Allele
Frequencies,
Types of
Selection &
HardyWeinberg
Evolution of
Populations:
Chapter 16
Evolution of Populations
Notes
Tuesday, 2/21/12
Take out Notebook
Turn in: Natural Selection of Antibiotic
Resistance Lab and Adaptation Project
Evolution of Populations
Notes
How is evolution defined in
genetic terms?
Evolution is any change in the relative
frequency of alleles in the gene pool of
a population. (also called
microevolution)
Remember: Alleles are
different forms of genes.
 Microevolution: The
frequency of an allele in a
gene pool of a population
depends on many factors and
can change over time.
 Over long periods of time,
microevolution can lead to
macroevolution- change
from one species to another.
What is a GENE POOL?
A gene pool is
the combined
genetic
information of all
members of a
particular
population.
www.darwinawards.com
What is relative frequency?
 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.
Expressed in percent.
Relative Frequency Practice!
 In a population of 50 students, there are 40 alleles
for hitchhiker’s thumb and 60 alleles for a straight
thumb. What is the relative frequency of alleles for
hitchhiker’s thumb?
 40/100 = 0.40 = 40%
 What is the relative frequency of alleles for
straight thumb?
 60/100 = 0.60 = 60%
How do allele frequencies change
during evolution?
 http://glencoe.mcgraw-
hill.com/sites/9834092339/student_view0/chapter20/animati
on_-_mechanisms_of_evolution.html
How do allele frequencies change during
evolution?
1.
Natural Selection which acts on the phenotype
rather than the genotype of an organism.
2.
Mutations (change in the DNA) which are
constantly being generated in a gene pool.
3.
Genetic Drift- Random change in allele frequency.
4.
Gene Flow- The movement of alleles into or out of
the gene pool.
5.
Non-random Mating – when females prefer one
phenotype over another. Also called sexual
selection.
Remind us: What is Natural
Selection?
Individuals with favorable genetic
variations will survive and reproduce.
Individuals that lack adaptations will
most likely die before they ever
reproduce. Thus, alleles that are
favorable will become more common
and those that are not will decrease.
Remind us: What are Mutations?
Changes in the DNA and therefore in the genes!
Genetic Drift is random change in allele
frequencies in small populations.
 Genetic Drift is usually caused by
natural disasters like a fire or flood.
 Genetic Drift can cause evolution
due to chance rather than natural
selection.
Genetic Drift causes…
the founder effect- when a migration of a small
subgroup of a population causes a change in allele
frequencies. (also called bottleneck affect)
Imagine: The Island of Bio Students!
Genetic Drift often leads to a
decrease in variation.
Based on this info and what you know about
evolution, do you think genetic drift will help a
population survive or will it cause the
population to go extinct? Why?
Remember: Populations with less variation
are likely to go extinct!
• Cheetahs in Africa are one of the most extreme
examples of genetic drift. The Cheetahs alive today
are the descendants of only a few cheetahs. This
means that Cheetahs are VERY genetically similar.
• They have less
resistance to
disease and are
more likely to go
extinct.
Lizard Evolution on Islands shows
genetic drift in action!
 http://www.futurity.org/science-technology/lizards-evolution-
keeps-ancestors-close/
Gene Flow: the
movement of
alleles into or out
of a population.
Immigrants add
new alleles.
Emigrants take
alleles away
Gene Flow
Animation:
http://nortonbooks.com/
college/biology/animatio
ns/ch17a01.htm
Non-random Mating – when females prefer one
phenotype over another. Also called sexual
selection.
Sexual Selection can cause
sexual dimorphism: when
males and females of the
same species look noticeably
different from each other
Example of sexual selection: Peacocks
• Female peacocks prefer
males with bright beautiful
tails, so over time male
peacocks have evolved to
have very showy tails.
• Scientists have linked
female preference to
important traits such as
health, size, and strength.
In peacocks, scientists
have linked tail size and
color to nutrition.
This means that females
do not just want to mate
with a male with a pretty
tail for the look of himfemales want to mate
with males with pretty
tails because they could
have only produced this
tail if he was able to find
nutritious food. The
female wants a male who
is healthy enough to find
food and can provide her
offspring with nutritious
food too.
Sexual Selection and natural selection may
often operate in opposing directions
Males have traits like showy plumage in
spite of their potential costs such as
increased visibility to predators.
Reminder: When reading and
answering questions out of the
book….
READ CAREFULLY!!!!!!!
Don’t just copy the sentences in
bold.
Try to understand what the
question is asking and answer it!
If evolution is change in allele
frequency in a population, what if
some alleles are lethal?
Will the population survive? Will
the alleles stay in the population?
