1. How Populations Evolve
chapter 16
How Populations Evolve

2. Chapter 16 At a Glance
16.1 How Are Populations, Genes, and Evolution Related?
16.2 What Causes Evolution?
16.3 How Does Natural Selection Work?
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3. 16.1 How Are Populations, Genes, and Evolution Related?
Evolutionary changes occur from generation to generation, causing descendants to differ from their ancestors
Evolution is a property not of individuals, but of populations
A population is a group that includes all members of a species living in a given area
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4. 16.1 How Are Populations, Genes, and Evolution Related?
The actions and fates of individuals determine which characteristics will be passed to descendant populations
Inheritance provides the link between the lives of individual organisms and the evolution of populations
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5. 16.1 How Are Populations, Genes, and Evolution Related?
Genes and the environment interact to determine traits
Each cell of every organism contains genetic information encoded in the DNA of its chromosomes
The combined DNA in an organism’s set of chromosomes is its genome
A gene is a segment of DNA located at a particular place on a chromosome
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6. 16.1 How Are Populations, Genes, and Evolution Related?
Genes and the environment interact to determine traits (continued)
In any population of organisms, there are usually two or more alleles of each gene
Individuals whose alleles of a particular gene are the same are homozygous for that gene
Individuals whose alleles are different are heterozygous for that gene
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7. 16.1 How Are Populations, Genes, and Evolution Related?
Genes and the environment interact to determine traits (continued)
The specific alleles borne on an organism’s chromosomes (its genotype) interact with the environment to influence the development of its physical and behavioral traits (its phenotype)
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8. dominant, so heterozygous hamsters have black coats. Each chromosome
Figure 16-1
Coat-color allele B is
dominant, so heterozygous
hamsters have black coats.
Each chromosome
has one allele of the
coat-color gene.
phenotype
genotype BB Bb bb
Figure Alleles, genotype, and phenotype in individuals B B B b b b
chromosomes
homozygous
heterozygous
homozygous
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9. 16.1 How Are Populations, Genes, and Evolution Related?
Genes and the environment interact to determine traits (continued)
Coat color in hamsters illustrates the interaction between genotype and phenotype
Coat color is determined by two alleles in hamsters
The dominant allele encodes for an enzyme that catalyzes black pigment formation
The recessive allele encodes for an enzyme that catalyzes brown pigment
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10. 16.1 How Are Populations, Genes, and Evolution Related?
Genes and the environment interact to determine traits (continued)
Hamsters with at least one dominant allele (homozygous dominant or heterozygous) produce black pigment
Hamsters with two recessive alleles (homozygous recessive) produce brown pigment
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11. 16.1 How Are Populations, Genes, and Evolution Related?
The gene pool comprises all of the alleles in a population
Population genetics deals with the frequency, distribution, and inheritance of alleles in populations
A gene pool is a set that contains all the alleles of all the genes in all individuals of a population
Equal to the number of individuals that carry only a single copy of the allele
Equal to twice the number of individuals that carry two copies
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12. 16.1 How Are Populations, Genes, and Evolution Related?
The gene pool comprises all of the alleles in a population (continued)
For a given gene, the proportion of times a certain allele occurs in a population relative to the occurrence of all the alleles for that gene is called its allele frequency
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13. 16.1 How Are Populations, Genes, and Evolution Related?
The gene pool comprises all of the alleles in a population (continued)
Again, coat color in hamsters can be used to illustrate the idea of allele frequency
A population of 25 hamsters contains 50 alleles of the coat color gene (hamsters are diploid)
If 20 of those 50 alleles code for black coats, then the frequency of the black allele is 0.40, or 40% (20/50 = 0.40)
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14. Figure 16-2 Figure 16-2 A gene pool © 2017 Pearson Education, Ltd.
The gene pool of the
coat-color gene contains
20 copies of allele B and
30 copies of allele b.
Population: 25 individuals
Gene pool: 50 alleles
B B B B B B B B BB BB BB BB
B B B B b b b b Bb Bb Bb Bb
B B B B b b b b Bb Bb Bb Bb
B B B B b b b b Bb Bb Bb Bb
Figure A gene pool
b b b b b b b b bb bb bb bb
b b b b b b b b bb bb bb bb
b b
© 2017 Pearson Education, Ltd. bb

15. 16.1 How Are Populations, Genes, and Evolution Related?
