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Notes to Instructors
Chapter 23 The Evolution of Populations
What is the focus of these activities?
Both genetics and ecology (more specifically, natural selection) play roles at many levels
in the continued evolution of life. Chapter 23 presents these ideas:
1. Genetic mutation in nature occurs randomly. Mutations in genes can produce
changes in
2. Change in environmental conditions can affect an organism’s ability to survive
and reproduce; that is, it can affect natural selection.
• Organisms with mutations that decrease their ability to survive and/or reproduce
will produce fewer offspring. As a result, the relative frequencies of these genes
Students need to understand, however, that the environment does not directly select for
and increase the number of advantageous phenotypes or organisms (and their genes).
It is more appropriate to think of alterations in gene frequency occurring because of the
What are the particular activities designed to do?
Activity 23.1 A Quick Review of Hardy-Weinberg Population Genetics
This activity is designed to help students understand the logic behind the Hardy-Weinberg
Notes to Instructors 159
Activity 23.2 What effects can selection have on populations?
This activity is designed to help students understand how natural selection can lead to the
What misconceptions or difficulties can these activities reveal?
Activity 23.1
Most of the difficulties students have solving Hardy-Weinberg problems result from a
lack of understanding of basic probability.
Question 4, Part A: Some students are concerned about which allele is given the
frequency pand which is given the frequency q. It helps to tell them that pis usually the
For question 4e, many students know what the binomial expansion is. They may even
recognize that (p2+ 2pq + q2= 1) is the binomial expansion of (p+ q= 1). However,
many will not understand what this mathematical reality has to do with random mating.
Using the Punnett square raises another difficulty: how to determine what pand qare for
the males alone and the females alone in a population. Pose the following problem in
class: “Assume the frequency of the ballele in the population is 10% and the frequency of
the Ballele is 90%. If females make up half of the population, what is the frequency of
the ballele in the female population alone?” A large percentage of the class will answer
Question 6, Part B: Many students don’t understand why one can randomly assign only
two alleles to each diploid organism in a population. For example, assume we want to
know the allele frequency in an isolated subpopulation of birds. The population contains
only six individuals: one of genotype TT, two of genotype Tt, and three of genotype tt.
160 Notes to Instructors
Activity 23.2
To answer any of these questions, students have to make some assumptions. In particular,
they have to ask themselves whether or not the characteristic under selection could be
heritable. If the characteristic is heritable and selection removes some types more often
than others, then the population is likely to change over time.
Note: For scenario I, some students may answer that no selection is going on in this situation.
When asked to explain, they may say, “There is no evidence of selection against any of the
mice.” Given the information provided, this is true. However, these students need a nudge to
recognize that all living organisms, plants included, can be affected by selection.
After working through several of the scenarios, some students may get the idea that answering
these types of questions can be done by formula: First assume that the characteristic being
affected is heritable, and then ask what would happen to the population over time if many
organisms with a particular characteristic were removed. This strategy works well only if the
Notes to Instructors 161
characteristics could be heritable. In some cases (for example, scenario IV), however, it is not
reasonable to assume the characteristic is heritable. That it is not heritable usually becomes
Answers
Activity 23.1 A Quick Review of Hardy-Weinberg Population
Genetics
Part A. Review Chapter 23 of Campbell Biology, 9th edition. Then complete the
discussion by filling in the missing information.
If evolution can be defined as a change in gene (or more appropriately, allele)
frequencies, is it conversely true that a population not undergoing evolution should
maintain a stable gene frequency from generation to generation? This was the question
that Hardy and Weinberg answered independently.
1. Definitions. Complete these definitions or ideas that are central to understanding
the Hardy-Weinberg theorem.
a. Population: An interbreeding group of individuals of the same species.
2. The Hardy-Weinberg theorem. The Hardy-Weinberg theorem states that in a
3. Assumptions. The assumptions required for the theorem to be true are listed on
page 472 of Campbell Biology, 9th edition, and are presented here in shortened form.
a. The population is very large.
4. The Hardy-Weinberg proof. Consider a gene that has only two alleles, R
(dominant) and r(recessive). The sum total of all Rplus all ralleles equals all the
alleles at this gene locus or 100% of all the alleles for that gene.
Let p= the percentage or probability of all the Ralleles in the population
Let q= the percentage or probability of all the ralleles in the population
If all R+ all ralleles = 100% of all the alleles, then
a. What percentage of the gametes in the females (alone) carry the Ballele? 50%
d. What percentage of the gametes in the males carry the ballele? 50%
e. Given the preceding case and all the Hardy-Weinberg assumptions, calculate the
probabilities of the three possible genotypes (RR, Rr, and rr) occurring in all
possible combinations of eggs and sperm for the population.
