Download Answers to End-of-Chapter Questions – Brooker et al ARIS site

Answers to End-of-Chapter Questions – Brooker et al ARIS site
Chapter 24
Test Yourself Questions
1. Population geneticists are interested in the genetic variation in populations. The most common type
of genetic change that can cause polymorphism in a population is
a. a deletion of a gene sequence.
b. a duplication of a region of a gene.
c. a rearrangement of a gene sequence.
d. a single-nucleotide substitution.
e. an inversion of a segment of a chromosome.
Answer: d. The most common type of genetic change in a population is a single nucleotide change.
These can result in single nucleotide polymorphisms (SNPs).
2. The Hardy-Weinberg equation characterizes the genotype frequencies and allele frequencies
a. of a population that is experiencing selection for mating success.
b. of a population that is extremely small.
c. of a population that is very large and not evolving.
d. of a community of species that is not evolving.
e. of a community of species that is experiencing selection.
Answer: c. The Hardy-Weinberg equation mathematically describes the distribution of genotypes based
on allele frequencies in a non-evolving population, a group of individuals of the same species that
potentially interbreed. Any factor that alters allele frequency will also alter genotype frequencies and
thus change the distribution of the population.
3. Considering the Hardy-Weinberg equation, what portion of the equation would be used to calculate
the frequency of individuals who do not exhibit a disease but are carriers of a recessive genetic
disorder?
a. q
b. p2
c. 2pq
d. q2
e. both b and d
Answer: c. The frequency of the carriers is also the frequency of heterozygotes in the population. The
frequency of the heterozygotes is determined by calculating 2pq.
4. Which of the following does not alter allele frequencies?
a. selection
b. immigration
c. mutation
d. inbreeding
e. emigration
Answer: d. Though inbreeding will alter genotype frequencies, decreasing the number of heterozygotes
in the population, it does not directly alter allele frequencies.
5. Which of the following statements is correct regarding mutations?
a. Mutations are not important in evolution.
b. Mutations provide the source for genetic variation that other evolutionary forces may act
upon.
c. Mutations occur at such a high rate that they promote major changes in the
gene pool from one generation to the next.
d. Mutations are insignificant when considering evolution of a large population.
e. Mutations are of greater importance in larger populations than in smaller populations.
Answer: b. Though mutation does provide the raw material for genetic variation in a population, it does
not, by itself, drive major evolutionary changes. Other factors, such as natural selection, migration, and
random genetic drift will work on the variation produced by mutation to alter the genetic makeup of a
population.
6. In a population of fish, body coloration varies from a light shade, almost white, to a very dark shade
of green. If changes in the environment resulted in decreased predation of individuals with the lightest
coloration, this would be an example of __________ selection.
a. disruptive
b. stabilizing
c. directional
d. sexual
e. artificial
Answer: c. Because the selection is favoring one extreme of the population distribution over the other,
the type of selection indicated would be directional.
7. Considering the same population of fish described in question 6, if the stream environment included
several areas of sandy, light-colored bottom areas and lots of dark-colored vegetation, both the lightand dark-colored fish would have selective advantages and increased survival. This type of scenario
could explain the occurrence of
a. genetic drift.
b. disruptive selection.
c. mutation.
d. stabilizing selection.
e. sexual selection.
Answer: b. Due to disruptive selection, populations that occupy a heterogeneous environment may
evolve two or more phenotypes that are adapted to different areas of the environment.
8. The microevolutionary force most sensitive to population size is
a. mutation.
b. migration.
c. selection.
d. genetic drift.
e. all of the above.
Answer: d. Genetic drift is random changes in allele frequencies that occur from one generation to the
next. The smaller a population is, the more dramatic this sampling effect can be on allele frequencies.
9. The neutral theory of evolution differs primarily from Darwinian evolution in that
a. the neutral theory states natural selection does not exist.
b. the neutral theory states that most of the genetic variation in a population is due to neutral
mutations that do not alter phenotypes.
c. neutral variation alters survival and reproductive success.
d. neutral mutations are not affected by population size.
e. both b and c
Answer: b. The neutral theory suggests that most of the genetic variation in a population is due to
neutral mutations that do not alter phenotypes and, therefore, do not affect reproductive success or
survival. The spread of neutral mutations is due to random genetic drift.
10. Populations that experience inbreeding may also experience
a. a decrease in fitness due to an increased frequency of recessive genetic diseases.
b. an increase in fitness due to increases in heterozygosity.
c. very little genetic drift.
d. no apparent change.
e. increased mutation rates.
Answer: a. Though it may not occur in all cases of inbreeding, the most common effect in a natural
population is a decrease in fitness, survival, and reproductive success, due to an increase in
homozygosity and an increase in the frequency of recessive genetic diseases.
