4. The children of bodybuilders tend to be much more athletic, on average, than other
children because the characteristics and abilities gained by their parents have been
passed on to the children.
Answer: This is a Lamarckian statement. The parents cannot pass on traits they
Activity 22.3 How would you evaluate these explanations of
Darwin’s ideas?
Unfortunately, even today some people get or give the impression that acquired
characteristics can be inherited. As a result, we need to be very careful about how we
state our understanding of evolution and evolutionary theory.
To test understanding of Darwin’s ideas, this question was included on an exam.
Activity 22.3 157
4-point question:
In two or three sentences describe Darwin’s theory of descent with modification and
the mechanism, natural selection, that he proposed to explain how this comes about.
Four student answers to the question are given. Based on what you know about
Student 1. Darwin saw that populations increased faster than the ability of the land to
support them could increase, so that individuals must struggle for limited resources. He
proposed that individuals with some inborn advantage over others would have a better
chance of surviving and reproducing offspring and so be naturally selected. As time passes,
these advantageous characteristics accumulate and change the species into a new species.
Student 2. Darwin’s theory of evolution explains how new species arise from already
existing ones. In his mechanism of natural selection, organisms with favorable traits tend
to survive and reproduce more successfully, while those that lack the traits do not.
Beneficial traits are passed on to future generations in this manner, and a new species will
be created in the end!
Grade: 3As noted above, Darwin did not explain how new species arise from already
Student 3. Descent with modification using natural selection was Darwin’s attempt at
explaining evolution. An organism is modified by its surroundings, activities, and lifestyle.
These modifications, by natural selection, make the organism better suited to its life.
Grade: 0In the first sentence, the student restates the information available in the
Student 4. Darwin’s theory states that organisms can become modified by environmental
conditions or use or disuse features and that the modifications can be passed down to
succeeding generations. He proposes that nature selects for a characteristic trait that is
beneficial to the survival of the organisms and that organisms would pass on this trait.
Grade: 0The first sentence of this answer indicates that this student has a Lamarckian
158 Activity 22.3