Evolution is the change of allele frequencies within a population
A casual observer might define evolution on the basis of changes in the outward appearance or behaviors of the members of a population
A population geneticist, however, defines evolution as the changes in allele frequencies that occur in a gene pool over time
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16. 16.1 How Are Populations, Genes, and Evolution Related?
Evolution is the change of allele frequencies within a population (continued)
If allele frequencies change from one generation to the next, the population is evolving
Evolution is change in the genetic makeup of populations over generations
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17. 16.1 How Are Populations, Genes, and Evolution Related?
The equilibrium population is a hypothetical population in which evolution does not occur
Considering the characteristics of a population in which evolution does not occur makes it easier to understand what causes populations to evolve
The Hardy-Weinberg principle is a mathematical model proposed independently in 1908 by an English mathematician, Godfrey H. Hardy, and a German physician, Wilhelm Weinberg
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18. 16.1 How Are Populations, Genes, and Evolution Related?
The equilibrium population is a hypothetical population in which evolution does not occur (continued)
The Hardy-Weinberg principle showed that under certain conditions, allele and genotype frequencies in a population will remain constant no matter how many generations pass
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19. 16.1 How Are Populations, Genes, and Evolution Related?
The equilibrium population is a hypothetical population in which evolution does not occur (continued)
An equilibrium population is the term used for this hypothetical non-evolving population in which allele frequencies do not change as long as the following conditions are met:
There must be no mutation
There must be no gene flow
The population must be very large
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20. 16.1 How Are Populations, Genes, and Evolution Related?
The equilibrium population is a hypothetical population in which evolution does not occur (continued)
An equilibrium population is the term used for this hypothetical non-evolving population in which allele frequencies do not change as long as the following conditions are met (continued):
Mating must be random, with no tendency for certain genotypes to mate with specific other genotypes
There can be no natural selection
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21. 16.1 How Are Populations, Genes, and Evolution Related?
The equilibrium population is a hypothetical population in which evolution does not occur (continued)
Violation of one or more of these five conditions may allow changes in allele frequencies, and the population will evolve
Few natural populations are truly in equilibrium
The Hardy-Weinberg conditions are useful starting points for studying the mechanisms of evolution
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22. 16.2 What Causes Evolution?
Population genetics predicts that the Hardy-Weinberg equilibrium can be disturbed by deviations from any of its five conditions
There are, predictably, five major causes of evolutionary change
Mutation
Gene flow
Small population size
Nonrandom mating
Natural selection
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23. 16.2 What Causes Evolution?
Mutations are the original source of genetic variability
A population remains in evolutionary equilibrium only if there are no mutations (changes in DNA sequence)
Most mutations occur during cell division, when a cell makes a copy of its DNA
Inherited mutations are rare but important
The cumulative effect of mutations is essential to evolution
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24. 16.2 What Causes Evolution?
Mutations are the original source of genetic variability (continued)
A population remains in evolutionary equilibrium only if there are no mutations (changes in DNA sequence) (continued)
Mutations are not goal directed
Mutations arise spontaneously, not as a result of or in anticipation of environmental necessity
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25. Figure 16-3 Mutations occur spontaneously
Start with bacterial
colonies that have never
been exposed to antibiotics.
Use velvet to transfer
colonies to identical positions
in three dishes containing the
antibiotic streptomycin.
Figure Mutations occur spontaneously
Incubate the dishes.
Only streptomycin-
resistant colonies grow;
the few colonies are in
the exact same positions
in each dish.
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26. 16.2 What Causes Evolution?
Gene flow between populations changes allele frequencies
Gene flow is the movement of alleles between populations
However, alleles can move between populations even if organisms do not
Pollen (sperm) and seeds from flowering plants can move and distribute alleles
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27. 16.2 What Causes Evolution?
Gene flow between populations changes allele frequencies (continued)
The evolutionary effect of gene flow is to increase the genetic similarity of different populations of a species
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28. 16.2 What Causes Evolution?
