17.1 Genes and Variation
Name:
Biology 5.0
Date:
Period:
Lesson Objectives
 Define evolution in genetic terms.
 Identify the main sources of genetic variation in a population.
 State what determines the number of phenotypes for a trait.
Lesson Summary
Genetics Joins Evolutionary Theory - Darwin’s original ideas can now be understood in genetic terms.
 Researchers discovered that traits are controlled by genes and that many genes have at least two forms,
or alleles. The combination of different alleles is an individual’s genotype. Natural selection acts on
phenotype, not genotype.
 Genetic variation and evolution are studied in populations. Members of a population share a common
group of genes, called a gene pool.
 Allele frequency is the number of times an allele occurs in a gene pool compared with the number of
times other alleles for the same gene occur. In genetic terms, evolution is any change in the allele
frequency in a population.
Sources of Genetic Variation - The three main sources of genetic variation are mutations, genetic
recombination during sexual reproduction, and lateral gene transfer.
 A mutation is any change in a sequence of DNA.
 Most heritable differences are due to genetic recombination during sexual reproduction. This occurs
during meiosis when each chromosome in a pair moves independently. Genetic recombination also
occurs during crossing-over in meiosis.
 Lateral gene transfer is the passing of genes from one organism to another organism that is not its
offspring.
Single-Gene and Polygenic Traits - The number of different phenotypes for a given trait depends on how
many genes control the trait.
 A single-gene trait is controlled by one gene. An example in snails is the presence or absence of dark
bands on their shells.
 A polygenic trait is controlled by two or more genes, and each gene often has two or more alleles. An
example of a human polygenic trait is height.
Genetics Joins Evolutionary Theory
Complete each statement by writing the correct word or words.
1. Natural selection works on an organism’s
2. A(n)
rather than its
.
consists of all the genes, including the alleles for each gene, that are
present in a population.
3. A gene pool typically contains different
for each heritable trait.
4. The number of times that an allele occurs in a gene pool compared with the number of times other alleles for
the same gene occur is called the
of the population.
1
Use the circle graph of a sample mouse population to answer the following questions.
5. In the diagram below, use circles to represent the alleles within each segment of the population. Draw the B
alleles as solid circles and the b alleles as outline circles.
a. The total number of individuals in this population is
; the total number of alleles is
.
b. How many alleles for black fur are in the sample population and what percentage of allele frequency
does that represent?
c. How many alleles for brown fur are in the sample population and what percentage of allele frequency
does that represent?
d. Describe how a geneticist might be able to tell that this population is evolving.
e. Can you determine whether an allele is dominant or recessive on the basis of the ratio of phenotypes in
the population? Explain your answer.
Sources of Genetic Variation
6. What are mutations? When do they affect evolution?
7. How does sexual reproduction affect a population’s genetic variation?
8. Identify two ways in which genes can be recombined during meiosis.
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9. Why is genetic variation important to the process of evolution?
Single Gene and Polygenic Traits
10. Label the two graphs to show which represents a single-gene trait and which represents a polygenic trait.
Write True if the statement is true. If the statement is false, change the underlined word or words to make the
statement true.
11.
The number of phenotypes produced for a given trait depends on how many genes
control the trait.
12.
Height in humans is an example of a single-gene trait.
13.
Each gene of a polygenic trait often has two or more phenotypes.
14.
A single polygenic trait often has many possible genotypes.
15.
A symmetrical bell-shaped graph is typical of polygenic traits.
Use the Venn diagram to compare and contrast single-gene traits and polygenic traits.
Single-Gene Traits
Both
Polygenic Traits
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Concept Map A concept map helps you see how the topics you read about are related to one another. Use the
words and phrases below to fill in the empty spaces in the concept map.
Genetic recombination
Chromosomes
Genes
Crossing-over
Sources of Genetic
Variation
include
Lateral gene
transfer
Mutations
happens during
involve changes in
Meiosis
Single-Gene and Polygenic Traits
Polygenic Traits Some traits are controlled by only one gene. They
are called single-gene traits. Other traits are controlled by two or
more genes. They are called polygenic traits. Human height is an
example of a polygenic trait.
1. Plot the points and draw a curve connecting the points.
What shape is the curve you drew?
a. bell curve
b. one-sided curve
c. irregular curve
2.
a.
b.
c.
d.
Height in cm
Number of Students
155–159
160–164
165–169
170–174
175–179
180–184
185-189
190–194
1
2
6
10
10
6
2
1
What is the average height for this population?
155–164 cm
165–174 cm
170–179 cm
185–194 cm
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17.2 Evolution as Genetic Change in Populations
Lesson Objectives
 Explain how natural selection affects single-gene and polygenic traits.
 Describe genetic drift.
 Explain how different factors affect genetic equilibrium.
