Download Chapter 25: Population Genetics

Chapter 25: Population Genetics
Student Learning Objectives
Upon completion of this chapter you should be able to:
1. Understand the concept of a population and polymorphism in populations.
2. Apply the Hardy-Weinberg equation to calculate the frequency of alleles and
genotypes in a population.
3. Understand microevolution and the factors that affect it.
4. Distinguish between the various patterns of natural selection.
5. Understand the concept of genetic drift.
6. Know how migration and nonrandom mating change allele frequencies in a
population and how they influence Hardy-Weinberg equilibrium.
7. Know the various mechanisms by which organisms acquire new genetic variations.
25.1 Genes in Populations and the Hardy-Weinberg Equation
Overview
The majority of the text has examined the relationship between genes and the individual.
This chapter explores the bigger picture of population genetics. In the first section we explore
some of the general features of populations and gene pools, as well as the mathematical
calculations of allele and genotype frequencies. In order to study population genetics we need to
first develop a mathematical model that examines the allele frequencies of a population that is
stable from one generation to the next. Once that is established it is possible to assess the
influences of factors that may alter the allele frequency of a population. This is the basis of the
Hardy-Weinberg equilibrium. While the Hardy-Weinberg formula (page 625) appears too
mathematical, in reality it is a theoretical model of a stable population. As you will observe in the
next section, stable populations are rare in the natural world since there are many factors that
influence the frequencies of alleles in a population. However, an understanding of the equation is
needed in order to assess the influence of these factors.
Outline of Key Terms
Allele frequencies
Genotype frequencies
Hardy-Weinberg equation
Equilibrium
Disequilibrium
Population genetics
Gene pool
Population
Local population (deme)
Polymorphism
Polymorphic
Monomorphic
Single-nucleotide polymorphism
(SNP)
Focal Points


Calculations of allele and genotype frequencies (pages 624-625)
Discussion of Hardy-Weinberg equilibrium (pages 625-627)
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Exercises and Problems
For each of the following, match the term to its correct definition.
_____ 1.
Population
_____ 2.
Gene pool
_____ 3.
Allele frequencies
_____ 4.
Genotype frequencies
a.
b.
c.
d.
The sum of all of the genes within a population.
A group of members of the same species that can interbreed with one another.
The percent that an allele is represented in a population.
The percent of individuals that are homozygous recessive, homozygous dominant or
heterozygous in a population.
Match each of the following Hardy-Weinberg equation terms with its correct definition.
_____ 5. p2
a. The genotype frequency of homozygous dominant individuals.
_____ 6. q2
b. The genotype frequency of heterozygous individuals.
_____ 7. 2pq
c. The genotype frequency of homozygous recessive individuals.
For questions 8 and 9, use the following information: In a certain population of 200 people, the
number of individuals with AA, Aa, and aa genotypes is 60, 100, and 40, respectively.
8. What is the frequency of AA individuals?
9. What is the frequency of the A allele?
For questions 10 and 11, use the following information: In a human population, a recessive
autosomal disorder occurs in approximately 1 in 10,000 births.
10. What is the frequency of the recessive allele?
11. What is the percentage of carriers?
25.2 Overview of Microevolution
Overview
The term microevolution describes changes in a population’s gene pool from generation
to generation. Such change is based on two related phenomena: 1) introduction of new genetic
variation into a population, and 2) actions of mechanisms that alter existing genetic variation in a
population. The previous section introduced the fact that a stable population possesses certain
characteristics under the Hardy-Weinberg equilibrium. Yet in the natural world, few populations
are in equilibrium. The factors that contribute to disequilibrium are outlined in this section.
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Outline of Key Terms
Microevolution
Focal Points