Allele Frequencies and
Sickle Cell Anemia
Students know why alleles that
are lethal in a homozygous
individual may be carried in a
heterozygote and thus
maintained in a gene pool.
How Sickle Cell Works
Sickle Cell and Pain
Malaria Infects Red Blood Cells
A Mutation Story
http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html
1. What is the name of the deadly disease?
2. What causes Malaria?
3. How is having one mutated gene beneficial to Africans?
4. How many genes does an individual need to have to express
sickle cell anemia?
5. How can a mutation be harmful in one environment and helpful in
another?
6. Why would a mutation persist if it kills people?
7. Why are there more people with sickle cell anemia in one part of
the world than in other parts?
Allele Frequency and Sickle Cell
Anemia Lab
Weds-Thurs, Feb. 2/22-2/23
Allele Frequency and Sickle Cell
Anemia Background Info
Read popcorn style!
Friday, 2/24/12
Warm-Up: Write down Table of
Contents!
Finish LAB!
CHECK THE WEBSITE FOR
ASSIGNMENTS!!!
Go to www.mahs.org
 STUDENTS
 CLASSES/HOMEWORK
 SCIENCE  BIOLOGY!!!
Why are alleles that are lethal in a
homozygous individual maintained
in the gene pool?
(Use Sickle Cell and Malaria as an
example.)
Types of Selection and Hardy
Weinberg Notes
(Ch. 16.2)
Monday, 2/27/12
In Single- Gene Traits,
… there are two phenotypes,
whereas in polygenic traits,
there are multiple phenotypes.
Natural Selection acts
differently on each of these.
Is height a
single-gene
trait or a
polygenic
trait?
Polygenic Trait!
Is Sickle Cell Anemia a single gene
trait or a polygenic trait?
Single
Gene Trait!
Natural Selection on Single-Gene
Traits can lead to…
…changes in allele frequencies and
therefore, evolution!
This was shown in the Peppered
Moth Simulation.
Natural Selection on Polygenic
Traits occurs in three ways:
1. Disruptive Selection
2. Stabilizing Selection
3. Directional Selection
Directional Selection
 When individuals at one end of the
curve have a higher fitness than those
at the other end.
 Examples: Finch bills, peppered moths
Animation of Directional Selection:
 http://wps.prenhall.com/wps/media/objects/487/499541/C
DA22_2/CDA22_2b/CDA22_2b.htm
Stabilizing Selection
 When individuals near the center of the curve
have higher fitness than individuals at either
end of the curve- selection against both
extremes.
 Examples: human baby size, lizard size, number of
children
Animation of Stabilizing Selection:
 http://wps.prenhall.com/esm_freeman_biosci_1/0,6452,499573-
,00.html
Disruptive
Selection
 When individuals at the lower and upper ends
of the curve have higher fitness than those in
the middle. This could cause the population to
split into two distinct subgroups.
 Examples: duck bills, sexual dimorphism, sickle
cell anemia
Animation of Disruptive Selection:
 http://wps.prenhall.com/wps/media/objects/487/499541/CDA22_2/
CDA22_2d/CDA22_2d.htm
CA BIO STANDARD- Evolution
Students know the conditions
for Hardy-Weinberg
equilibrium in a population and
why these conditions are not
likely to appear in nature.
Hardy-Weinberg Principle
 Allele frequencies in a population will
remain constant unless one or more factors
causes those frequencies to change.
 When allele frequencies remain constant, it is
called genetic equilibrium.
 If there is genetic equilibrium, evolution will not
occur.
Five conditions required to
maintain genetic equilibrium:
1. Random mating
2. Large population
3. No movement into or out of the
population
4. No mutations
5. No natural selection
Conditions necessary for Hardy
Weinberg Equilibrium
 http://www.phschool.com/science/biology_place/labbench
/lab8/intro.html
Animation of H-W Conditions
 http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flas
h/life4e_15-6-OSU.swf
Hardy Weinberg Equation
Students know how to solve the
Hardy-Weinberg equation to
predict the frequency of
genotypes in a population, given
the frequency of phenotypes.
Hardy Weinberg Cheat Sheet!
 Frequency of dominant allele = p
 Frequency of recessive allele = q
 The sum of the two alleles in a
population = 100%
or…
p+q=1
Hardy Weinberg Cheat Sheet!
p2 + 2pq + q2 = 1
 p2 = frequency of AA homozygotes
 2pq = frequency of Aa heterozygotes
 q2 = frequency of aa homozygotes
 1 = frequency of all genotypes
Hardy Weinberg Sample Problems
 http://www.phschool.com/science/biology_place/labbench
/lab8/samprob1.html
H-W Sample Problem: Albinism
 http://anthro.palomar.edu/synthetic/sample.htm
Hardy Weinberg Problem Set
15 points