Download Evolution of Populations

Name
Class
Date
Evolution
of Populations
Evolution
Q: How can populations evolve to form new species?
WHAT I KNOW
17.1 How
do genes make
evolution possible?
17.2 What
makes a
population’s gene
pool change?
17.3 How do
new species form?
17.4 What can
genes tell us about
an organism’s
evolutionary
history?
WHAT I LEARNED
SAMPLE ANSWER:
There are
different variations of the
same gene.
Evolution occurs
when the allele frequency in
the gene pool of a population
changes over time.
SAMPLE ANSWER:
Over time useful
traits (and the genes that
control them) accumulate in a
population.
SAMPLE ANSWER:
Genetic changes
can affect the number and
types of possible phenotypes
organisms in a population can
have. These changes provide
the variation that populations
need to evolve.
SAMPLE ANSWER:
The genome
of a species changes enough
that it becomes a new species.
SAMPLE ANSWER:
Speciation
sometimes occurs when
populations become
reproductively isolated.
SAMPLE ANSWER:
SAMPLE ANSWER: A molecular
clock uses mutation rates
in DNA to estimate the time
that two species have been
evolving independently.
Some species’
genomes are very similar.
These species are closely
related. The opposite is true
for species with very different
genomes.
SAMPLE ANSWER:
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Class
Date
17.1 Genes and Variation
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
For Questions 1–4, complete each statement by writing the correct word or words.
1. Natural selection works on an organism’s
phenotype
rather than its
genotype
2. A(n) gene pool consists of all the genes, including the alleles for each gene, that are
present in a population.
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268
.
Name
Class
alleles
3. A gene pool typically contains different
Date
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
allele frequency
of the population.
Use the circle graph of a sample mouse population to answer Questions 5–8.
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. The total number of individuals in this population is
number of alleles is
50
25
; the total
.
Sample Population
12 individuals:
heterozygous
black (Bb)
12
12
8
9 individuals:
homozygous
brown (bb)
4 individuals:
homozygous
black (BB)
18
6. How many alleles for black fur are in the sample population and what percentage of allele
frequency does that represent?
20 B alleles, 40 percent
7. How many alleles for brown fur are in the sample population and what percentage of
allele frequency does that represent?
30 b alleles, 60 percent
8. Describe how a geneticist might be able to tell that this population is evolving.
The frequency of alleles will change.
9. Can you determine whether an allele is dominant or recessive on the basis of the ratio of
phenotypes in the population? Explain your answer.
No, because the phenotypic ratio depends on the allele frequencies of the dominant
and recessive alleles, and the frequency of alleles has nothing to do with whether the
allele is dominant or recessive.
Sources of Genetic Variation
10. What are mutations? When do they affect evolution?
A mutation is any change in the genetic material of a cell. Mutations only affect
evolution when they occur in germ line cells that produce eggs or sperm and if they
produce a change in phenotype that affects fitness.
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11. How does sexual reproduction affect a population’s genetic variation?
Genetic recombination during sexual reproduction can produce many different phenotypes through the production of new and unique genetic combinations.
12. Identify two ways in which genes can be recombined during meiosis.
by independent assortment of chromosomes and by gene swapping during meiosis
13. What is lateral gene transfer? How does it affect variation?
Lateral gene transfer occurs when genes are passed from one organism to another
organism that is not its offspring. It can occur between organisms of the same or different species. Lateral gene transfer increases variation when a species picks up new
genes from a different species.
Single Gene and Polygenic Traits
Relative Frequency
of Phenotype (%)
Frequency of Phenotype
14. Label the two graphs to show which represents a single-gene trait and which represents a
polygenic trait.
100
80
60
40
20
0
Without bands
With bands
Phenotype
Phenotype (height)
Single-Gene Trait
Polygenic Trait
For Questions 15–19, write True if the statement is true. If the statement is false, change the
underlined word or words to make the statement true.
True
polygenic
15. The number of phenotypes produced for a given trait depends on how
many genes control the trait.