Because the offspring types represent all possible genotypes for this gene, it follows that
p2+ 2pq + q2= 1 or 100% of all genotypes for this gene
Activity 23.1 163
Female gametes and probabilities
r (q)R (p)
Part B. Use your understanding of the Hardy-Weinberg proof and theorem to
answer the questions.
1. According to the Hardy-Weinberg theorem, p+ q= 1 and p2+ 2pq + q2= 1. What
does each of these formulas mean, and how are the formulas derived?
p+ q= 1: If you add all the dominant alleles for a gene to all the recessive alleles for
2. Assume a population is in Hardy-Weinberg equilibrium for a given genetic
autosomal trait. What proportion of individuals in the population are heterozygous
for the gene if the frequency of the recessive allele is 1%?
Assume that Dis the dominant allele and dis the recessive allele. Because all the
3. About one child in 2,500 is born with phenylketonuria (an inability to metabolize the
amino acid phenylalanine). This is known to be a recessive autosomal trait.
a. If the population is in Hardy-Weinberg equilibrium for this trait, what is the
frequency of the phenylketonuria allele?
Assume Pis the normal allele and pis the phenylketonuria allele. The frequency
b. What proportion of the population are carriers of the phenylketonuria allele (that
is. what proportion are heterozygous)?
4. In purebred Holstein cattle, about 1 calf in 100 is spotted red rather than black. The
trait is autosomal and red is a recessive to black.
a. What is the frequency of the red alleles in the population?
b. What is the frequency of black homozygous cattle in the population?
164 Activity 23.1
c. What is the frequency of black heterozygous cattle in the population?
5. Assume that the probability of a sex-linked gene for color blindness is 0.09 = qand
the probability of the normal allele is 0.91 = p. This means that the probability of X
chromosomes carrying the color blindness allele is 0.09 and the probability of X
chromosomes carrying the normal allele is 0.91.
a. What is the probability of having a color-blind male in the population?
Remember, males have only one X chromosome. Color blindness is a sex-linked
b. What is the probability of a color-blind female?
Unlike males, females have two copies of the X chromosome. As a result, they
6. The ear tuft allele in chickens is autosomal and produces feathered skin projections
near the ear on each side of the head. This gene is dominant and is lethal in the
homozygous state. In other words, homozygous dominant embryos do not hatch
from the egg. Assume that in a population of 6,000 chickens, 2,000 have no ear tufts
and 4,000 have ear tufts. What are the frequencies of the normal versus ear tuft
alleles in this population?
This gene is lethal in the homozygous condition. Therefore, homozygotes that are
produced do not hatch and do not appear in the population. The population contains
Activity 23.1 165
If these are in Hardy-Weinberg equilibrium, we would expect the following
7. How can one determine whether or not a population is in Hardy-Weinberg
equilibrium? What factors need to be considered?
To determine whether a population is in Hardy-Weinberg equilibrium, you need to
be able to calculate the numbers of individuals in the population that are
8. Is it possible for a population’s genotype frequencies to change from one generation
to the next but for its gene (allele) frequencies to remain constant? Explain by
providing an example.
There are a number of ways that this is possible. Here is one example of how it
could occur: Assume inbreeding has occurred in two populations of mice. In one
166 Activity 23.1
In each of the following scenarios, choose which assumption of the Hardy-Weinberg Law
is being violated.
1. In a particular region of the coast, limpets (a type of mollusc) live on near shore
habitats that are uniformly made up of brown sandstone rock. The principle
predators of these limpets are shorebirds. The limpets occur in two morphs, one with
a light-colored shell and one with a dark-colored shell. The shorebirds hunt by sight
and are able to see the light ones on the dark sandstone easier than the dark ones.
2. In Chen caerulescens (a species of goose), the white body form, the snow goose
and the blue body form, the blue goose, occasionally coexist. In these areas of
contact, white-by-white and blue-by-blue matings are much more common than
white-by-blue matings.
3. Prior to the Mongolian invasions which occurred between the 6th and 16th centuries,
the frequency of blood type B across Europe was close to zero. The frequency of
blood type B among the Mongols was relatively high. Today, it is possible to see
fairly high frequencies of blood type B in the Eastern European countries and a
gradual decrease in the frequency of blood type B as one moves from the Eastern
European countries to the Western European countries, such as France and England.
No migration—Mongolian invasions were more common in Eastern European
Activity 23.1 167
23.1 Test Your Understanding
Activity 23.2 What effects can selection have on populations?
1. What effects can natural selection have on populations? For example, what types of
selection can occur in a population, and how does each affect a population?
