Chapter 11: The Evolution of
Populations
11.1: Genetic Variation Within
Populations
• Objectives: Describe the significance of
genetic variation within a population.
• Identify sources of genetic variation.
• Warm Up: What are the chances that you
can put together a good team if just ten
players try out? What about 30 players?
• Words to Know: Gene Pool, Allele
Frequency, Phenotype, Gene, Allele, Meiosis,
Gamete
Genetic Variation
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Natural selection acts on different phenotypes (physical characteristics) in a
population.
In order to have different phenotypes, a population must have genetic variation.
A population with a lot of variation likely has a wide range of phenotypes.
Genetic Variation is stored in a population’s Gene Pool – the combined alleles
(genes) of all of the individuals in a population.
An Allele Frequency is a measure of how common a certain allele is in the
population.
What is the relationship between allele frequencies and a gene pool?
Genetic Variation Comes from
Several Sources
• Mutation
• A mutation is a
random change in the
DNA of a gene.
• This change can form
a new allele.
• If these changes are
in reproductive cells
they can be passed
on to new
generations.
Genetic Variation Comes from
Several Sources
• Recombination
• New allele combinations
form in offspring through
a process called
recombination.
• Most recombination
occurs during meiosis
(producing 4 sex cells) in
crossing over.
• This shuffling results in
many new combinations
of gametes (sex cellssperm and egg) .
• Why aren’t mutations in
nonreproductive cells
sources of genetic
variation?
11.2: Natural Selection in
Populations
• Objectives: Describe how natural selection acts on
the distribution of traits in a population.
• Explain three ways natural selection can change the
distribution of a trait in a population.
• Warm Up: When a drought caused more large0geak
ground finches to survive than small-beaked ground
finches both of the same species, what was natural
selection acting upon?
• Words to Know: Normal Distribution,
Microevolution, List 3 Ways Natural Selection can
change the Distribution of a trait (with definition)
1.Directional Selection, 2.Stabilizing Selection,
3.Disruptive Selection
Natural Selection Acts on
Distributions of Traits
• If we were to line up a group of
people on a football field by
height, relatively few people
would fall at the extreme tall
and short ends. A majority of
people would fall in the middle
range.
• When frequency is highest
near the mean value (middle)
and decreases towards each
extreme, it is called a Normal
Distribution.
• The graphed result is a bell
shaped curve.
• What other types of data might
follow a normal distribution?
Natural Selection Can Change
the Distribution of a Trait
• Macroevolution is
the observable
change in the allele
frequencies of a
population over time.
• Microevolution
occurs on a small
scale, within a single
population.
• This can result in one
of three paths.
1. Directional Selection
• Directional Selection favors phenotypes at one extreme of a trait’s
range.
• Ex: Drug Resistant Bacteria.
– This phenotype was selected Against when there were no antibiotics.
– Now that there are antibiotics, this phenotype is now more successful and
selected for.
• Directional selections moves the curve on the graph left or right.
2. Stabilizing Selection
• In Stabilizing Selection the intermediate phenotype (median) is
favored.
• Ex: Gall flies and their predators.
– Woodpeckers feed on larger gall fly larva.
– Parasitic wasps lay eggs on the smaller gall fly larva.
– Only the gall fly larva that are medium size have a chance to survive.
• Stabilizing Selection on a graph looks like Normal Selection.
3. Disruptive Selection
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Disruptive Selection occurs when
both extreme phenotypes are
favored, while individuals with
intermediate phenotypes are selected
against by something in nature.
Ex: Feather color in male lazuli
buntings (a bird).
– Feathers range from dull brown
to bright blue.
– Dull Brown and Bright blue
feather buntings have a better
chance of getting a mate than
regular
blue birds do.
The brown males are successful
because the aggressive bright blue
males do not see them as a threat.
The regular blue birds are frequently
attacked because they are seen as a
threat.
The graph in disruptive selection has
two curves, one at each extreme.
If bluish brown coloring became
advantageous for young males, what
type of selection would likely occur in
a lazuli bunting population?
Frequency of
individuals
LE 23-12
Original
population
Evolved
population
Directional selection
Original population
Phenotypes (fur color)
Disruptive selection
Stabilizing selection
11.3: Other Mechanisms of
Evolution
• Objectives: Explain how gene flow,
genetic drift, and sexual selection can
lead to the evolution of populations.
