Download Evolution of Populations

15-3 Shaping Evolutionary Theory:
Georgia Performance Standards:
SB5b: Explain the history of life in terms of
biodiversity, ancestry, and the rates of evolution.
SB5d: Relate natural selection to changes in
organisms.
Essential Questions:
1.
2.
Why is important to understand evolutionary theory?
What is the role of natural selection in speciation?
Gene Pools:
• All members of a population can interbreed, they
share a common group of genes, called a gene
pool.
– A gene pool is the combined genetic information
of all the members of a particular population.
• Typically contains two or more alleles—or forms of a
certain gene—for each inheritable trait.
– The relative frequency of an allele is the
number of times that allele occurs in a gene pool
compared with the number of times other alleles
occur.
Relative Frequencies of Alleles
Section 16-1
Sample Population
Frequency of Alleles
allele for brown fur
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
allele for black fur
Evolution as Genetic Change
• Natural selection on single-gene traits
can lead to changes in allele
frequencies and, thus, to evolution.
– Ex: Color Mutations
Single-Gene and Polygenic Traits
• Inheritable variation can be expressed in a
variety of ways.
• The number of phenotypes produced
for a given trait depends on how many
genes control the trait
Single-gene trait
• Trait controlled by a
single gene
• Variation in this gene
leads to only two distinct
phenotypes
• The number of
phenotypes a given trait
has is determined by how
many genes control the
trait.
In humans, having a widow’s
peak or not having a widow’s
peak is controlled by a single
gene with two alleles. As a result,
only two phenotypes are
possible.
Polygenic Traits:
• Most traits are controlled by
two or more genes and are,
therefore, called polygenic
traits.
• Each gene of a polygenic
trait often has two or more
alleles.
• As a result, one polygenic
trait can have many possible
genotypes and even more
possible phenotypes.
EX: height (A bell-shaped curve is
also called a normal distribution)
Evolution as Genetic Change
• Natural selection
– does not act
directly on genes,
but on phenotypes.
– affects which
individuals having
different
phenotypes survive
and reproduce and
which do not.
– determines which
alleles are passed
from one
generation to the
next.
– can change the
relative
frequencies of
alleles in a
population over
time.
Exactly what factors change the relative
frequencies of alleles in a population?
• In genetic terms, any factor that causes alleles
to be added to or removed from a population
will change the relative frequencies of alleles.
• Evolution is any change in the relative
frequencies of alleles in a population’s gene
pool.
• Evolution acts on populations, not on
individuals.
Natural Selection on Single-Gene Traits
• Natural selection on single-gene traits
can lead to changes in allele
frequencies and, thus, to evolution.
– EX: Color Mutations (organisms of one color
may produce fewer offspring than organisms
of another color.
Equilibrium v/s Evolution
• The Hardy-Weinberg principle states that
allele frequencies in a population will remain
constant unless one or more factors cause those
frequencies to change.
• The situation in which allele frequencies remain
constant is called genetic equilibrium (juh-netik ee-kwih-lib-ree-um).
• If the allele frequencies do not change, the
population will not evolve.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 This equation allows us to determine the
equilibrium frequency of each genotype in
the population.
 Homozygous dominant (p2)
 Heterozygous (2pq)
 Homozygous recessive (q2)
13.8 The gene pool of a nonevolving population
remains constant over the generations
• Hardy-Weinberg equilibrium
states that the shuffling of
genes during sexual
reproduction does not alter
the proportions of different
alleles in a gene pool
– To test this, let’s look at an
imaginary, nonevolving
population of blue-footed
boobies
Webbing
No webbing
Figure 13.8A
• We can follow alleles in a population to
observe if Hardy-Weinberg equilibrium
exists
Phenotypes
Genotypes
WW
Ww
ww
Number of animals
(total = 500)
320
160
20
Genotype frequencies
320/
500
= 0.64
Number of alleles
in gene pool
(total = 1,000)
640 W
Allele frequencies
800/
1,000
160/
500
20/
= 0.32
160 W + 160 w
= 0.8 W
200/
1,000
500
= 0.04
40 w
= 0.2 w
Figure 13.8B
Recombination
of alleles from
parent generation
SPERM
EGGS
WW
p2 = 0.64
WW
qp = 0.16
Ww
pq = 0.16
ww
q2 = 0.04
Next generation:
Genotype frequencies
Allele frequencies
0.64 WW
0.32 Ww
0.8 W
0.04 ww
0.2 w
Figure 13.8C
Understanding Evolution: Problem-based discussion
Population genetics calculations and Hardy Weinberg
1) What are the genotype frequencies at the flower-color locus in this
generation?
2) What are the allele frequencies at the flower-color locus in this generation?
3) If the individuals in this generation mate randomly, what would you expect the
genotype frequencies to be in the next generation?
