14
47) Anopheles mosquitoes, which carry the malaria parasite, cannot live above elevations of 5,900 feet.
In addition, oxygen availability decreases with higher altitude. Consider a hypothetical human
population that is adapted to life on the slopes of Mt. Kilimanjaro in Tanzania, a country in equatorial
Africa. Mt. Kilimanjaro’s base is about 2,600 feet above sea level and its peak is 19,341 feet above sea
level. If the incidence of the sickle-cell allele in the population is plotted against altitude (feet above sea
level), which of the following distributions is most likely, assuming little migration of people up or
down the mountain?
A)
B)
C)
D)
48) If global warming permits mosquitoes to live at higher altitudes than they currently do, then in
which direction should the entire plot in the correct distribution below be shifted?
a. b.
c. d.
A) to the right
B) to the left
C) upward
D) downward
In a very large population, a quantitative trait has the following distribution pattern:
49) What is true of the trait whose frequency distribution in a large population appears in the previous
figure? It has probably undergone
A) directional selection.
B) stabilizing selection.
C) disruptive selection.
D) normal selection.
50) If the curve in the previous figure shifts to the left or to the right, there is no gene flow, and the
population size consequently increases over successive generations. Which of the following is (are)
probably occurring?
1. immigration or emigration
2. directional selection
3. adaptation
4. genetic drift
5. disruptive selection
A) 1 only
B) 4 only
C) 2 and 3
D) 4 and 5
E) 1, 2, and 3
Scenario Questions
The following questions refer to the following paragraph.
HIV’s genome of RNA includes the code for reverse transcriptase (RT), an enzyme that acts early in
infection to synthesize a DNA genome off of an RNA template. The HIV genome also codes for
protease (PR), an enzyme that acts later in infection by cutting long viral polyproteins into smaller,
functional proteins. Both RT and PR represent potential targets for antiretroviral drugs. Drugs called
nucleoside analogs (NA) act against RT, whereas drugs called protease inhibitors (PI) act against PR.
51) Which of the following represents the treatment option most likely to avoid the production of drug-
resistant HIV (assuming no drug interactions or side effects)?
A) using a series of NAs, one at a time, and changed about once a week
B) using a single PI, but slowly increasing the dosage over the course of a week
C) using high doses of NA and a PI at the same time for a period not to exceed one day
D) using moderate doses of NA and two different PIs at the same time for several months
52) Within the body of an HIV-infected individual who is being treated with a single NA, and whose
HIV particles are currently vulnerable to this NA, which of these situations can increase the virus’
relative fitness?
1. mutations resulting in RTs with decreased rates of nucleotide mismatch
2. mutations resulting in RTs with increased rates of nucleotide mismatch
3. mutations resulting in RTs that have proofreading capability
A) 1 only
B) 2 only
C) 3 only
D) 1 and 3
E) 2 and 3
53) HIV has nine genes in its RNA genome. Every HIV particle contains two RNA molecules, and each
molecule contains all nine genes. If, for some reason, the two RNA molecules within a single HIV
particle do not have identical sequences, then which of these terms can be applied due to the existence of
the nonidentical regions?
A) homozygous
B) gene variability
C) nucleotide variability
D) average heterozygosity
E) All but one of the responses are correct.
54) Every HIV particle contains two RNA molecules. If two genes from one RNA molecule become
detached and then, as a unit, get attached to one end of the other RNA molecule within a single HIV
particle, which of these is true?
A) There are now fewer genes within the viral particle.
B) There are now more genes within the viral particle.
C) A point substitution mutation has occurred in the retroviral genome.
D) The retroviral equivalent of crossing over has occurred, no doubt resulting in a heightened positive
effect.
E) One of the RNA molecules has experienced gene duplication as the result of translocation.
55) In a hypothetical population’s gene pool, an autosomal gene, which had previously been fixed,
undergoes a mutation that introduces a new allele, one inherited according to incomplete dominance.
Natural selection then causes stabilizing selection at this locus. Consequently, what should happen over
the course of many generations?
A) The proportions of both types of homozygote should decrease.
B) The proportion of the population that is heterozygous at this locus should remain constant.
C) The population’s average heterozygosity should decrease.
D) The two homozygotes should decrease at different rates.
Use this information to answer the following questions.