• Warm Up: What effect has immigration
had on the genetic variation of the U.S.
population?
• Words to Know: Gene Flow,
Speciation, Genetic Drift, Bottleneck
Effect, Founder Effect, Sexual Selection
Gene Flow
• When an organism joins a new
population and reproduces, its
alleles become part of that
population’s gene pool.
• At the same time its genes are
removed from its previous
population.
• The movement of alleles from
one population to another is
called Gene Flow.
• Gene flow increases genetic
variation.
• A lack of gene flow can cause
speciation.
• How does gene flow affect
neighboring populations?
Genetic Drift
• Genetic Drift is the
change in allele
frequencies due to
chance.
• Two processes
commonly cause
populations to
become small enough
for genetic drift to
occur.
Bottleneck Effect
• The Bottleneck Effect is
genetic drift that occurs after
an event greatly reduces the
size of a population.
• Disasters such as
earthquakes, floods, or fires
may reduce the size of the
pop. Drastically. The result is
that a small surviving pop. Is
unlikely to be representative of
the original pop.
• Ex: Overhunting of northern
elephant seals during the
1800’s.
– The population got reduced to
about 20 seals that did not
represent the genetic diversity
of the original population.
Founder Effect
• The Founder Effect is genetic drift that occurs after a
small number of individuals colonize a new area.
• The gene pools of these new populations are often very
different from those of the larger population.
• Ex: The Amish of Lancaster Pennsylvania have a high
rate of a rare form of Dwarfism.
– Because they are such a small community, this trait is common
in the population.
Effects of Genetic Drift
• The population loses
genetic variation.
• Less likely to have
individuals to adapt.
• Alleles that are lethal
can become more
common.
• What is genetic drift
more likely to occur in
smaller populations?
Sexual Selection
• Mating has an important effect
on the evolution of a population.
• Sexual Selection occurs when
certain traits increase mating
success.
• There are two types of sexual
selection:
– 1. Intrasexual Selection
involves competition among
males. Whoever wins get the
girl.
– 2. Intersexual Selection occurs
when males display certain traits
that attract the females, such as
peacock feathers.
• Male Irish elks, which are now
extinct, had 12-foot-wide antlers.
Describe how sexual selection
could have caused such an
exaggerated trait to evolve.
11.4: Hardy-Weinberg
Equilibrium
• Objectives: Identify the conditions that
define Hardy-Weinberg Equilibrium
• Explain the predictive value of the
Hardy-Weinberg equation.
• Warm Up: What does it mean to be in a
state of equilibrium?
• Words to Know: Hardy-Weinberg
Equilibrium.
Hardy-Weinberg Equilibrium
• Hardy and Weinberg showed
that genotype frequencies in a
population will stay the same
over time as long as 5
conditions are met:
– 1. Must be a very Large
Population (NO genetic drift).
– 2. No emigration or immigration
(NO gene flow).
– 3. No mutations (NO new
alleles).
– 4. Must have Random Mating
(NO sexual selection allowed).
– 5. No Natural Selection (all traits
must equally aid in survival).
• Real populations rarely meet all
five requirements
• How are models used by
population biologists?
The Hardy-Weinberg Equation
• For traits in a simple dominant-recessive
systems, biologists can predict genotype
frequencies using the Hardy-Weinberg equation.
• P2 + 2pq + q2 = 1
–  p = frequency of the dominant allele
–  q = frequency of the recessive allele.
• In Hardy-Weinberg Equilibrium, the equation
ALWAYS equals 1.
• If the equation DOES NOT equal 1, evolution is
occurring.
Hardy Weinberg Equation
There are Five Factors that can
Lead to Evolution
• 1. Genetic Drift – allele frequency can change due to
chance alone.
• 2. Gene Flow – the movement of alleles from one place
to another change the allele frequencies of the population.
• 3. Mutation – new alleles can form through mutations,
and these create the genetic variation needed for
evolution.
• 4. Sexual Selection – certain traits may improve mating
success which cause an increase in that allele frequency.
• 5. Natural Selection – Certain traits may be an
advantage for survival so alleles for these traits increase in
frequency.
• Why do real populations rarely reach Hardy-Weinberg
equilibrium?
Isolation of Populations
• If gene flow between two populations stops for any reason, the
population are said to be isolated.
