Download Ch. 16 Evolution of Populations

16-1 Genes
and
Genes
and Variation
16-1
Variation
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How Common Is Genetic
Variation?

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Many genes have at
least two forms, or
alleles.
All organisms have
genetic variation
An individual organism is
heterozygous for many
genes.
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Genetic variation is
studied in populations.
A population is a group of
individuals of the same
species that interbreed.
A gene pool consists of
all genes, including all
the different alleles, that
are present in a
population.
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Variation and Gene Pools
Variation and Gene Pools
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The relative frequency of an allele is the
number of times the allele occurs in a gene
pool, compared with the number of times
other alleles for the same gene occur.
Relative frequency is often expressed as a
percentage.
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Variation and Gene Pools

Gene Pool for Fur Color in Mice
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Variation and Gene Pools

In genetic terms, evolution is any change in
the relative frequency of alleles in a
population.
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Sources of Genetic Variation

The two main sources of genetic variation
are
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
mutations
genetic shuffling that results from sexual
reproduction.
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Sources of Genetic Variation

Mutations
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A mutation is any change in a sequence of DNA.
Mutations occur because of mistakes in DNA
replication or as a result of radiation or chemicals
in the environment.
Mutations do not always affect an organism’s
phenotype.
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
Gene Shuffling
 Most heritable differences
are due to gene shuffling.
 Crossing-over increases
the number of genotypes
seen in offspring.
 Sexual reproduction
produces different
phenotypes, but does not
change the relative
frequency of alleles in a
population.
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Sources of Genetic Variation
Single-Gene and Polygenic
Traits

The number of phenotypes produced for a
given trait depends on how many genes
control the trait.
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Single-Gene and Polygenic
Traits