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Understanding Evolution: Problem-based discussion
Population genetics calculations and Hardy Weinberg
4) If the individuals in this generation mate randomly, what would you expect the
allele frequencies to be in Generation 2?
5) If the resulting individuals in Generation 2 mate randomly, what would you
expect the allele frequencies to be in Generation 3?
6) Is this population evolving? Why or why not?
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Process of Speciation:
• Factors such as natural selection and
chance events can change the relative
frequencies of alleles in a population.
• But how do these changes lead to the
formation of new species, or
speciation?
Process of Speciation:
• A species as a group of organisms that
breed with one another and produce fertile
offspring.
– They share a common gene pool.
– A genetic change that occurs in one individual can
spread through the population as that individual and
its offspring reproduce.
– If a genetic change increases fitness, that allele will
eventually be found in many individuals of that
population.
Genes and Variation:
• Genetics, molecular biology, and
evolutionary theory work together to
explain how inheritable variation
appears and how natural selection
operates on that variation
Checkpoint Questions:
1. Describe how natural selection can affect traits
controlled by single genes.
2. Describe three patterns of natural selection on
polygenic traits. Which one leads to two distinct
phenotypes?
3. How does genetic drift lead to a change in a
population’s gene pool?
4. What is the Hardy-Weinberg principle?
5. How are directional selection and disruptive
selection similar? How are they different?
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Genetic Drift
 A change in the allelic frequencies in a
population that is due to chance
 In smaller populations, the effects of genetic
drift become more pronounced, and the
chance of losing an allele becomes greater.
Examples: Founder Effect & Bottleneck
Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Understanding Evolution: Problem-based discussion
Population genetics simulation of drift
1) Explain what is shown on the x- and y-axes.
2) Choose two lines on graph A, one that goes to the top of the graph and one
that goes to the bottom. For each line, explain what the line represents and how
it changes over time. Also, explain what it means when a line goes to the top of
the graph versus what it means when a line goes to the bottom of the graph.
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Understanding Evolution: Problem-based discussion
Population genetics simulation of drift
3) Explain the difference between the simulations that generated graphs A and
B.
4) Why do y-values fluctuate in each graph?
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Understanding Evolution: Problem-based discussion
Population genetics simulation of drift
5) In graph A, how many of the trials resulted in a frequency of A = 1.0 and how
many resulted in a frequency of a = 1.0? Why might this occur?
6) The populations represented by graph A and graph B demonstrate very
different behavior. What concept regarding genetic drift does this illustrate?
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Understanding Evolution: Problem-based discussion
Population genetics simulation of drift
7) If one allele were under selection in graph B, how would this graph differ?
8) For each population represented on graph A, would you expect the population to be
in Hardy-Weinberg equilibrium? For graph B?
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Understanding Evolution: Problem-based discussion
Population genetics simulation of drift
9) How would each of these graphs differ if starting allele frequencies in the
simulations were not 50/50—for example, if the populations began with 30% A
alleles and 70% a alleles? Would drift occur?
Ribozyme structure comes from Scott, W.G., Finch, J.T., Klug, A. (1995) The crystal structure of
an all-RNA hammerhead ribozyme: a proposed mechanism for RNA catalytic cleavage. Cell 81:
991-1002
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Founder Effect
 Occurs when a small sample of a
population settles in a location separated
from the rest of the population
 Alleles that were uncommon in the original
population might be common in the new
population.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Bottleneck
 Occurs when a population declines to a
very low number and then rebounds
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Gene Flow
 Increases genetic variation within a
population and reduces differences between
populations
Nonrandom Mating
 Promotes inbreeding and could lead
to a change in allelic proportions
favoring individuals that are
homozygous for particular traits
Sources of Genetic Variation
• The two main sources of genetic variation
are mutations and the genetic shuffling
that results from sexual reproduction.
– Sexual reproduction can thus produce many
different phenotypes, but this does not
change the relative frequency of alleles in a
population. (Card deck analogy)
Checkpoint Questions:
1. What two processes can lead to inherited
variation in populations?
2. How does the range of phenotypes differ
between single-gene traits and polygenic
traits?
3. What is a gene pool? How are allele
frequencies related to gene pools?
4. How could you distinguish between a species
in which there is a lot of variation and two
separate species?
Evolution as Genetic Change
Natural Selection on
Polygenic Traits
• Fitness varies in
polygenic traits.
• Where fitness varies,
natural selection can
act.
• Natural selection
can affect the
distributions of
phenotypes in any
of three ways:
– directional selection
– stabilizing selection
– disruptive selection.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
Natural Selection
 Acts to select
the individuals
that are best
adapted for
survival and
reproduction
Evolution
Chapter
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15.3 Shaping Evolutionary Theory
 Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
Evolution
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15.3 Shaping Evolutionary Theory
 Directional selection makes an organism
more fit.