Lesson Summary
How Natural Selection Works - Natural selection on a single-gene trait can lead to changes in allele
frequencies and changes in phenotype frequencies. For polygenic traits, populations often exhibit a range of
phenotypes for a trait. When graphed, this range usually forms a bell curve, with fewer individuals exhibiting
the extreme phenotypes than those with the average (in the case of beak size, the extremes may be tiny and
large beaks). Natural selection on polygenic traits can cause shifts to the bell curve depending upon which
phenotype is selected for.
 Directional selection takes place when individuals at one end of the bell curve have higher fitness than
those near the middle or at the other end of the curve. For example, when large seeds are plentiful, largebeaked birds in a population may be selected for.
 Stabilizing selection takes place when individuals near the middle of the curve have higher fitness than
individuals at either end.
 Disruptive selection takes place when individuals at the upper and lower ends of the curve have higher
fitness than individuals near the middle.
Genetic Drift In small populations, alleles can become more or less common simply by chance. This kind of
change in allele frequency is called genetic drift.
 The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a
population.
 The founder effect is a change in allele frequency that may occur when a few individuals from a population
migrate to and colonize a new habitat.
Evolution Versus Genetic Equilibrium If allele frequencies in a population do not change, the population is in
genetic equilibrium. Evolution is not taking place.
 The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless
one or more factors cause those frequencies to change. These factors include: non-random mating, small
population size, immigration or emigration, mutations, and natural selection.
 Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring. For example, many
species exhibit non-random mating patterns. Sexual selection, or the process in which an individual chooses
its mate based on heritable traits (such as size or strength), is a common practice for many organisms.
How Natural Selection Works
1. If a trait made an organism less likely to survive and reproduce, what would happen to the allele for that
trait?
2. If a trait had no effect on an organism’s fitness, what would likely happen to the allele for that trait?
5
Use the table showing the evolution of a population of mice to answer the following questions.
Initial Population
Generation 10
Generation 20
Generation 30
90%
80%
70%
40%
10%
20%
30%
60%
3. Is the trait for fur color a single-gene trait or a polygenic trait? Explain your answer.
4. Describe how the relative frequency of fur color alleles is changing in this population and propose one
explanation for this change.
5. Suppose a mutation causes a white fur phenotype to emerge in the population. What might happen to the
mouse population after 40 generations?
6. What effect does stabilizing selection have on variation in a population?
Match the type of selection with the correct situation.
Type of Selection
7.
Directional
8.
Stabilizing
9.
Disruptive
Situation
A. Individuals at the upper and lower ends of the curve
have higher fitness than individuals near the middle.
B. Individuals at one end of the curve have higher fitness
than individuals in the middle or at the other end.
C. Individuals near the center of the curve have higher
fitness than individuals at either end.
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Draw the missing line in the graph on the right to show how disruptive selection affects beak size.
Genetic Drift - complete each statement by writing the correct word or words.
10. In small populations, random changes in
is called genetic drift.
11. A situation in which allele frequencies change as a result of the migration of a small subgroup of a
population is known as the
.
12. The
is a change in allele frequency following a dramatic
reduction in the size of a population.
Number of Birds in
Population
13. Label each graph that shows stabilizing selection and disruptive selection.
Percentage of
Human
Population
original
populatio
Beak Size
Birth Weight
_______________________________
original
population
_______________________________
14. With which type of selection do organisms in the middle of the curve have the highest fitness?
15. In disruptive selection, organisms on which part of the curve have the lowest fitness?
16. How does the curve change in stabilizing selection?
a. The curve becomes shorter and wider.
b. The curve becomes taller and narrower.
c. The curve moves to the right.
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17. The plants in an area have either very small or very large seeds. Birds with small beaks can eat small seeds,
and birds with large beaks can eat large seeds. Birds with smaller beaks and birds with larger beaks become
more common than birds with medium beaks. What type of selection is this?
a. Stabilizing
b. Directional
c. Disruptive
18. Suppose a population of insects live in a sandy habitat. Some of the insects have tan bodies and some have
green bodies. Over time, the habitat changes to a grass-filled meadow. Use the ideas of natural selection to
explain how and why the insect population might change.
Evolution Versus Genetic Equilibrium
19. What does the Hardy-Weinberg principle state?
20. What is genetic equilibrium?
21. Explain how sexual selection results in non-random mating.
The Hardy-Weinberg Principle lists the five conditions in which evolution does not occur in a population.
Read the chart below. Use phrases in the word box to identify each condition that leads to genetic equilibrium.
The first row has been completed for you.
large population
random mating
no natural selection
no mutations
no movement
The Hardy-Weinberg Principle
The more individuals in the population, the smaller the effect of
genetic drift.
No changes to genes means new alleles are not introduced into
the population’s gene pool.
Each individual in a population has the same chance of passing
on its alleles.