A single-gene trait is
controlled by one
gene that has two
alleles. Variation in
this gene leads to
only two possible
phenotypes.
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Single-Gene and Polygenic
Trait
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The allele for a widow’s peak is dominant
over the allele for a hairline with no peak.
However, the presence of a widow’s peak
may be less common in a population.
In real populations, phenotypic ratios are
determined by the frequency of alleles as
well as by whether the alleles are dominant
or recessive.
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Single-Gene and Polygenic
Trait
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Many traits are
controlled by two or
more genes and are
called polygenic traits.
One polygenic trait can
have many possible
genotypes and
phenotypes.
Height in humans is a
polygenic trait.
Normal Distribution
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Which of the following statements is TRUE?
a) The relative frequency of an allele is not related
to whether the allele is dominant or recessive.
b) Mutations always affect an organism's
phenotype.
c) Crossing over decreases the number of different
genotypes that appear in an offspring.
d) Evolution does not affect the frequency of genes
in a gene pool.
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Most inheritable differences are a result of
a)
b)
c)
d)
gene shuffling.
frequency of alleles.
mutations.
DNA replication.
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The main sources of inherited variation are
a)
b)
c)
d)
gene shuffling and mutations.
gene pools and frequencies.
single-gene and polygenic traits.
genotypes and phenotypes.
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
A widow's peak in humans is an example of
a(an)
a)
b)
c)
d)
invariable trait.
single-gene trait.
polygenic trait.
mutation.
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A graph of the length of the little finger on the left
hand versus the number of people having fingers
of a particular length is a bell-shaped curve. This
indicates that finger length is a
a)
b)
c)
d)
single-gene trait.
polygenic trait.
randomly inherited trait.
strongly selected trait.
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16-2Evolution
Evolution
as Genetic
as Genetic
Change Change
16-2
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16-2 Evolution as Genetic
Change
Evolution as Genetic Change
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Natural selection affects which individuals
survive and reproduce and which do not.
If an individual dies without reproducing, it
does not contribute its alleles to the
population’s gene pool.
If an individual produces many offspring, its
alleles stay in the gene pool and may
increase in frequency.
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16-2 Evolution as Genetic
Change
Evolution as Genetic Change
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Evolution is any change over time in the
relative frequencies of alleles in a
population.
Populations, not individual organisms, can
evolve over time.
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Natural Selection on
Single-Gene Traits
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Natural selection on single-gene traits can
lead to changes in allele frequencies and
thus to evolution.
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Natural Selection on
Single-Gene Traits
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Organisms of one color
may produce fewer
offspring than
organisms of other
colors.
Those most visible to
predators will be less
likely to survive and
reproduce.
Therefore, that allele
will become rare.
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pology/HumanGeneticEvolution/NaturalSelection/ketlwell.jpg
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Natural Selection on
Single-Gene Traits
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Black lizards may
warm up faster on cold
days. This may give
them energy to avoid
predators. In turn, they
may produce more
offspring.
The allele for black
color will increase in
relative frequency.
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Natural Selection on
Polygenic Traits
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Natural selection can affect the distributions
of phenotypes in any of three ways:
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directional selection
stabilizing selection
disruptive selection
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Natural Selection on
Polygenic Traits
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Directional Selection
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When individuals at one
end of the curve have
higher fitness than
individuals in the middle
or at the other end,
directional selection
takes place.
The range of phenotypes
shifts as some individuals
survive and reproduce
while others do not.
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Natural Selection on
Polygenic Traits
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Stabilizing Selection
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When individuals near
the center of the curve
have higher fitness than
individuals at either end
of the curve, stabilizing
selection takes place.
This keeps the center of
the curve at its current
position, but it narrows
the overall graph.
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Natural Selection on
Polygenic Traits
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Human babies born
at an average mass
are more likely to
survive than babies
born either much
smaller or much
larger than average.
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Natural Selection on
Polygenic Traits
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Disruptive Selection
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When individuals at the
extreme ends of the
curve have higher
fitness than individuals
near the middle
If the pressure of natural
selection is strong
enough and long
enough, the curve will
split, creating two
distinct phenotypes.
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Natural Selection on
Polygenic Traits
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If average-sized
seeds become
scarce, a bird
population will split
into two groups: one
that eats small seeds
and one that eats
large seeds.
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Genetic Drift
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A random change in allele frequency is
called genetic drift.
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Genetic Drift
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In small populations, individuals that carry a
particular allele may leave more descendants
than other individuals do, just by chance.
Over time, a series of chance occurrences of
this type can cause an allele to become
common in a population.
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Genetic Drift
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Genetic drift may occur when a small group
of individuals colonizes a new habitat.
Individuals may carry alleles in different
relative frequencies than did the larger
population from which they came.
The new population will be genetically
different from the parent population.
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Genetic Drift
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When allele frequencies change due to
migration of a small subgroup of a
population it is known as the founder
effect.
Original Population
Founding
Populations
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Descendent
Populations
Evolution Versus Genetic
Equilibrium
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The Hardy-Weinberg principle states that
allele frequencies in a population will
remain constant unless one or more factors
cause those frequencies to change.
When allele frequencies remain constant it
is called genetic equilibrium.
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Evolution Versus Genetic
Equilibrium
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Five conditions are required to maintain
genetic equilibrium from generation to
generation:
1) there must be random mating,
2) the population must be very large,
3) there can be no movement into or out of the
population,
4) there can be no mutations, and
5) there can be no natural selection.
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Evolution Versus Genetic
Equilibrium
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Random Mating
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Random mating ensures that each individual has
an equal chance of passing on its alleles to
offspring.
In natural populations, mating is rarely
completely random. Many species select mates
based on particular heritable traits.
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Evolution Versus Genetic
Equilibrium
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Large Population
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Genetic drift has less effect on large populations
than on small ones.
Allele frequencies of large populations are less
likely to be changed through the process of
genetic drift.
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Evolution Versus Genetic
Equilibrium
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No Movement Into or Out of the Population
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Because individuals may bring new alleles into a
population, there must be no movement of
individuals into or out of a population.
The population's gene pool must be kept
together and kept separate from the gene pools
of other populations.
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Evolution Versus Genetic
Equilibrium
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No Mutations
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If genes mutate, new alleles may be introduced
into the population, and allele frequencies will
change.
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Evolution Versus Genetic
Equilibrium
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No Natural Selection
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All genotypes in the population must have
equal probabilities of survival and
reproduction.
No phenotype can have a selective
advantage over another.
There can be no natural selection operating
on the population.
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Which of the following patterns of natural selection
on polygenic traits favors both extremes of a bell
curve?
a)
b)
c)
d)
stabilizing selection
disruptive selection
directional selection
genetic drift
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Which of the following events could lead to genetic
drift?
a) A few new individuals move into a large, diverse
population.
b) A few individuals from a large, diverse population
leave and establish a new population.
c) Two large populations come back together after a
few years of separation.
d) The mutation rate in a large population increases
due to pollution.
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The situation in which allele frequencies remain
constant in a population is known as
a)
b)
c)
d)
genetic drift.
the founder effect.
genetic equilibrium.
natural selection.
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Which of the following conditions is required to
maintain genetic equilibrium in a population?
a)
b)
c)
d)
movement in or out of the population
random mating
natural selection
small population
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According to the Hardy-Weinberg principle, no
evolution will take place if
a) all five of the Hardy-Weinberg conditions are met.
b) any one of the Hardy-Weinberg conditions is met.
c) at least three of the Hardy-Weinberg conditions are
met.
d) none of the Hardy-Weinberg conditions are met.
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END OF SECTION
The Process of Speciation
16-3
16-3 The Process of Speciation
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The Process of Speciation
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Natural selection and chance events can
change the relative frequencies of alleles in
a population and lead to speciation.
Speciation is the formation of new species.
A species is a group of organisms that
breed with one another and produce fertile
offspring.
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Isolating Mechanisms
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What factors are involved in the formation
of new species?
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The gene pools of two populations must become
separated for them to become new species.
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Isolating Mechanisms
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As new species evolve, populations
become reproductively isolated from each
other.
When the members of two populations
cannot interbreed and produce fertile
offspring, reproductive isolation has
occurred.
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Isolating Mechanisms
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Reproductive isolation can develop in a
variety of ways, including:
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behavioral isolation
geographic isolation
temporal isolation
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Isolating Mechanisms