Allele frequencies may change by chance in small populations
Allele frequencies in populations can be changed by chance events other than mutations
If bad luck prevents some members of a population from reproducing, their alleles will ultimately be removed from the gene pool
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29. 16.2 What Causes Evolution?
Allele frequencies may change by chance in small populations (continued)
Genetic drift is the process by which chance events change allele frequencies
In a small population of hamsters, if randomly chosen individuals are the only ones that mate, the resulting population may be very different from the original one
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30. Figure 16-4 Figure 16-4 Genetic drift © 2017 Pearson Education, Ltd.
(a) Generation 1
frequency of B = 50%
frequency of b = 50% BB BB BB BB BB
In each generation, only two
randomly chosen individuals
breed; their offspring form
the entire next generation. Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb bb bb bb bb bb
(b) Generation 2
frequency of B = 25%
frequency of b = 75% Bb Bb Bb Bb Bb Bb Bb Bb Bb Bb bb bb bb bb bb
Figure Genetic drift bb bb bb bb bb
(c) Generation 3
frequency of B = 0%
frequency of b = 100% bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
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31. 16.2 What Causes Evolution?
Allele frequencies may change by chance in small populations (continued)
Population size matters
Genetic drift occurs more rapidly and has a greater effect on small populations, where chance may dictate that the alleles of only a few individuals are passed on
In very small populations, drift can result in the complete loss of an allele in only a few generations, even if it is the more frequent one
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32. 16.2 What Causes Evolution?
Allele frequencies may change by chance in small populations (continued)
Population size matters (continued)
Computer modeling can show the effect of genetic drift on small populations when allele frequencies change randomly
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33. Figure 16-5 The effect of population size on genetic drift
1.0
In the large population,
allele frequencies remain
relatively constant.
frequency of allele B
0.8
0.6
0.4
0.2 10 20 30 40 50
(a) Population size = 2,000
1.0
frequency of allele B
0.8
0.6
0.4
0.2 20 40 60 80
100
(b) Population size = 40
Figure The effect of population size on genetic drift
1.0
frequency of allele B
0.8
0.6
In the small population,
one allele may become
extinct in a few
generations.
0.4
0.2 20 40 60 80
100
generations
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(c) Population size = 4

34. Why does genetic drift affect a small population more than it affects a large population?
A small population will be left with more allele variations.
A chance event is more likely to eliminate an allele from a small population, leaving it with reduced allelic variation.
Genetic drift always kills off small populations.
A small population will experience gene flow.
Question: 16-9
Answer: b
Diff: Moderate
Text Ref: Section 16.2
Skill: Conceptual
Notes:
This question is used to explain the impact of genetic drift on populations of different sizes. There is a greater chance that alleles can be eliminated by genetic drift when the population is small, reducing allelic variation.
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35. Why does genetic drift affect a small population more than it affects a large population?
A small population will be left with more allele variations.
A chance event is more likely to eliminate an allele from a small population, leaving it with reduced allelic variation.
Genetic drift always kills off small populations.
A small population will experience gene flow.
Question: 16-9
Answer: b
Diff: Moderate
Text Ref: Section 16.2
Skill: Conceptual
Notes:
This question is used to explain the impact of genetic drift on populations of different sizes. There is a greater chance that alleles can be eliminated by genetic drift when the population is small, reducing allelic variation.
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36. 16.2 What Causes Evolution?
Allele frequencies may drift in small populations
A population bottleneck can cause genetic drift
Population bottleneck and founder effect are two causes of genetic drift
In a population bottleneck, a population is drastically reduced by a natural catastrophe or overhunting
Only a few individuals are available to contribute genes to the next generation
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37. 16.2 What Causes Evolution?
Allele frequencies may drift in small populations (continued)
A population bottleneck can cause genetic drift (continued)
A population bottleneck can rapidly change allele frequencies and reduce genetic variation
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38. Animation: Bottleneck Effect
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39. 16.2 What Causes Evolution?
Allele frequencies may drift in small populations (continued)
A bottleneck has been documented in the northern elephant seal
Hunted almost to extinction in the 1800s, the elephant seals had been reduced to only 20 individuals by the 1890s
Since the late nineteenth century, the population has increased in number to more that 200,000 individuals
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40. 16.2 What Causes Evolution?
