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Population Genetics
Ch 17 and Ch 18.2
Genes and Variation
• Combine Darwin’s theory
of Natural Selection with
Mendel’s studies on
• Which phenotype will be
more often expressed in
a population?
• For example why might
the black coat show up
more in a population
than the brown?
Which moth is more likely to survive
due to the theory of Natural
How would happen to the black
moth’s population here?
Natural selection never acts on genes. Why?
The organisms that have phenotypes that are better suited to
their environment, will produce more offspring.
Organisms more adapted to their environment, can pass
these traits onto their offspring.
Populations and Gene Pools
• Population – a group of
individuals of the same
species that mate and
produce offspring.
• Gene pool- consists of
all the genes (different
alleles), found in that
• Allele frequency – the
number of times an
allele occurs in a
Evolution, in genetic terms involves a change in the frequency of
alleles in a population over time.
Populations evolve, not individuals.
Some alleles affect the species
ability to survive.
Determing Allele Frequency
You have a population of 25 mice. How many alleles for coat color
are found in this population?
Allele Frequencies of Different
Blood types
Location of O Blood type globally
Location of A blood type globally
Location of B blood type globally
Sources of Genetic Variation
• Mutations
• Genetic Recombination
in Sexual Reproduction
– Crossing over
• Lateral Gene Transfer
– Bacteria swapping
plasmid DNA
What determines the number of
phenotypes for a given trait?
• Single gene trait
– Absence or presence of
a band on shells
• Polygenic traits
– Over 180 genes help
code for height
How many alleles are found in each example?
How does Natural Selection Work?
What changes Allele Frequencies
besides Natural Selection?
Lesson Overview
The Process of Speciation
Genetic Drift
• Random change in an allele frequency in small
populations. (Change is not caused by Natural Selection)
– Genetic Bottlenecks – change in allele frequency
after a dramatic reduction in size of the
• Disaster such as disease
Potato Famine in Ireland
Irish poor grew one variety of
potatoes called lumpers, which were
harvested in the fall.
Airborne fungus wiped out the crop
in September of 1845 and wiped out
the potato crops for the next 6 years.
Genetic Drift
• The Founder Effect – allele frequencies change
as a result of the migration of a small group of
a population.
Finches on G. Islands
Random loss of alleles; leads to reduced genetic variation
Smaller population that changed from the original population found on
South America.
Allele frequency in a Population
• A population that does not evolve should have
allele frequencies that do not change in a
population – Genetic Equilibrium
• Hardy-Weinberg Principle – allele frequencies
in a population should remain constant unless
one or more factors cause those frequencies
to change.
– Used to make predictions about allele frequencies
in populations. (Similar to what Mendel did with offspring)
What factors would cause a change
in allele frequency?
• Five conditions that affect genetic equilibrium
– Nonrandom mating
• Sexual Selection – genes that are used for selection of a
mate, are not longer in equilibrium
– Small Population Size
• Genetic drift has a greater chance of affecting small
populations. Why?
– Immigration and Emigration
• Addition and removal of alleles from a population
Factors that affect genetic
equilibrium? (cont.)
– Mutations
• New alleles are introduced into gene pool
– Natural Selection
• A allele makes the organisms better able to survive,
genetic equilibrium will be disrupted. More organism
will show up with the better adapted trait over time.
How do you determine allele
frequency using Hardy-Weinberg?
• Hardy-Weinberg Equation
– p2 +2pq + q2 = 1
• q represents the frequency for the recessive
allele (a)
• P represents the frequency for the
homozygous dominant allele (A)
• pq represents the frequency for heterozygous
Solving allele frequency with HardyWeinberg Equation
Remember that p + q = 1
Always solve for q first and then p. Why?
Example. In a population of mice, the recessive brown coat has
an allele frequency of .36.
We need to solve for q. .36 represents q2. Why?
To Solve for q take the √ of .36
How would you solve for p? (Remember p+q =1)
How would you determine the allele frequency for
Within a population of butterflies, the color brown (B) is
dominant over the color white (b). And, 40% of all butterflies
are white. Calculate the following:
The frequency of the recessive allele.
The frequency of homozygous dominant individuals.
The percentage of butterflies in the population that are
In the U.S., 16% of the population is Rh-, due to a homozygous
recessive pair of alleles.