No new alleles are introduced into the population’s gene pool by
new individuals.
No phenotype can have a selective advantage over another—all
individuals have equal fitness.
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Read the descriptions below. Use what you know about the Hardy-Weinberg principle to identify what is
happening.
22. Genetic drift strongly affects the population.
A. small population
B. random mating
C. no movement into or out of the population
D. no natural selection
23. Lions with a darker fur color have the same chance to reproduce as lions with a lighter fur color.
A. large population
C. random mating
B. no mutations
D. no natural selection
24. Fitness is basically the same among individuals in the population.
A. large population
C. random mating
B. no mutations
D. no natural selection
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17.3 The Process of Speciation
Lesson Objectives
 Identify the types of isolation that can lead to the formation of new species.
 Describe the current hypothesis about Galápagos finch speciation.
Lesson Summary
Isolating Mechanisms Speciation is the formation of new species. For one species to evolve into two new
species, the gene pools of two populations must become separated, or reproductively isolated. Reproductive
isolation occurs when members of two populations do not interbreed and produce fertile offspring.
Reproductive isolation can develop through behavioral, geographic, or temporal isolation.



Behavioral isolation occurs when populations have different courtship rituals or other behaviors involved
in reproduction.
Geographic isolation occurs when populations are separated by geographic barriers, such as mountains or
rivers.
Temporal isolation occurs when populations reproduce at different times.
Speciation in Darwin’s Finches Peter and Rosemary Grant’s work supports the hypothesis that speciation in
the Galápagos finches was, and still continues to be, a result of the founder effect and natural selection.
 Speciation in Galápagos finches may have occurred in a sequence of events that involved the founding of a
new population, geographic isolation, changes in the gene pool, behavioral isolation, and ecological
competition.
 For example, a few finches may have flown from mainland South America to one of the islands. There, they
survived and reproduced. Some birds may have crossed to a second island, and the two populations became
geographically isolated. Seed sizes on the second island could have favored birds with larger beaks, so the
population on the second island evolved into a population with larger beaks. Eventually, these large-beaked
birds became reproductively isolated and evolved into a new species.
Isolating Mechanisms
1. What is speciation?
2. What does it mean for two species to be reproductively isolated from each other?
3. What must happen in order for a new species to evolve?
4. Many finch characteristics appear in bell-shaped distributions typical of
traits.
5. The ancestors of the Galápagos Island finches originally came from the continent of
.
6. The populations of finches on separate islands are
isolated from one another by
large stretches of open water.
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Populations Become Isolated - Read the chart below. Then, use phrases in the word box to identify each
process that leads to speciation. The first row has been completed for you.
changes in the gene pool
geographic isolation
founders arrive
competition
behavioral isolation
Process
How It Leads to Speciation
A population arrives in a new place.
Populations are separated by a geographic barrier and do not share a gene
pool.
Populations evolve new traits in response to natural selection in their
environments.
Species evolve in a way that reduces competition between them.
Groups within a population are separated by different courtship rituals.
Speciation in Darwin’s Finches - Charles Darwin proposed that the different species of finches on the
Galapagos Islands once had a common ancestor. Over time, he said, natural selection led to each species of
finch. A current hypothesis says that speciation in the Galapagos finches occurred through the founding of a
new population, geographic isolation, changes in the new population’s gene pool, behavioral isolation, and
ecological competition. Use the phrases in the box to complete the chart.
insects that live inside dead wood
small seeds
large, thick-shelled seeds
Galápagos Islands Finches
Shape of head
and beak
Main food
Feeding
adaptation
Habitat
fruits
Parrot-like beak
uses cactus spines
pointed crushing
beak
large crushing beak
trees
trees
ground
ground
7. How does the large crushing beak help the fourth finch survive?
8. Circle the finch that would be least likely to survive if the insect population decreased.
9. Both the first and second finch live in trees. Suppose one season there is less fruit in the trees. Which finch
is more likely to survive and reproduce?
A. the first finch
B. the second finch
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Term
Definition
The number of times an allele appears in a gene pool, compared to the
total number of alleles in that pool for the same gene.
A common group of genes, and all their alleles, shared by a population
A trait controlled by two or more genes
A trait controlled by only one gene
A change in an allele’s frequency following a dramatic reduction in
population size
Form of natural selection in polygenic traits in which the entire curve
shifts because some individuals are more successful at surviving
Form of natural selection in polygenic traits in which the curve splits in
two because individuals at the two ends are more successful at surviving
than those in the center
A change in an allele’s frequency that occurs as a result of a migration
of a small part of the population
A random change in an allele’s frequency
When all of the alleles in a population remain the same
The process of selecting a mate based on traits like strength or color
Form of natural selection in polygenic traits in which the center of the
curve stays in the same position because the individuals in the center are
more successful at surviving
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