Factors that govern microevolution (Table 25.1)
Exercises and Problems
Complete the following sentences with the most appropriate term(s):
1. Microevolution describes changes in a population’s ________ from generation to generation.
2. Random mutations can introduce new alleles into a population, but at a ________ rate.
3. ________ is the phenomenon in which the environment favors individuals that possess certain
traits.
4. The change in genetic variation from generation to generation due to random sampling error is
called ________.
5. ________ is the phenomenon in which individuals select mates based on their phenotypes or
genetic lineage.
25.3 Natural Selection
Overview
The theory of evolution by natural selection was proposed in the 1850s by Charles
Darwin and Alfred Russell Wallace. The theory posits that the conditions found in nature result in
the survival and reproduction of individuals that are best suited to their environment. This process
will lead to changes in allele frequencies from one generation to the next. This is based on the
concept of Darwinian fitness, which is the relative likelihood that one genotype will contribute to
the gene pool of the next generation compared with other genotypes. Note that Darwinian fitness
does not necessarily mean physical fitness. Rather, it is a measure of reproductive success. An
extremely fertile genotype may have a higher Darwinian fitness than a less fertile genotype that
appears more physically fit.
Geneticists have found that natural selection can occur in several ways, depending on the
relative fitness values of the different genotypes and on the variation of environmental effects. In
this section, we will consider four different patterns of natural selection: directional, stabilizing,
disruptive, and balancing selection. We will also examine an ongoing example of natural
selection—a change in the beak size of Darwin’s finches due to drought conditions.
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Outline of Key Terms
Balancing selection
Heterozygote advantage
Selection coefficient (s)
Negative frequency-dependent
selection
Natural selection
Darwinian fitness
Directional selection
Mean fitness of the population
Stabilizing selection
Disruptive selection
Focal Points

Directional selection (Figures 25.6 and 25.8); stabilizing selection (Figure 25.9);
disruptive selection (Figure 25.10); and balancing selection (pages 633-634)
Exercises and Problems
For questions 1 to 6, choose the model of selection from the diagram below:
_____ 1.
Favors the survival of individuals with intermediate phenotypes.
_____ 2.
Separates a population into distinct phenotypic classes.
_____ 3.
Favors one extreme of the phenotype.
_____ 4.
Occurs in species that occupy diverse environments.
_____ 5.
Resistance to antibiotics typically occurs in this manner.
_____ 6.
Clutch size in birds is an example of this type of selection.
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For questions 7 to 11, complete the following sentences with the most appropriate term(s):
7. Darwinian fitness values are denoted by the variable ________.
8. For genetic variation involving a single gene, balancing selection may arise when the
heterozygote has a higher fitness that either corresponding homozygote. This situation is
called ________ or ________.
9. Balancing selection may also be caused by ________, a pattern in which the fitness of a
genotype decreases when its frequency becomes higher.
10. The change in beak size in the Galapagos finches that the Grants observed is an example of
natural selection, and most likely due to ________ selection.
11. A heterozygote for the sickle-cell allele, HbAHbS, has a higher level of fitness compared to
the HbAHbA homozygote, because the heterozygote has a better chance of survival if
infected with the malarial parasite, ________.
25.4 Genetic Drift
Overview
The concept of random genetic drift, or simply genetic drift, was developed in the 1930s
by the geneticist Sewall Wright. It refers to changes in allelic frequencies in a population due to
random fluctuations. Over the long run, genetic drift can lead to the loss or fixation of a particular
allele. The rate at which this occurs depends on the population size and on the initial allele
frequencies (Figure 25.16).
In nature, geography and population size can influence how genetic drift affects the
genetic composition of a species in different ways. In general, small isolated populations tend to
be more genetically disparate in relation to other populations. Changes in population size may
influence genetic drift in one of two main ways: 1) the bottleneck effect, where a population is
reduced dramatically in size by natural disasters or human destruction of habitat; and 2) the
founder effect, which involves separation of a small group of individuals from a larger
population, and establishment of a colony in a new location.
Outline of Key Terms
Random genetic drift (genetic drift)
Bottleneck effect
Founder effect
Focal Points


A hypothetical simulation of genetic drift (Figure 25.16)
The bottleneck effect, an example of genetic drift (Figure 25.17)
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Exercises and Problems
For questions 1 to 4, use the following information: The mutation rate is given by , and the
number of individuals in a population is given by N. Assume equal numbers of males and females
contribute to the new generation.
1. What is the expected number of new mutations in a given gene?
2. If a new mutation has arisen, what is the probability that the new allele will be fixed due to
genetic drift?
3. What is the probability that the new allele will be eliminated?
4. If a population has 1000 breeding members, what is the average number of generations
required to achieve fixation?
For questions 5 to 6, complete the following sentences with the most appropriate term(s):
5. The African cheetah population has lost a substantial amount of its genetic variation due to a
________ effect.
6. The high frequency of dwarfism in the Old Order Amish of Lancaster County, Pennsylvania,
is due to the ________ effect.
25.5 Migration
Overview
Migration is the movement of individuals from one location to another. Population
geneticists are particularly interested in the phenomenon of gene flow. This occurs when
individuals migrate from one population to another with different allele frequencies, and the
migrants are able to breed successfully with the members of the recipient population. Thus gene
flow depends not only on migration, but also on the ability of the migrants’ alleles to be passed to
subsequent generations. Migration can be unidirectional or bidirectional. Depending on its rate,
migration tends to reduce differences in allele frequencies between neighboring populations and
increase genetic diversity within a population.
Outline of Key Terms
Conglomerate
Gene flow
Focal Points