Allele frequencies may drift in small populations (continued)
A bottleneck has been documented in the northern elephant seal (continued)
Biochemical analysis shows that present-day northern elephant seals are almost genetically identical
Despite their numbers, their lack of genetic variation leaves them little flexibility to evolve if their environmental circumstances change
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41. Figure 16-6 Population bottlenecks reduce variation
The gene pool of a
population contains
equal numbers of
red, blue, yellow, and
green alleles.
A bottleneck event
drastically reduces the
size of the population.
By chance, the gene
pool of the reduced
population contains
mostly blue and a
few yellow alleles.
After the population
grows and returns to
its original size, blue
alleles predominate;
red and green alleles
have disappeared.
Figure Population bottlenecks reduce variation
(a) Simulation of a population bottleneck
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(b) Elephant seals

42. 16.2 What Causes Evolution?
Allele frequencies may drift in small populations (continued)
Isolated founding populations may produce bottlenecks
The founder effect occurs when isolated colonies are founded by a small number of organisms
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43. 16.2 What Causes Evolution?
Allele frequencies may drift in small populations (continued)
Isolated founding populations may produce bottlenecks (continued)
By chance, the allele frequencies of founders may differ from those of the original population
Over time, the new population may exhibit allele frequencies that differ from the original population
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44. Figure 16-7 A human example of the founder effect
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45. 16.2 What Causes Evolution?
Mating within a population is almost never random
The effects of nonrandom mating can play a significant role in evolution because organisms seldom mate strictly randomly
Some species lack mobility and tend to stay near their birthplace
The offspring of such species have parents living in the same area and may even be related
Such sexual reproduction between relatives is called inbreeding
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46. 16.2 What Causes Evolution?
Mating within a population is almost never random (continued)
Because relatives are genetically similar, inbreeding tends to increase the number of individuals that inherit the same alleles from both parents
Results in both parents being homozygous for many genes
Increase in homozygotes can have harmful effects
Increased occurrence of genetic diseases or disorders
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47. 16.2 What Causes Evolution?
Mating within a population is almost never random (continued)
Nonrandom mating in animals can also arise if individuals have preferences or biases that influence their choice of mate
The snow goose comes in two “color phases”: some snow geese are white, and others are blue-gray; mate choice is not random, but based on color
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48. Figure 16-8 Nonrandom mating among snow geese
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49. 16.2 What Causes Evolution?
Mating within a population is almost never random (continued)
Nonrandom mating in animals can also arise if individuals have preferences or biases that influence their choice of mate (continued)
The birds exhibit a strong tendency to mate with a partner of the same color
Assortative mating is a preference for mates that are similar
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50. 16.2 What Causes Evolution?
Mating within a population is almost never random (continued)
Neither inbreeding nor assortative mating by themselves will alter allele frequencies in a population
Nonrandom mating, however, will change the distribution of genotypes and, therefore, of phenotypes in a population
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51. 16.2 What Causes Evolution? All genotypes are not equally beneficial
In a hypothetical equilibrium population, individuals of all genotypes survive and reproduce equally well
No genotype has any advantage over the others in a hypothetical equilibrium population
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52. 16.2 What Causes Evolution?
All genotypes are not equally beneficial (continued)
The individuals who carry the alleles are favored by natural selection
Natural selection is the process in which individuals with traits that help them survive and reproduce leave more offspring than do individuals without those traits
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53. Table 16-1
Table 16-1
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54. Evolutionary change occurs in association with all of the following except _____.
mutations
gene flow
small population size
random mating
Question: 16-6
Answer: d
Diff: Moderate
Text Ref: Sections 16.1 and 16.2
Skill: Factual
Notes:
The conditions that cause a population to evolve can be difficult for students not only to remember but also to understand. It is important to go over these conditions so that students understand the impact of each one.
It is also important to discuss the fact that few populations experience equilibrium. This question is a good introduction to the rest of Chapter 16 and can help students understand the mechanisms of evolution.
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55. Evolutionary change occurs in association with all of the following except _____.
mutations
gene flow
small population size
random mating
Question: 16-6
Answer: d
Diff: Moderate
Text Ref: Sections 16.1 and 16.2
Skill: Factual
Notes:
The conditions that cause a population to evolve can be difficult for students not only to remember but also to understand. It is important to go over these conditions so that students understand the impact of each one.