Conceptual Questions
1. Explain the five conditions that are required for Hardy-Weinberg equilibrium.
Answer:
 The population is so large that allele frequencies do not change due to random sampling error.
 The members of the population mate with each other without regard to their genotypes and
phenotypes.
 No migration occurs between different populations.
 No survival or reproductive advantage exists for any of the genotypes.
 There are no new mutations.
2. List and define the four patterns of natural selection.
Answer:
 Directional selection – This pattern of selection favors individuals at one extreme of a phenotypic
distribution that have greater reproductive success in a particular environment..
 Stabilizing selection – This pattern of selection favors the survival of individuals with intermediate
phenotypes; the extreme values of a trait are selected against..
 Disruptive selection – This pattern of selection (also known as diversifying selection) favors the
survival of two or more different genotypes that produce different phenotypes..
 Balancing selection – This pattern of selection maintains genetic diversity in a population. Over
many generations, balancing selection can create a situation known as a balanced polymorphism,
in which two or more alleles are kept in balance.
3. Define the founder effect.
Answer: The founder effect occurs when a small group of individuals separates from a larger population
and establishes a colony in a new location. This effect has two important consequences. First, the
founding population, which is relatively small, is expected to have less genetic variation than the larger,
original population from which it was derived. Second, as a matter of chance, the allele frequencies in
the founding population may differ markedly from those of the original population.
Experimental Questions
1. What hypothesis is tested in the Seehausen and van Alphen experiment?
Answer: The two species of cichlids used in the experiment are distinguishable by coloration, and the
researchers were testing the hypothesis that the females make mate choices based on this variable.
2. Describe the experimental design for this study, illustrated in Figure 24.9. What was the purpose of
conducting the experiment under the two different light conditions?
Answer: Individual females were placed in tanks that contained one male from each species. The males
were held in small glass tanks to limit their movement but allow the female to see each of the males.
The researchers recorded the courtship behavior between the female and males and the number of
positive encounters between the female and each of the different males. This procedure was conducted
under normal lighting and under monochromatic lighting that obscured the coloration differences
between the two species. Comparing the behavior of the females under normal light conditions and
monochromatic light conditions allowed the researchers to determine the importance of coloration in
mate choice.
3. What were the results of the experiment in Figure 24.9?
Answer: The researchers found that the female was more likely to select a mate from her own species
in normal light conditions. However, under monochromatic light conditions the species-specific mate
choice was not seen. Females were as likely to choose males of the other species as they were males
of their own species. This indicated that coloration is an important factor in mate choice in these species
of fish.
Collaborative Questions
1. Discuss four sources of new genetic variation in a population.
Answer:
New alleles - When random mutations arise, they introduce new alleles into a population. These new
mutations may be beneficial, neutral, or deleterious. If a new mutation is beneficial it is more likely to be
selected for and it may remain in the population. If the mutation is deleterious, the mutation along with
the individual may be removed from the gene pool.
Gene duplication - Sometimes abnormal crossing over or a transposition occurs, which results in an
increase in the number of copies of a gene. This may allow mutation and natural selection to tinker with
one of the gene copies without affecting the function of the other copies of that gene. This factor can
increase the variability of an individual and enhance its fitness.
Exon shuffling - Abnormal crossover events and transposable elements may promote gene
rearrangements in which one or more exons from one gene are inserted into another gene. The protein
encoded by such a gene may display a novel function.
Horizontal gene transfer - This is when a gene of one species is introduced into the genome of another.
As a result of this, the species that receives the new gene may acquire the use of that introduced gene,
thus increasing its variability and fitness.
2. Discuss various patterns of natural selection that lead to environmental adaptation and also discuss
sexual selection.
Answer:
Directional selection - This is when an extreme phenotype is selected for which makes the organism
better suited to survive in its environment. As a result of this, the extreme phenotype will become
predominant in the population. This can occur either through new mutation or through a prolonged
environmental change. In addition to selecting for a certain phenotype, the opposite end of the extreme
is removed from the gene pool.
Stabilizing selection - In this type of selection, individuals with intermediate phenotypes are selected for
and organisms with extreme phenotypes are selected against and removed from the population. This
selection tends to prevent major changes in the phenotypes of organisms.
Disruptive selection - This type of selection favors both extremes and removes the intermediate
phenotype. It is also known as diversifying selection.
Balancing selection - This type of selection results in a balanced polymorphism in which two or more
alleles are stably maintained in a population. Examples include heterozygote advantage, as in the
sickle-cell allele, and negative frequency-dependent selection, as in the pollinators of Dactylorhiza
sambucina.
Sexual selection - This is a type of natural selection that is directly aimed at reproductive success. It can
occur by any of the previous four mechanisms. Male coloration in African cichlids is an example.