Evolution
Chapter
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15.3 Shaping Evolutionary Theory
 Disruptive selection is a process that splits
a population into two groups.
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Sexual selection operates in populations
where males and females differ significantly
in appearance.
 Qualities of sexual attractiveness
appear to be the opposite of qualities
that might enhance survival.
Natural
Selection
Isolating Mechanisms
• What happens to a gene pool as one species
evolves into one or more species?
• 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.
– At that point, the populations have separate gene
pools.
Reproductive Isolation
• Develops in a variety of ways:
– Postzygotic isolation (behavioral isolation) = occurs
when two populations are capable of interbreeding but
have differences in courtship rituals or other types of
behavior.
– Allopatric Speciation (geographic isolation) = two
populations are separated by geographic barriers such as
rivers, mountains, or bodies of water.
• do not guarantee the formation of new species
– Prezygotic isolation (temporal isolation) = two or more
species reproduce at different times.
Section 16-3
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
• Courtship ritual in blue-footed boobies is an
example of one kind of prezygotic barrier,
behavioral isolation
• Many plant species have
flower structures that
are adapted to specific
pollinators
– This is an example of
mechanical isolation,
another prezygotic
barrier
Figure 14.2A, B
• Hybrid sterility is one type of postzygotic
barrier
– A horse and a
donkey may
produce a hybrid
offspring, a mule
– Mules are sterile
Figure 14.2C
Evolution
Chapter
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15.3 Shaping Evolutionary Theory
 Prezygotic isolation
prevents reproduction
by making fertilization
unlikely.
 Prevents genotypes
from entering a
population’s gene pool
through geographic,
ecological, behavioral,
or other differences
Eastern meadowlark and Western meadowlark
Evolution
Chapter
15
15.3 Shaping Evolutionary Theory
 Postzygotic isolation occurs when fertilization
has occurred but
a hybrid offspring
cannot develop
or reproduce.
 Prevents
offspring survival
or reproduction
Liger
Evolution
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15.3 Shaping Evolutionary Theory
Allopatric Speciation
 A physical barrier divides one population
into two or more populations.
Abert squirrel
Kaibab
squirrel
Evolution
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15.3 Shaping Evolutionary Theory
Sympatric Speciation
 A species evolves into a new species
without a physical barrier.
 The ancestor species and the new species
live side by side during the speciation
process.
Checkpoint Questions:
1. How is reproductive isolation related to the
formation of new species?
2. What type of isolating mechanism was
important in the formation of different
Galápagos finch species?
3. Explain how behavior can play a role in the
evolution of species.
4. Leopard frogs and tree frogs share the same
habitat. Leopard frogs mate in April; tree frogs
mate in June. How are these species isolated
from each other?
Adaptive Radiation
• Studies of fossils or of living organisms
can show that a single species or a
small group of species has evolved into
several different forms that live in
different ways.
• This process is known as adaptive
radiation.
– Implies common descent
Convergent Evolution
• Unrelated organisms that come to
resemble one another, is called
convergent evolution.
• Natural selection may mold different body
structures, such as arms and legs, into
modified forms, such as wings or flippers.
– EX: Streamlined body of penguin, shark,
dolphin
Coevolution
• The process by which two species evolve in
response to changes in each other over time
is called coevolution.
• An evolutionary change in one organism may
also be followed by a corresponding change in
another organism.
• EX: Many flowering plants, for example, can
reproduce only if the shape, color, and odor of
their flowers attract a specific type of pollinator.
Developmental Genes and Body Plans
• First, molecular studies show that homologous hox
genes establish body plans in animals as different as
insects and humans
• Second, major evolutionary changes—such as the
different numbers of wings, legs, and body segments in
insects—may be based on hox genes.
• Finally, geneticists are learning that even small changes
in the timing of genetic control during embryonic
development can make the difference between long legs
and short ones
Changes in
developmental genes
are one major pattern of
macroevolution.
• Fossil evidence shows
that some ancient insects
(top left) had no wings, but
others (top right) had
winglike structures on
many body segments.
• In modern insects
(bottom), genes may turn
off wing development in all
except one or two body
segments.
Rates of Evolution:
• Evolution has often
proceeded at different
rates for different
organisms at different
times during the long
history of life on
Earth.
(Rate of Evolution)
• Gradualism - slow,
steady change in a
particular line of
descent.
• Punctuated
equilibrium - long,
stable periods
interrupted by brief
periods of more
rapid change
Patterns of Evolution
Is this punctuated equilibrium or gradualism?
Checkpoint Questions:
• What is macroevolution? Describe two patterns
of macroevolution.
• What role have mass extinctions played in the
history of life?
• Use an example to explain the concept of
coevolution.
• How might hox genes contribute to variation?
• Compare and contrast the theories of
gradualism and punctuated equilibrium.