Chapter 23
The Evolution of Populations
AP Biology
2007-2008
5 Types of Selection
Acts to select the individuals that are best adapted
for survival and reproduction; resulting in alleles
being passed to the next generation in proportions
different than frequencies in the present generation
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



Stabilizing
Disruptive (Diversifying)
Directional
Sexual
Artificial
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 Stabilizing selection: operates
to eliminate extreme
expressions of a trait when the
average expression leads to
higher fitness. (Birth Weights)
 Directional selection: An
extreme trait makes an
organism more fit making one
phenotype replace another
(Glacier Lilies/pesticides and
insects)
 Disruptive selection: a process
that splits a population into two
groups resulting in balanced
polymorphism (black-bellied
seed cracker finches)
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Effects of Selection
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
giraffe neck
horse size
human birth weight
rock pocket mice
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Natural selection in action (Directional Selection)
 Insecticide &
drug resistance
insecticide didn’t
kill all individuals
 resistant survivors
reproduce (selective
advantage)
 resistance is inherited
 insecticide becomes
less & less effective

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Sexual Selection
 Acting on reproductive success
attractiveness to potential mate
 fertility of gametes
 successful rearing of offspring
Acts in all sexually
reproducing species
 the traits that get you mates


 sexual dimorphism (intersexual selection:
Survival doesn’t matter
if you don’t reproduce!
Female picks best looking male)
influences both morphology &
behavior (intrasexual selection: males
Fight, winner is more desirable to female)
 can act in opposition to natural selection

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Artificial Selection
 Humans breeding plants and animals
by seeking individuals with desired
traits as a breeding stock

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animals, plants, etc.
Artificial selection
This is not just a
process of the
past…
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It is all
around
us today
Artificial selection
 Artificial breeding can use variations in
populations to create vastly different
“breeds” & “varieties”
“descendants” of wild mustard
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“descendants” of the wolf
Preserving Variation in a Population
 Balanced Polymorphism
 Geographic Variation
 Sexual Reproduction
 Outbreeding
 Diploidy
 Heterozygote Advantage
 Frequency-Dependent Selection
 Evolutionary Neutral Traits
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Balanced Polymorphism
 The presence of two or more
phenotypically distinct forms of a trait
in a single population of a species.
each morph is
better adapted in a
different area, but
both varieties
continue to exist
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Heterozygote Advantage
• Heterozygote advantage occurs when
heterozygotes have a higher fitness
than do both homozygotes;
preserving multiple alleles in a
population
• Natural selection will tend to maintain
two or more alleles at that locus
• The sickle-cell allele causes
mutations in hemoglobin but also
confers malaria resistance
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Heterozygote Advantage
 In tropical Africa, where malaria is common:

homozygous dominant (normal)
 die or reduced reproduction from malaria: HbHb

homozygous recessive
 die or reduced reproduction from sickle cell anemia: HsHs

heterozygote carriers are relatively free of both: HbHs
 survive & reproduce more, more common in population
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Frequency of sickle cell allele
& distribution of malaria
Frequency-Dependent Selection
 AKA: minority advantage
decrease of the more common
phenotypes, increase of the less
common phenotypes
 ex: predator-prey relationships allow
the less common phenotypes to
succeed and reproduce

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Geographic Variation
 Graded variation in the phenotype of an
organism (cline): variation in
appearances due to differences in
environments

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ex. north-south cline
Sexual Reproduction
 Variation due to shuffling and
recombination of alleles during meiosis
and fertilization
independent assortment
 crossing over
 random fertilization

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Outbreeding
 the mating of organisms within one
species that are not closely related;
maintaining variation and a strong gene
pool
This is one result you get when
you Google Image
“outbreeding”
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This is true
example
“outbreeding”
Diploidy
 Diploidy (2n) maintains genetic
variation in the form of hidden
recessive alleles, that could be
advantageous when conditions change
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Evolutionary Neutral Traits
 traits that seem to have no selective
advantage

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ex.) blood types
Causes of Evolution of a Population
 Genetic Drift

Bottleneck effect, flounder effect
 Gene Flow
 Mutations
 Nonrandom Mating
 Natural Selection
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Genetic Drift
 Chance events changing frequency of
traits in a population

not adaptation to environmental conditions
 not selection

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces
population to small number & then
population recovers & expands
again but from a limited gene pool
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Founder effect
 When a new population is started
by only a small group of individuals
just by chance some rare alleles may
be at high frequency;
others may be missing
 skew the gene pool of
new population

 human populations that
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started from small group
of colonists
 example:
colonization of New World
albino deer Seneca Army Depot
Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance event

 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
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Gene Flow
 the movement of alleles into or out of a
population via migration of fertile
individuals or gametes between
populations
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Mutations
 changes in genetic material; increasing
diversity either at one loci or several
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Nonrandom Mating
 Rarely is mating
completely random in a
population.
 Usually individuals mate
with individuals in close
proximity.
 This promotes
inbreeding and could
lead to a change in
allelic proportions
favoring individuals that
are homozygous for
particular traits
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Forces of evolutionary change
 Natural selection

traits that improve survival
or reproduction will accumulate
in the population
 adaptive change
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Natural Selection
 Selection acts on any trait that affects
survival or reproduction
predation selection
 physiological selection
 sexual selection

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Measuring
Evolution of Populations
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5 Agents of evolutionary change
Mutation
Gene Flow
Genetic Drift
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Non-random mating
Selection
Populations & gene pools
 Concepts
a population is a localized group of
interbreeding individuals
 gene pool is collection of alleles in the
population

 remember difference between alleles & genes!

allele frequency is how common is that
allele in the population
 how many A vs. a in whole population
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Evolution of populations
 Evolution = change in allele frequencies in a
population
 Hardy Weinberg Equilibrium states that allele and
genotype frequencies in a population will remain
constant from generation to generation in the
absence of evolutionary influences.