A large population of laboratory animals has been allowed to breed randomly for a number of
generations. After several generations, 25% of the animals display a recessive trait (aa), the same
percentage as at the beginning of the breeding program. The rest of the animals show the dominant
phenotype, with heterozygotes indistinguishable from the homozygous dominants.
56) What is the most reasonable conclusion that can be drawn from the fact that the frequency of the
recessive trait (aa) has not changed over time?
A) The population is undergoing genetic drift.
B) The two phenotypes are about equally adaptive under laboratory conditions.
C) The genotype AA is lethal.
D) There has been a high rate of mutation of allele A to allele a.
E) There has been sexual selection favoring allele a.
57) What is the estimated frequency of allele A in the gene pool?
A) 0.25
B) 0.50
C) 0.75
58) What proportion of the population is probably heterozygous (Aa) for this trait?
A) 0.05
B) 0.25
C) 0.50
D) 0.75
Use this information to answer the following questions.
You are studying three populations of birds. Population A has ten birds, of which one is brown (a
recessive trait) and nine are red. Population B has 100 birds, of which ten are brown. Population C has
30 birds, and three of them are brown.
59) In which population would it be least likely that an accident would significantly alter the frequency
of the brown allele?
A) population A
B) population B
C) population C
D) They are all the same.
E) It is impossible to tell from the information given.
60) Which population is most likely to be subject to the bottleneck effect?
A) population A
B) population B
C) population C
D) They are all equally likely.
E) It is impossible to tell from the information given.
Use the following information to answer the following questions.
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele
constitutes 20% of the β hemoglobin alleles in the human gene pool.
61) What should be the proportion of heterozygous individuals in populations that live here?
A) 0.04
B) 0.16
C) 0.20
D) 0.32
E) 0.80
62) If the sickle-cell allele is recessive, what proportion of the population should be susceptible to
sickle-cell anemia under typical conditions?
A) 0.04
B) 0.16
C) 0.20
D) 0.32
E) 0.80
63) In the United States, the parasite that causes malaria is not present, but African-Americans whose
ancestors were from equatorial Africa are present. What should be happening to the sickle-cell allele in
the United States, and what should be happening to it in equatorial Africa?
A) stabilizing selection; disruptive selection
B) disruptive selection; stabilizing selection
C) disruptive selection; directional selection
D) directional selection; disruptive selection
E) directional selection; stabilizing selection
64) With respect to the sickle-cell allele, what should be true of the β hemoglobin locus in U.S.
populations of African-Americans whose ancestors were from equatorial Africa?
1. The average heterozygosity at this locus should be decreasing over time.
2. There is an increasing heterozygote advantage at this locus.
3. Diploidy is helping to preserve the sickle-cell allele at this locus.
4. Frequency-dependent selection is helping to preserve the sickle-cell allele at this locus.
A) 1 only
B) 1 and 3
C) 2 and 3
D) 1, 2, and 3
E) 1, 2, and 4
65) Considering the overall human population of the U.S. mainland at the time when the slave trade
brought large numbers of people from equatorial Africa, what was primarily acting to change the
frequency of the sickle-cell allele in the overall U.S. population?
A) natural selection
B) gene flow
C) genetic drift
D) founder effect
E) Two of the responses are correct.
66) The sickle-cell allele is pleiotropic (i.e., it affects more than one phenotypic trait). Specifically, this
allele affects oxygen delivery to tissues and affects one’s susceptibility to malaria. Under conditions of
low atmospheric oxygen availability, individuals heterozygous for this allele can experience life-
threatening sickle-cell “crises.” Such individuals remain less susceptible to malaria. Thus, pleiotropic
genes/alleles such as this can help explain why
A) new advantageous alleles do not arise on demand.
B) evolution is limited by historical constraints.
C) adaptations are often compromises.
D) chance events can affect the evolutionary history of populations.
In the year 2500, five male space colonists and five female space colonists (all unrelated to each other)
settle on an uninhabited Earthlike planet in the Andromeda galaxy. The colonists and their offspring
randomly mate for generations. All ten of the original colonists had free earlobes, and two were
heterozygous for that trait. The allele for free earlobes is dominant to the allele for attached earlobes.
67) Which of these is closest to the allele frequency in the founding population?