From this data determine:
a. The frequency of the recessive allele
b. the frequency of the dominant allele
c. The percentage of homozygous dominants and
percentage of heterozygotes in the population.
Allele W for white wool is dominant over allele w for black wool.
In a sample of 900 sheep, 891 are white and 9 are black.
Estimate the allele frequencies in this sample.
For w:
For W:
For Ww:
In a given population, only the "A" and "B" alleles are present
in the ABO system; there are no individuals with type "O"
blood or with O alleles in this particular population. If 200
people have type A blood, 75 have type AB blood, and 25 have
type B blood, what are the allele frequencies of this population
(i.e., what are p and q)?
Lesson Overview
The Process of Speciation
Speciation is the formation of a new species. A species is a
population whose members can interbreed and produce
fertile offspring.
How does one species become two?
When populations become reproductively isolated, they can
evolve into two separate species. Reproductive isolation can
develop in a variety of ways, including behavioral isolation, or
geographic isolation.
Lesson Overview
The Process of Speciation
Isolating Mechanisms
Reproductive isolation occurs when a population splits into
two groups and the two populations no longer interbreed.
When populations become reproductively isolated, they can
evolve into two separate species.
Lesson Overview
The Process of Speciation
Behavioral Isolation
Behavioral isolation occurs when two populations that are
capable of interbreeding develop differences in courtship
rituals or other behaviors.
Lesson Overview
The Process of Speciation
Geographic isolation occurs when two populations are
separated by geographic barriers such as rivers, mountains, or
bodies of water.
An example: Kaibab squirrel is a subspecies of the Abert’s
squirrel that formed when a small population became
isolated on the north rim of the Grand Canyon.
Why would this cause speciation?
Lesson Overview
The Process of Speciation
Testing Natural Selection in Nature
What did the Grants’ scientific investigation show about
Galápagos finches? (pages 496-497)
Founder effect – allele frequencies change as a result of the
migration of a small subgroup of a population.
Variation within a species increases the likelihood that the
species can adapt and survive environmental change.
Lesson Overview
The Process of Speciation
Testing Natural Selection in Nature
Darwin hypothesized that the Galápagos finches had
descended from a common ancestor.
He proposed that natural selection shaped the beaks of
different bird populations as they became adapted to eat
different foods.
Lesson Overview
The Process of Speciation
A Testable Hypothesis
Peter and Rosemary Grant from Princeton University realized
that Darwin’s hypothesis rested on two testable assumptions:
For beak size and shape to evolve, there must be enough
heritable variation in those traits to the material for natural
• research found that there was a lot of diversity
Differences in beak size and shape must produce differences
in fitness.
Lesson Overview
The Process of Speciation
Natural Selection
The Grants’ data showed individual finches with differentsized beaks had different chances of surviving drought.
When food was scarce, individuals with the largest beaks
were more likely to survive.
The Grants observed that average beak size in that finch
population increased dramatically over time.
Lesson Overview
The Process of Speciation
What is a current hypothesis about Galápagos finch
Founders Arrive
Many years ago, a few finches from
South America—species M—
arrived on one of the Galápagos
Because of the founder effect, the
allele frequencies of this founding
finch population could have
differed from those in the South
American population.
Lesson Overview
The Process of Speciation
Geographic Isolation
Over time, GI and NS enabled the
island finch population to evolve
into a new species – Species A.
These species (A) crossed to
another island.
As a result we have two
populations who are
geographically isolated.
Lesson Overview
The Process of Speciation
Changes in Gene Pools
Over time, populations on
each island adapted to local
Natural selection could have
caused two distinct
populations to evolve (A and
B), each characterized by a
new phenotype.
Lesson Overview
The Process of Speciation
Behavioral Isolation
Over time, populations on
each island adapted to local
A few birds (B) come back to
main island, but since they
have evolved differently, they
don’t mate with each other.
We now have two distinct
Lesson Overview
The Process of Speciation
Competition and Continued Evolution
Birds that are most different from
each other have the highest
fitness. More specialized birds
have less competition for food.
Over time, species evolve in a way
that increases the differences
between them, and new species
may evolve (C, D, and E).
This process could have produced
the `13 different finch species
found today.