It is also important to discuss the fact that few populations experience equilibrium. This question is a good introduction to the rest of Chapter 16 and can help students understand the mechanisms of evolution.
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56. 16.3 How Does Natural Selection Work?
Natural selection stems from unequal reproduction
“Survival of the fittest” was a term coined by Herbert Spencer to summarize the process that Darwin named natural selection
Natural selection favors traits that increase possessors’ survival only to the extent that improved survival leads to improved reproduction
A trait that improves survival may, for example, increase the likelihood that an individual survives long enough to reproduce
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57. 16.3 How Does Natural Selection Work?
Natural selection stems from unequal reproduction (continued)
“Survival of the fittest” was a term coined by Herbert Spencer to summarize the process that Darwin named natural selection (continued)
The main driver of natural selection is differences in reproduction
Individuals bearing certain alleles leave more offspring (who inherit those alleles) than do other individuals with different alleles
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58. 16.3 How Does Natural Selection Work?
Natural selection stems from unequal reproduction (continued)
“Survival of the fittest” was a term coined by Herbert Spencer to summarize the process that Darwin named natural selection (continued)
Individuals with greater lifetime reproductive success are said to have greater fitness than do individuals with lower reproductive success
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59. 16.3 How Does Natural Selection Work?
Natural selection acts on phenotypes
Natural selection does not act directly on the genotypes of individual organisms
Natural selection acts on phenotypes, the structures and behaviors displayed by the members of a population
The selection of phenotypes inevitably affects the genotypes present in a population
Phenotypes and genotypes are closely tied
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60. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others
Successful phenotypes are those that have the best adaptations to their particular environment
Adaptations are characteristics that help an individual survive and reproduce
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61. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
An environment has nonliving and living components
Nonliving environmental factors include such things as climate, the availability of water, and the concentration of minerals in the soil
Living environmental components consist of other organisms
Organisms must deal with nonliving and living factors in order to survive
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62. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Competition acts as an agent of selection
Competition, one of the major sources of natural selection, is an interaction among organisms competing for scarce resources
It is most intense among members of the same species because they all require the same things
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63. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Competition acts as an agent of selection (continued)
Competition, one of the major sources of natural selection, is an interaction among organisms competing for scarce resources (continued)
No two competing organisms have such similar requirements for survival as do members of the same species
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64. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Both predators and prey act as agents of selection
When two species interact extensively, each exerts strong selective pressures on the other
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65. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Both predator and prey act as agents of selection (continued)
When one evolves a new feature or modifies an old one, the other typically evolves new adaptations in response
This process in which species mutually affect one another’s evolution is called coevolution
Predator–prey relationships are the most familiar form of coevolution
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66. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Both predator and prey act as agents of selection (continued)
Predation is an interaction in which one organism (the predator) kills and eats another organism (the prey)
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67. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Both predator and prey act as agents of selection (continued)
Coevolution between predators and prey is akin to a “biological arms race”
Wolf predation selects against slow, careless deer
Alert swift deer select against slow, clumsy wolves
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68. Newts of the genus Taricha are poisonous, deterring their predators from eating them. The common garter snake, a predator, has evolved a resistance to the newt toxins. This situation is an example of _____.
competition
natural selection
sexual selection
coevolution
Question: 16-12
Answer: d
Diff: Moderate
Text Ref: Section 16.3
Skill: Application
Notes:
Students will learn more about how organisms exercise selective pressure on one another when they discuss community interactions in later chapters, but this is an important concept regarding evolution to understand now.
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69. Newts of the genus Taricha are poisonous, deterring their predators from eating them. The common garter snake, a predator, has evolved a resistance to the newt toxins. This situation is an example of _____.
competition
natural selection
sexual selection
coevolution
Question: 16-12
Answer: d
Diff: Moderate
Text Ref: Section 16.3
Skill: Application
Notes:
Students will learn more about how organisms exercise selective pressure on one another when they discuss community interactions in later chapters, but this is an important concept regarding evolution to understand now.
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70. 16.3 How Does Natural Selection Work?