Selection can be directional, stabilizing, or disruptive. If selection removes
2. Examine the scenarios on the following pages. For each scenario:
a. Decide whether or not natural selection is operating. In doing this, indicate
whether there is variability in the population(s).
If no, what does this imply about evolution?
If yes, what is the nature of the variation? For example, what characteristics
must the variation have for selection to operate on it?
b. Is there any indication that members of the population(s) differ in fitness?
If no, what does this imply about the operation of natural selection?
If yes, describe the difference in fitness.
c. Given your answers to parts a and b, what trend should characterize the future
behavior or composition of the population(s)?
Be sure to indicate any assumptions you make in answering the questions.
The key to analyzing these scenarios is to ask first what is being selected against
Scenario I. A particular species of mouse feeds on the seeds of a single species of
cherry tree. When the mice eat a seed, they digest it completely. The mice choose
seeds of intermediate and large sizes, leaving the very small seeds of the cherry
tree uneaten.
168 Activity 23.2
a. Selection is occurring. The variation is in cherry seed size. For selection to be
operating, the assumption is that seed size is a heritable trait.
Scenario IIa. Small island A contains three separate populations of a single species
of cherry tree. The seed size varies between trees. That is, some trees produce seeds
that are all in the small size ranges, others produce seeds all in the middle size
ranges, and others produce seeds in the large size ranges. A small population of mice
is introduced to the island. The mice eat cherries and are the only predators on the
cherry trees. When the mice eat a cherry, they completely digest it and the pit or seed
inside it. The mice choose medium and large seeds and leave the smallest seeds
uneaten.
a. Selection is occurring. The variation is in cherry seed size. For selection to be
operating, the assumption is that seed size is a heritable trait.
Scenario IIb. Would your answer for Scenario IIa change given the following
information? Explain. As you continue to study the populations of trees, you note
that the viability of the seeds varies with size such that the viability of the small seeds
is less than that of the middle-sized seeds, which is less than that of the largest seeds.
a. Again, selection is occurring. The variation is in cherry seed size. For selection to
be operating, the assumption is that seed size is a heritable trait.
Activity 23.2 169
Scenario IIIa. Small island B contains three separate populations of a single
species of cherry tree. Unlike the species on island A, in this species seed size varies
within trees. That is, each tree produces seeds that range in size from large to small.
A small population of mice is introduced to the island. These are the only predators
on the cherry trees. When the mice eat a cherry, they digest it and the pit or seed
completely. The mice choose medium and large seeds and leave the smallest seeds
uneaten.
a. Again, selection could be occurring if you assume that seed size is a heritable
trait. If each flower on a tree is independently pollinated to produce its seeds,
Scenario IIIb. Would your answer for Scenario IIIa change given the following
information? Explain. As you continue to study these populations, you note that
the viability of the seeds does not vary with size. Over time, however, you find that
the trees that grow from the smallest seeds produce fewer large seeds.
a. This information provides additional evidence that seed size is heritable and not
the result of growth conditions. As a result, selection is occurring.
Scenario IV. After a severe spring ice storm, about half of the finches (small birds)
in a population are found dead. Examination of the dead birds indicates that they
vary in age from young to old. About 60% of the dead are new fledglings (just left
the nest); about 20% are over 3 years of age (old for this species).
170 Activity 23.2
a. I’d argue that this is not selection. The vast majority of the dead birds are the
very young (just fledged and without parental support) and the very old.
The figure below shows the distribution of a phenotypic characteristic in a population of
organisms. Also shown are three portions (A, B, and C) of that distribution.
Use the following set of answers
A. Portion A only
B. Portion B only
C. Portion C only
D. Portions A and C simultaneously
E. Portions B and C simultaneously
Activity 23.2 171
23.2 Test Your Understanding
AB C
1. Male sticklebacks with bright red coloring are favored by female stickleback as
mates. However, the bright red color makes the males more likely to be seen by
predators.
D. In this example, the bright red color is both selected against (by predators)
2. On Island Z in the Galápagos the plant population contains only two species. One of
the two plant species produces very large seeds and the other produces small seeds.
A species of seed-eating finch has lived on the island for many years. This established
species has large beaks and prefers large seeds. A small population of a different species
of seed eating finch has migrated to the island. The beak size in the new species varies
over a continuum from relatively small to large. How will evolution of the new finch
species be affected?
C. The established species with large beaks are likely to be better competitors for the
3. In a species of plant-eating land snail, the shell color is variable and ranges from
very light to intermediate to very dark in color. The snails are preyed upon by birds
that use sight to find their prey. A small population of these snails is moved to an
island where the food plants in their preferred habitat are either very light in color or
very dark.
B. In this scenario the landscape is either light or dark. As a result, light snails would
172 Activity 23.2