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hypothetical: what conditions would cause allele
frequencies to not change?
non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
If all five conditions are met then a
population is considered to be in
equilibrium and NO EVOLUTION is
occurring
1) no genetic drift
2) no gene flow
3) no nonrandom mating
4) no mutations
5) no natural selection
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Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population

preserves allele frequencies
 Serves as a model (null hypothesis: no
relationship between two results)


natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on
a population
 measuring evolutionary change
G.H. Hardy
AP mathematician
Biology
W. Weinberg
physician
The Hardy-Weinberg Equation
 enables us to calculate frequencies of
alleles in a population
p2 + 2pq + q2 = 1 or
 p= dominant allele
 q= recessive allele

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p+q=1
A
a
A
AA
Aa
a
Aa
aa
Hardy-Weinberg theorem
 Counting Alleles
assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q

 frequencies must add to 1 (100%), so:
p+q=1
BB
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Bb
bb
Hardy-Weinberg theorem
 Counting Individuals



frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
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Bb
bb
H-W formulas
 Alleles:
p+q=1
B
 Individuals:
p2 + 2pq + q2 = 1
BB
BB
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b
Bb
Bb
bb
bb
Using Hardy-Weinberg equation
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
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What are
the genotype
Must
assume
population
is in frequencies?
H-W equilibrium!
Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.16
BB
Bb
bb
p2=.20
=.74
BB
2pq=.64
2pq=.10
Bb
q2=.16
bb
Null hypothesis
Sampled data
How do you
explain
the data?
AP
Biology
Insert
Any
Practice
Questions??
Problems
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2005-
 1. Given a population in HardyWeinberg equilibrium with allele
frequencies A =0.9 and a = 0.1,
determine the frequencies of the three
genotypes AA, Aa and aa.
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 3. 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 allelic frequencies
in this sample.
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6. 1 in 1,700 US Caucasian newborns have cystic fibrosis. C
for normal is dominant over c for cystic fibrosis.





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a) What percentage of the above population has cystic fibrosis
(cc)?
b) Calculate the frequencies of the C and c alleles.
c) Calculate the frequencies of the normal (CC) and carrier (Cc)
genotypes.
d) How many of the 1,700 population members are normal
(CC)? Carriers (Cc)?
e) It has been found that a carrier is better able to survive
diseases with severe diarrhea. What would happen to the
frequency of the “c” if there was an epidemic of cholera or
other type of diarrhea producing disease? Would “c” increase
or decrease?
9. In a tropical forest there is a species of bird that has a
variable tail length. Long is incompletely dominant over
short. In one population of 2000 birds, 614 have long tails,
973 have medium length tails, and 413 birds have short
tails.


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a) What is the frequency of each allele in the population?
b) Is the population in Hardy Weinberg equilibrium?
 THE END
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Speciation and Reproductive Isolation
 Species: members in a population who
have the potential to interbreed in nature
and produce viable, fertile offspring
Reproductive isolation: one group of
genes becomes isolated from one another
to begin a separate evolutionary history
 Speciation: anything that fragments a
population and isolates a small group of
individuals

 Allopatric
 Sympatric
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 Allopatric Speciation: caused by
geographic isolation
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 Sympatric Speciation: caused by anything
besides geographic isolation

polyploidy: a cell has two or more complete sets
of chromosomes

habitat isolation: two organisms live in the same
area but rarely encounter one another

behavioral isolation: two species do not mate
because of differences in courtship behavior

temporal isolation: populations may mate or
flower at different seasons or different times of day

reproductive isolation: closely related species
unable to mate because of a variety of reasons
 prezygotic barriers
 postzygotic barriers
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 reproductive
isolation
Patterns of Evolution
 Divergent
 Convergent
 Parallel
 Coevolution
 Adaptive Radiation
 Gradualism
 Punctuated
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Divergent Evolution
 Occurs when a population becomes
isolated from the rest of the species,
exposed to new selective pressures,
and evolves into a new species
allopatric speciation
 sympatric speciation

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Convergent evolution
 Flight evolved in 3 separate animal groups
evolved similar “solution” to similar “problems”
 analogous structures

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Convergent evolution
 Fish: aquatic vertebrates
 Dolphins: aquatic mammals
similar adaptations to
life in the sea
 not closely related

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Those fins & tails
& sleek bodies are
analogous structures!
Parallel Evolution
 Convergent evolution in common niches


filling similar ecological roles in similar
environments, so similar adaptations were selected
but are not closely related
marsupial
mammals
placental
mammals
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Parallel types across continents
Niche
Burrower
Placental Mammals
Australian Marsupials
Mole
Marsupial mole
Anteater
Numbat
Anteater
Nocturnal
insectivore
Mouse
Climber
Marsupial mouse
Spotted cuscus
Lemur
Glider
Stalking
predator
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Chasing
predator
Sugar glider
Flying
squirrel
Ocelot
Tasmanian cat
Wolf
Tasmanian “wolf”
Coevolution
 Two or more species reciprocally
affect each other’s evolution

predator-prey
 disease & host
competitive species
 mutualism

 pollinators & flowers
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Adaptive Radiation
 the emergence of
numerous species
from a common
ancestor introduced
into an environment,
filling a niche
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