Some phenotypes reproduce more successfully than others (continued)
Antibiotic resistance illustrates key points about natural selection
Natural selection does not cause genetic changes in individuals
Natural selection acts on individuals, but it is population that are changed by evolution
Evolution by natural selection is not progressive: it does not make organisms “better”
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71. Methicillin-resistant Staphylococcus aureus is a bacterium that causes staph infections in hospitals. How do bacteria gain resistance to an antibiotic?
Treatment with an antibiotic causes the bacteria to mutate to adapt to the antibiotic.
The antibiotic stimulates the growth of bacterial cells.
Some of the bacteria already have a mutation that confers resistance to the antibiotic, allowing them to survive and pass on the advantageous gene to their offspring.
The hospital environment causes the bacteria to mutate.
Question: 16-1
Answer: c
Diff: Hard
Text Ref: Chapter 16 Case Study
Skill: Application
Also relates to: Chapter 15
Notes:
This question picks up where Chapter 15 leaves off and emphasizes the importance of variation to the process of natural selection. Emphasize how mutations are already present in a population prior to a change in the environment. Many students think the environment causes the mutation.
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72. Methicillin-resistant Staphylococcus aureus is a bacterium that causes staph infections in hospitals. How do bacteria gain resistance to an antibiotic?
Treatment with an antibiotic causes the bacteria to mutate to adapt to the antibiotic.
The antibiotic stimulates the growth of bacterial cells.
Some of the bacteria already have a mutation that confers resistance to the antibiotic, allowing them to survive and pass on the advantageous gene to their offspring.
The hospital environment causes the bacteria to mutate.
Question: 16-1
Answer: c
Diff: Hard
Text Ref: Chapter 16 Case Study
Skill: Application
Also relates to: Chapter 15
Notes:
This question picks up where Chapter 15 leaves off and emphasizes the importance of variation to the process of natural selection. Emphasize how mutations are already present in a population prior to a change in the environment. Many students think the environment causes the mutation.
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73. 16.3 How Does Natural Selection Work?
Sexual selection favors traits that help an organism mate
Sexual selection is a type of selection that acts on traits that help an organism acquire a mate
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74. 16.3 How Does Natural Selection Work?
Sexual selection favors traits that help an organism mate (continued)
Sexual selection can be driven either by sexual contests among males or by female preference for particular male phenotypes
Male–male competition for access to females can favor the evolution of features that provide an advantage in fights or ritual displays of aggression
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75. Figure 16-9
Figure Competition between males favors the evolution, through sexual selection, of structures for ritual combat
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76. 16.3 How Does Natural Selection Work?
Sexual selection favors traits that help an organism mate (continued)
Female mate choice provides a second source of sexual selection
In some animal species, females often seem to prefer males with the most elaborate ornaments or most extravagant displays
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77. Figure 16-10
Figure The peacock’s showy tail has evolved through sexual selection
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78. 16.3 How Does Natural Selection Work?
Sexual selection favors traits that help an organism mate (continued)
One hypothesis is that male structures, colors, and displays that do not enhance survival might instead provide a female with an outward sign of a male’s condition
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79. 16.3 How Does Natural Selection Work?
Sexual selection favors traits that help an organism mate (continued)
A female that chooses the brightest, most ornamented male is also choosing the healthiest, most vigorous male
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80. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways
Directional selection favors individuals with an extreme value of a trait and selects against both average individuals and individuals at opposite extremes
Stabilizing selection favors individuals with the average value of a trait and selects against individuals with extreme values
Disruptive selection favors individuals at both extremes of a trait and selects against individuals with intermediate values
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81. Figure 16-11 Three ways that selection affects a population over time
Directional selection
Stabilizing selection
Disruptive selection
Larger-than-average
sizes favored.
Average sizes
favored.
Smaller-than-average and larger-
than-average sizes favored.
before
selection
percent of population
time
Figure Three ways that selection affects a population over time
after
selection
Average phenotype shifts
to larger size over time.
Average phenotype does not
change; phenotypic variability
declines.