A) 0.1 a, 0.9 A
B) 0.2 a, 0.8 A
C) 0.5 a, 0.5 A
D) 0.8 a, 0.2 A
E) 0.4 a, 0.6 A
68) If one assumes that Hardy-Weinberg equilibrium applies to the population of colonists on this
planet, about how many people will have attached earlobes when the planet’s population reaches 10,000?
A) 100
B) 400
C) 800
D) 1,000
E) 10,000
69) If four of the original colonists died before they produced offspring, the ratios of genotypes could be
quite different in the subsequent generations. This would be an example of
A) diploidy.
B) gene flow.
C) genetic drift.
D) disruptive selection.
E) stabilizing selection.
70) You are maintaining a small population of fruit flies in the laboratory by transferring the flies to a
new culture bottle after each generation. After several generations, you notice that the viability of the
flies has decreased greatly. Recognizing that small population size is likely to be linked to decreased
viability, the best way to reverse this trend is to
A) cross your flies with flies from another lab.
B) reduce the number of flies that you transfer at each generation.
C) transfer only the largest flies.
D) change the temperature at which you rear the flies.
E) shock the flies with a brief treatment of heat or cold to make them more hardy.
71) The volcano is currently dormant, but in a hypothetical future scenario, satellite cones at the base of
Mt. Kilimanjaro spew sulfurous gases and lava, destroying all life located between the base and 6,000
feet above sea level. As a result of this catastrophe, how should the frequency of the sickle-cell allele
change in the remnant human population that survives above 6,000 feet, and which phenomenon
accounts for this change in allele frequency?
A) decreases; disruptive selection
B) increases; genetic drift
C) decreases; gene flow
D) increases; nonrandom mating
E) decreases; bottleneck effect
72) Swine are vulnerable to infection by bird flu virus and human flu virus, which can both be present in
an individual pig at the same time. When this occurs, it is possible for genes from bird flu virus and
human flu virus to be combined. If the human flu virus contributes a gene for Tamiflu resistance
(Tamiflu is an antiviral drug) to the new virus, and if the new virus is introduced to an environment
lacking Tamiflu, then what is most likely to occur?
A) The new virus will maintain its Tamiflu-resistance gene, just in case of future exposure to Tamiflu.
B) The Tamiflu-resistance gene will undergo mutations that convert it into a gene that has a useful
function in this environment.
C) If the Tamiflu-resistance gene involves a cost, it will experience directional selection leading to
reduction in its frequency.
D) If the Tamiflu-resistance gene confers no benefit in the current environment, and has no cost, the
virus will become dormant until Tamiflu is present.
End-of-Chapter Questions
The following questions are from the end-of–chapter “Test Your Understanding” section in Chapter 23
of the textbook.
73) Natural selection changes allele frequencies because some ________ survive and reproduce more
successfully than others.
A) alleles
B) loci
C) gene pools
D) species
E) individuals
74) No two people are genetically identical, except for identical twins. The main source of genetic
variation among human individuals is
A) new mutations that occurred in the preceding generation.
B) genetic drift due to the small size of the population.
C) the reshuffling of alleles in sexual reproduction.
D) geographic variation within the population.
E) environmental effects.
75) Sparrows with average-sized wings survive severe storms better than those with longer or shorter
wings, illustrating
A) the bottleneck effect.
B) disruptive selection.
C) frequency-dependent selection.
D) neutral variation.
E) stabilizing selection.
76) If the nucleotide variability of a locus equals 0%, what is the gene variability and number of alleles
at that locus?
A) gene variability = 0%; number of alleles = 0
B) gene variability = 0%; number of alleles = 1
C) gene variability = 0%; number of alleles = 2
D) gene variability > 0%; number of alleles = 2
E) Without more information, gene variability and number of alleles cannot be determined.
77) There are 40 individuals in population 1, all with genotype A1A1, and there are 25 individuals in
population 2, all with genotype A2A2. Assume that these populations are located far from each other and
that their environmental conditions are very similar. Based on the information given here, the observed
genetic variation is most likely an example of
A) genetic drift.
B) gene flow.
C) disruptive selection.
D) discrete variation.
E) directional selection.
78) A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the gametes
produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equilibrium,
what proportion of the flies carry both A1 and A2?
A) 0.7
B) 0.49
C) 0.21
D) 0.42
E) 0.09