Quantitative discussion of migration (page 639)
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Exercises and Problems
Complete the following sentences with the most appropriate word or phrase:
After migration has occurred, the new population is called a (1) ________. To calculate
the allele frequencies in this new population, we need two kinds of information. First, we must
know the (2) ________ in the donor and recipient populations. Second, we must know the
proportion of the (3) ________ population that is due to (4) ________.
25.6 Nonrandom Mating
Overview
You may recall that in the first section of this chapter, we stated that one of the conditions
required to establish the Hardy-Weinberg equilibrium is random mating. This means that
individuals choose their mates regardless of their genotypes and phenotypes. However, in many
cases, especially in human populations, this condition is violated frequently. When mating is
based on phenotype, the process is called assortative mating. Positive assortative mating occurs
when individuals with similar phenotypes choose each other as mates, while in negative
assortative mating individuals with dissimilar phenotypes mate preferentially. When mating is
based on genotype, inbreeding or outbreeding occurs. The mating of two genetically related
individuals is called inbreeding, while outbreeding involves preferential mating between
unrelated individuals. Note that nonrandom mating may alter the genotype frequencies that would
be predicted by the Hardy-Weinberg equation. In general, positive assortative mating and
inbreeding increase homozygosity, whereas negative assortative mating and outbreeding increase
heterozygosity.
Outline of Key Terms
Nonrandom mating
Assortative mating
Inbreeding
Inbreeding coefficient (F)
Inbreeding depression
Outbreeding
Focal Points


A human pedigree containing inbreeding (Figure 25.18)
Quantitative discussion of nonrandom mating (pages 640-641)
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Exercises and Problems
For questions 1 to 6, match the term to its correct definition.
_____ 1.
Negative assortative mating
_____ 2.
Disequilibrium
_____ 3.
Coefficient of inbreeding
_____ 4.
Inbreeding
_____ 5.
Outbreeding
_____ 6.
Positive assortative mating
a. the mating of two genetically related individuals
b. has the ability to create hybrids that are heterozygous for many genes
c. individuals who mate due to similar phenotypes
d. individuals who mate based on dissimilar phenotypes
e. allele and genotype frequencies are not in Hardy-Weinberg equilibrium
f. the quantification of the degree of inbreeding
For questions 7 to 16, choose the most appropriate term for the definition. Note that this exercise
offers a review of the first six sections of this chapter.
_____ 7. A population is not evolving towards fixation of an allele.
_____ 8. The degree to which a genotype is selected against.
_____ 9. A fitness calculation in which the terms do not add up to 1.0.
_____ 10. The drastic reduction in size of a population.
_____ 11. Random changes in allele frequencies due to chance events.
_____ 12. Genetic variation that decreases the average fitness of a population.
_____ 13. Occurs due to the movement of individuals between populations.
_____ 14. Mating that produces homozygotes that are less fit, thereby decreasing the
reproductive success of the population.
_____ 15. The relative likelihood that a phenotype will survive and contribute to the next
generation’s gene pool.
_____ 16. Occurs when a small group of individuals establish a new population.
a.
b.
c.
d.
e.
inbreeding depression
founder effect
genetic drift
bottleneck effect
gene flow
f. mean fitness of the population
g. balanced polymorphism
h. Darwinian fitness
i. genetic load
j. selection coefficient
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25.7 Sources of New Genetic Variation
Overview
Earlier in the textbook we discussed various mechanisms by which genetic variation may
be generated. For example, independent assortment (Chapter 3) and crossing-over (Chapter 6)
during sexual reproduction can produce new allelic combinations in the offspring. This section
explores additional mechanisms through which organisms acquire new genetic variation. These
include: 1) mutations, which involve changes in gene sequences, chromosome structure and/or
chromosome number; 2) exon shuffling (by which new genes are created); 3) horizontal gene
transfer (by which new genes are acquired); and 4) changes in repetitive sequences. The last part
of this section examines the technique of DNA fingerprinting, which is used to identify
individuals and to study the genetic relationship between them.
Outline of Key Terms
Mutation rate
Exon shuffling
Horizontal gene transfer
Repetitive sequences
Microsatellite
Minisatellite
DNA fingerprinting
DNA profiling
Focal Points