• As they adapt to their new environments, their gene pools may
change.
1. Reproductive Isolation occurs when member of different
populations can no longer mate successfully with one another.
• Speciation is the rise of two or more species from on existing
species.
• Why is reproductive isolation considered to be the final stage in
speciation?
11.5: Speciation Through
Isolation
• Objectives: Explain how isolation of populations
can lead to speciation.
• Describe how populations can become isolated.
• Warm Up: How do we know that a three-foot-tall
Irish wolfhound and a six-inch-high Chihuahua are
the same species? At what point would the two
breeds become separate species?
• Words to Know: List the 4 ways Speciation can
Occur (with definition) 1. Reproductive Isolation
2. Behavioral Isolation 3. Geographic Isolation
4. Temporal Isolation.
Behavioral Barriers
2. Behavioral Isolation
is isolation caused by
differences in
courtship or mating
behaviors.
• Ex: Fireflies. Male
and female fireflies
produce patterns of
light flashes that
attract only their own
species.
Geographic Barriers
3. Geographic Isolation
involves physical barriers
that divide a population
into two or more groups.
• These can be rivers,
mountains, and dried
lakebeds.
• Ex: The isthmus of
Panama separated many
aquatic species that then
evolved separately.
Temporal Barriers
4. Temporal Isolation
exists when Timing
prevents reproduction
between populations.
• Some members of a
population may show
signs of courtship at
different times.
• Ex: flowers bloom at
different times of year.
• What are the differences
and similarities between
behavioral isolation and
temporal isolation?
11.6: Patterns in Evolution
• Objectives: Describe different types and
rates of evolution.
• Compare different types and rates of
extinction.
• Warm Up: What are some different
adaptations in plants and animals that seem
to relate directly to features of their
environment?
• Words to Know: Convergent Evolution,
Divergent Evolution, Coevolution, Extinction,
Punctuated Equilibrium, Adaptive Radiation
Evolution through Natural
Selection is NOT Random.
• Natural Selection is NOT random.
• Individuals with traits that are better adapted for their environment
have a better chance of surviving and reproducing than do
individuals without these traits.
• Natural selection always pushes traits in an advantageous
direction.
• The environment controls the direction taken by natural selection.
Convergent Evolution
• Different species often
must adapt to similar
environments.
• Evolution toward similar
characteristics in
unrelated species is
called Convergent
Evolution.
• Ex: Shark and Dolphins
are not related yet they
have evolved similar tail
fins.
Divergent Evolution
• When closely related
species evolve in different
directions, they become
increasing different in
Divergent Evolution.
• Ex: Red Fox and Kit fox:
though closely related
they have developed
different characteristics
based on the
environment they live in.
• Are the shells of turtles
and snails examples of
convergent or divergent
evolution? Explain.
Coevolution
• Coevolution is the
process in which two
or more species
evolve in response to
changes in each
other.
• Ex: Bees and
flowers.
Evolutionary Arms Race
• Coevolution can also occur in competitive relationships.
• Ex: Many plants produce defense chemicals to discourage herbivores
from eating them.
– Natural selection then favors the herbivores who can overcome the effect of
the toxins.
• Ex: Thick shells and spines of murex snails are an adaptive response to
predation by crabs.
– In turn, crabs have evolved strong claws.
• What do you think will happen in future generations of crabs and snails?
Species Can become Extinct
• The elimination of a species from Earth is Extinction.
• There are two types of extinctions involved in evolution:
– 1. Background extinctions – occur continuously at a slow rate.
(usually effect one or few species in a small space).
– 2. Mass Extinctions – many species are destroyed suddenly.
(ice age, meteors).
• What are the differences and similarities between
background extinctions and mass extinctions?
Speciation Often Occurs in
Patterns
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There are repeating patterns in the
history of life.
The Theory of Punctuated Equilibrium
states that episodes of speciation occur
suddenly in geologic time and are
followed by long periods of little
evolutionary change.
This opposed Darwin’s theory of
gradualism (slow, steady evolution).
Adaptive Radiation the diversification of
one ancestral species into many
descendant species.
Ex: Following the mass extinction at the
end of the Cretaceous Period 65 million
years ago, Adaptive radiation of
mammals occurred.
The adaptive radiation of mammals
followed the extinction of the dinosaurs.
How do these events support the theory
of punctuated equilibrium?
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