16. Height in humans is an example of a single-gene trait.
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alleles
Date
17. Each gene of a polygenic trait often has two or more phenotypes.
True
18. A single polygenic trait often has many possible genotypes.
True
19. A symmetrical bell-shaped graph is typical of polygenic traits.
20. Use the Venn diagram to compare and contrast single-gene traits and polygenic traits.
Single-Gene Traits
Controlled by only one
gene; may only have
two or three distinct
phenotypes
Polygenic Traits
Both
Controlled
by genes
Controlled by two or
more genes; may have
many phenotypes that
are not clearly distinct
from one another
21. Why is genetic variation important to the process of evolution?
Genetic variation is the raw material of evolution, which can lead to different
members of a population having different levels of fitness in a certain environment.
The variation allows species to adapt to changes in their environment. Without such
variation, the population would not evolve.
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Class
Date
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, large-beaked 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.
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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? Fewer copies of the allele would pass to future generations and the
allele could even disappear from the gene pool completely.
2. If a trait had no effect on an organism’s fitness, what would likely happen to the allele for
that trait? The allele would not be under pressure from natural selection, and its
frequency would probably stay about the same.
Use the table showing the evolution of a population of mice to answer Questions 3–5.
Initial Population
Generation 10
90%
10%
Generation 20
Generation 30
80%
70%
40%
20%
30%
60%
3. Is the trait for fur color a single-gene trait or a polygenic trait? Explain your answer.
The fur color is controlled by a single gene. There are only two phenotypes for this
trait, gray or black fur.
4. Describe how the relative frequency of fur color alleles is changing in this population and
propose one explanation for this change.
The lighter fur color allele is decreasing in frequency and the darker fur color allele is
increasing in frequency. Darker mice may be harder for predators to see, so they are
more likely to survive and reproduce.
5. Suppose a mutation causes a white fur phenotype to emerge in the population. What
might happen to the mouse population after 40 generations?
SAMPLE ANSWER:
If individuals with the new phenotype are more fit than the gray or
black mice, the white allele may increase in frequency in the population. Black mice
will likely continue to be more common than the other phenotypes.
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6. What effect does stabilizing selection have on variation in a population?
Stabilizing selection would generally reduce the variation in a population.
For Questions 7–9, match the type of selection with the correct situation.
Type of Selection
Situation
B
_____
7. Directional
A. Individuals at the upper and lower ends of the
curve have higher fitness than individuals near the
middle.
C
_____
8. Stabilizing
A
_____
9. Disruptive
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.
10.
Draw the missing line in the graph on the right to show how
disruptive selection affects beak size.
Disruptive Selection
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
Genetic Drift
For Questions 11–13, complete each statement by writing the correct word or words.
11. In small populations, random changes in allele frequencies is called genetic drift.
12. A situation in which allele frequencies change as a result of the migration of a small
subgroup of a population is known as the
13. The
bottleneck effect
founder effect
is a change in allele frequency following a dramatic
reduction in the size of a population.
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14. Complete the concept map.
Genetic Drift
can resultl ffrom
Founder Effect
Bottleneck Effect
caused
db
by
caused
db
by
a dramatic reduction in the
size of a population
the migration of a small
subgroup of a population
Evolution Versus Genetic Equilibrium
15. What does the Hardy-Weinberg principle state? Allele frequencies in a population
should remain constant unless one or more factors cause them to change.
16. What is genetic equilibrium? the situation in which allele frequencies remain constant
17. List the five conditions that can disturb genetic equilibrium and cause evolution to occur.
non-random mating, small population size, immigration or emigration, mutations,
and natural selection
18. Explain how sexual selection results in non-random mating.
When an individual practices sexual selection, or choosing a mate based on heritable
characteristics such as size and strength, this individual’s mate choice is not random.
19. 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.
In the original sandy habitat, tan insects may have been camouflaged from
predators, making them more successful than green insects. When the habitat
changes to a green, grassy meadow, individuals with green bodies may become
more successful at hiding from predators. The green-bodied insects may survive and
produce more offspring than the tan-bodied insects. Over time the frequency of the
green-bodied allele would probably increase.
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