Sources of new genetic variation (Table 25.2)
The process of exon shuffling (Figure 25.19)
Horizontal gene transfer from bacterium to eukaryote (Figure 25.20)
DNA fingerprinting (pages 644-645)
Exercises and Problems
Complete the following sentences with the appropriate terms(s):
1. DNA fingerprinting is also called __________.
2. __________ are also called short tandem repeats (STRs).
3. __________ refers to the process by which an exon and its flanking introns are inserted into a
gene, thus producing a new gene encoding a protein with an additional domain.
4. In __________, the repeat unit is 6 to 80 bp in length.
5. In __________, an organism incorporates genetic material from another organism without
being its offspring.
6. The __________ is defined as the probability that a gene will be altered by a new mutation.
7. Repetitive sequences may come from __________, which are genetic sequences that can move
from place to place within a species’ genome.
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Chapter Quiz
1. The sum of all of the alleles in a population is called what?
a. deme
b. genotype frequency
c. gene pool
d. fitness coefficient
2. Individuals who prefer dissimilar phenotypes for mating are said to be exhibiting
a. inbreeding.
b. positive assortative breeding.
c. disequilibrium.
d. negative assortative mating.
3. Which of the following is the measure of inbreeding?
a. mutation frequency
b. Darwinian fitness
c. fixation coefficient
d. selection coefficient
4. Which of the following forms of selection favors an intermediate phenotype?
a. directional selection
b. disruptive selection
c. stabilizing selection
d. none of the above
5. Which of the following is an example of genetic drift in which catastrophic events influence
the allele frequencies?
a. bottleneck effect
b. inbreeding
c. founder effect
d. migration
6. In a population of animals, 96% have brown bodies (BB or Bb) and only 4% are black-bodied
(bb). According to the Hardy-Weinberg equation, what percentage of the total population is
expected to be heterozygous?
a. 20%
b. 32%
c. 48%
d. 64%
e. 80%
7. In shorthorn cattle, individuals can be red (CRCR), white (CWCW), or roan (CRCW), which is a
mixture of red and white. A population of Shorthorns was sampled and found to contain 108 red,
48 white, and 144 roan animals. What is the frequency of the CR allele in the population?
a. 0.2
b. 0.3
c. 0.4
d. 0.5
e. 0.6
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8. A population in which two alleles are NOT evolving towards fixation is said to be an example
of which of the following?
a. disequilibrium
b. balanced polymorphism
c. monomorphism
d. inbreeding depression
9. The degree to which a genotype is being selected against is called the
a. selection coefficient.
b. mean fitness of the population.
c. Darwinian fitness.
d. fixation coefficient.
10. The variation that decreases the average fitness of a population is called the
a. genetic load.
b. mean fitness of the population.
c. Darwinian fitness.
d. fixation coefficient.
Answer Key for Study Guide Questions
This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers
in parentheses ( ) represent possible alternate answers to a problem, while answers marked with
an asterisk (*) indicate that the response to the question may vary.
25.1
1. b
2. a
3. c
4. d
5. a
6. c
7. b
8. 0.3
9. 0.55
10. 0.01 (q)
11. 1.98% (2pq)
25.2
1. gene pool
2. very low
3. Natural selection
4. genetic drift
5. Nonrandom mating
25.3
1. c
2. b
3. a
4. b
5. a
6. c
7. W
8. heterozygote advantage; overdominance
9. negative-frequency dependent selection
10. directional
11. Plasmodium falciparum
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25.4
1. 2N
2. 1/2N
3. 1 – 1/2N
25.5
1. conglomerate
2. allele frequencies
3. conglomerate
4. migrants
25.6
1. d
2. e
3. f
4. a
5. b
6. c
7. g
8. j
9. f
10. d
25.7
1. DNA profiling
2. Microsatellites
3. Exon shuffling
4. minisatellites
5. horizontal gene transfer
6. mutation rate
7. transposable elements
4. 4000 generations (4N)
5. bottleneck
6. founder
11. c
12. i
13. e
14. a
15. h
16. b
17. c
18. b
19. a
20. b
Quiz
1. c
2. d
3. c
4. c
5. a
6. b
7. e
8. b
9. a
10. a
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