Behavioral Isolation
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occurs when two
populations are
capable of
interbreeding but have
differences in
courtship rituals or
other reproductive
strategies that involve
behavior.
Figure 14.3B
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Isolating Mechanisms
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Geographic Isolation
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occurs when two populations are separated by
geographic barriers such as rivers or mountains.
A. harrisi
A. leucurus
Figure 14.4
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Isolating Mechanisms
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Geographic barriers do
not guarantee the
formation of new species.
If two formerly separated
populations can still
interbreed, they remain a
single species.
Potential geographic
barriers may separate
certain types of
organisms but not others.
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Isolating Mechanisms

Temporal Isolation
occurs when two or more species reproduce at
different times.
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Testing Natural Selection
in Nature
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Studies showing natural selection in action involve
descendants of the finches that Darwin observed in the
Galápagos Islands.
The finches Darwin saw were different, but he
hypothesized that they had descended from a common
ancestor.
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Testing Natural
Selection
in Nature

Peter and Rosemary Grant tested Darwin’s
hypothesis, which relied on two testable
assumptions:
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For beak size and shape to evolve, there must
be enough heritable variation in those traits to
provide raw material for natural selection.
Differences in beak size and shape must
produce differences in fitness, causing natural
selection to occur.
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Testing Natural Selection
in Nature
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The Grants tested these hypotheses on the
medium ground finch on Daphne Major,
one of the Galápagos Islands.
During the rainy season, there is plenty of
food.
During droughts, food becomes scarce.
Individual birds with different-sized beaks
had different chances of survival during a
drought.
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Testing Natural Selection
in Nature

When food was scarce, individuals with
large beaks were more likely to survive.
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Testing Natural Selection
in Nature


The Grants provided
evidence of the
process of evolution.
Beak size can be
changed by natural
selection.
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Speciation in Darwin's Finches
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Speciation in the Galápagos finches
occurred by:
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founding of a new population
geographic isolation
changes in new population's gene pool
reproductive isolation
ecological competition
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Speciation in Darwin's Finches

A few finches—
species A—travel
from South America
to one of the
Galápagos Islands.

There, they survive
and reproduce.
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Speciation in Darwin's Finches
Some birds from
species A cross
to a second
island.

Geographic Isolation
The two
populations no
longer share a
gene pool.
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Speciation in Darwin's Finches
Seed sizes on
the second island
favor birds with
large beaks.

Changes in the Gene
Pool
The population
on the second
island evolves
into population B,
with larger beaks.
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Speciation in Darwin's Finches

Reproductive
Isolation
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If population B birds
cross back to the first
island, they will not
mate with birds from
population A.
Populations A and B
are separate species.
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Speciation in Darwin's Finches

Ecological
Competition
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As species A and B
compete for available
seeds on the first
island, they continue
to evolve in a way that
increases the
differences between
them.
A new species—C—
may evolve.
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Speciation in Darwin's Finches

Continued Evolution

This process of isolation, genetic change, and
reproductive isolation probably repeated itself
often across the entire Galápagos island chain.
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Studying Evolution Since
Darwin


Scientific evidence supports the theory that
living species descended with modification
from common ancestors that lived in the
ancient past.
Scientists predict that as new fossils are
found, they will continue to expand our
understanding of how species evolved.
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When two species do not reproduce because of
differences in mating rituals, the situation is
referred to as
a)
b)
c)
d)
temporal isolation.
geographic isolation.
behavioral isolation.
reproductive isolation.
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The most important factor involved in the evolution
of the ground squirrels of the Grand Canyon
appears to be
a)
b)
c)
d)
temporal isolation.
geographic isolation.
behavioral isolation.
different food sources.
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One finding of the Grants' research on generations
of Galápagos finches was that
a) natural selection did not occur in the finches
b) natural selection can take place often and very
rapidly.
c) beak size had no effect on survival rate of the
finches.
d) natural selection was slow and permanent.
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All of the following played a role in speciation of
Galápagos finches EXCEPT
a)
b)
c)
d)
no changes in the gene pool.
separation of populations.
reproductive isolation.
natural selection.
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Beak size in the various groups of Galápagos
finches changed primarily in response to
a)
b)
c)
d)
climate.
mating preference.
food source.
availability of water.
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