Population divides into two
phenotypic groups over time.
trait, such as size
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82. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Directional selection shifts character traits in a specific direction
If environmental conditions change in a consistent way, a species may respond by evolving in a consistent direction
For example, if the climate becomes colder, mammal species may evolve thicker fur
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83. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Stabilizing selection acts against individuals that deviate too far from the average
Stabilizing selection commonly occurs when a trait is under opposing environmental pressures from two different sources
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84. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Stabilizing selection acts against individuals that deviate too far from the average (continued)
For example, Aristelliger lizards of intermediate body size are favored over the extremes
The smallest lizards have difficulty defending territory
The largest lizards are more likely to be eaten by owls
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85. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats
The population divides into two phenotypic groups over time
Beak size in black-bellied seedcrackers is subject to disruptive selection
Birds with large, stout beaks are able to crack the hard seeds found in their habitat
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86. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Beak size in black-bellied seedcrackers is subject to disruptive selection (continued)
Birds with small, pointy beaks are better adapted to process the soft seeds of the habitat
Birds with intermediate-size beaks have a lower survival rate than individuals with either large or small beaks
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87. Figure 16-12 Black-bellied seedcrackers
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88. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats (continued)
Black-bellied seedcrackers represent an example of balanced polymorphism, in which two or more phenotypes are maintained in a population
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89. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats (continued)
Each phenotype is favored by a separate environmental factor
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90. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats (continued)
Balanced polymorphism often occurs when environmental conditions favor heterozygotes
For example, normal and sickle-cell hemoglobin alleles coexist in malaria-prone regions of Africa
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91. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats (continued)
Balanced polymorphism often occurs when environmental conditions favor heterozygotes (continued)
Homozygotes for the sickle-cell allele suffer from severe anemia caused by clumping of the defective hemoglobin in their red blood cells, and often die
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92. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats (continued)
Ironically, the sickling of the red blood cells confers increased resistance to malaria, a deadly disease affecting red blood cells that is widespread in equatorial Africa
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93. 16.3 How Does Natural Selection Work?
Selection can influence populations in three ways (continued)
Disruptive selection adapts individuals within a population to different habitats (continued)
Heterozygotes have one allele for defective hemoglobin, which gives them increased resistance to malaria, and one allele for normal hemoglobin, which moderates their anemia, giving them a selective advantage over either type of homozygote
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94. A farmer uses an insecticide but still gets crop damage
A farmer uses an insecticide but still gets crop damage. Many of the targeted insects have developed insecticide resistance. What mode of natural selection has occurred?
Directional selection
Stabilizing selection
Artificial selection
Coevolution
Question: 16-13
Answer: a
Diff: Moderate
Text Ref: Section 16.3
Skill: Application
Notes:
This question shows students that natural selection can favor populations in different ways.
© 2017 Pearson Education, Ltd.

95. A farmer uses an insecticide but still gets crop damage
A farmer uses an insecticide but still gets crop damage. Many of the targeted insects have developed insecticide resistance. What mode of natural selection has occurred?
Directional selection
Stabilizing selection
Artificial selection
Coevolution
Question: 16-13
Answer: a
Diff: Moderate
Text Ref: Section 16.3
Skill: Application
Notes:
This question shows students that natural selection can favor populations in different ways.
© 2017 Pearson Education, Ltd.

96. In an island population of birds, the large birds eat the only seeds available, which are large, and the small birds feed on flower nectar. The medium-sized birds have a hard time eating both the seeds and the nectar. What mode of natural selection has occurred?
Directional selection
Stabilizing selection
Disruptive selection
Coevolution
Question: 16-14
Answer: c
Diff: Moderate
Text Ref: Section 16.3
Skill: Application
Notes:
This question shows students that natural selection can favor populations in different ways.
© 2017 Pearson Education, Ltd.

97. In an island population of birds, the large birds eat the only seeds available, which are large, and the small birds feed on flower nectar. The medium-sized birds have a hard time eating both the seeds and the nectar. What mode of natural selection has occurred?
Directional selection
Stabilizing selection
Disruptive selection
Coevolution
Question: 16-14
Answer: c
Diff: Moderate
Text Ref: Section 16.3
Skill: Application
Notes:
This question shows students that natural selection can favor populations in different ways.
© 2017 Pearson Education, Ltd.