Microevolution
Changing Allele Frequencies
Evolution
• Evolution is defined as a change in
the inherited characteristics of
biological populations over
successive generations.
• Microevolution involves the
change in allele frequencies that
occur over time within a
population.
• This change is due to four different
processes: mutation, selection
(natural and artificial), gene flow,
and genetic drift.
2
Determining Allele Frequency
• Examine the frog population
presented here.
• Their color is determined by a
single gene, which has two alleles
and phenotypically exhibits
incomplete dominance.
• CGCG is green, CG CR is blue, and
CR CR is red
• Calculate the allele frequency of
the gene pool in the diagram.
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Determining Allele Frequency
• These frogs are diploid, thus have two
copies of their genes for color.
Allele:
CG
CR
Green (11)
22
0
Blue (2)
2
2
Red (3)
0
6
Total:
24
8
Frequency:
p = 24 ÷ 32
p = ¾ = 0.75
q = 8 ÷ 32
q = ¼ = 0.25
• If allelic frequencies change, then
evolution is occurring.
• Let’s suppose 4 green frogs enter the
population (immigration). How do the
frequencies change?
4
Immigration:
Determining Allele Frequency
Recall that currently: CG = 0.75 & CR = 0.25
Allele:
Green (15)
Blue (2)
Red (3)
Total:
Frequency:
CG
30
2
0
32
p = 32 ÷ 40
p = 8/10 = 0.80
CR
0
2
6
8
q = 8 ÷ 40
q = 2/10 = 0.20
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Determining Allele Frequency
How do the allelic frequencies
change if 4 green frogs leave the
population instead of enter the
population? (emigration)
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Emigration:
Determining Allele Frequency
Recall that originally: CG = 0.75 & CR = 0.25
Allele:
Green (7)
Blue (2)
Red (3)
Total:
Frequency:
CG
14
2
0
16
p = 16 ÷ 24
p = 2/3 = 0.67
CR
0
2
6
8
q = 8 ÷ 24
q = 1/3 = 0.33
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Impact On Small vs. Large Population
Before 4 frogs joined
After 4 frogs joined
Compare the effect on the small population to 4 frogs joining a
much larger population.
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Impact Large Population
Before 4 frogs joined
After 4 green frogs joined
larger population
larger population
Allele:
CG
CR
Allele:
CG
CR
Green (22)
44
0
Green (26)
52
0
Blue (4)
4
4
Blue (4)
4
4
Red (6)
0
12
Red (6)
0
12
Total:
48
16
Total:
56
16
Frequency:
p = 48 ÷ 64
p = 3/4
= 0.75
q = 16 ÷ 64
q = 1/4
= 0.25
Frequency:
p = 5 ÷ 72
p = 56/72
= 0.78
q = 16 ÷ 72
q = 16/72
= 0.22
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Impact Small Population
Before 4 frogs joined
After 4 green frogs joined
Allele:
CG
CR
Allele:
CG
CR
Green (11)
22
0
Green (15)
30
0
Blue (2)
2
2
Blue (2)
2
2
Red (3)
0
6
Red (3)
0
6
Total:
24
8
Total:
32
8
Frequency:
p = 24 ÷ 32
p = ¾ = 0.75
q = 8 ÷ 32
q = ¼ = 0.25
Frequency:
p = 32 ÷ 40
p = 8/10 = 0.80
q = 8 ÷ 40
q = 2/10 = 0.20
In both cases the allele frequency for CG increases but it has a
bigger impact on the smaller population.
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Genetic Drift
Small populations can experience changes in allele frequencies
more dramatically than large populations. In very large populations
the effect can be insignificant. Also in small populations genes can
be lost more easily. When there is only one allele left for a
particular gene in a gene pool, that gene is said to be fixed , thus
there is no genetic diversity.
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Genetic Drift
• Genetic drift or allelic drift is the change in the
frequency of a gene variant (allele) in a population
due to random sampling in the absence of a selection
pressure.
• Genetic drift is important when populations are
dramatically reduced. Genes are lost and deleterious
genes can also increase.
• When there are few copies of an allele, the effect of
genetic drift is larger, and when there are many
copies the effect is smaller.
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Genetic Drift
• Genetic drift can be most
profound in populations that
are dramatically reduced
(bottle neck populations)
usually due to some
environmental catastrophe.
• Also genetic drift occurs when
a small population arrives at a
new habitat such as an island.
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Bottleneck Example
In 1900, the population of prairie
chickens in Illinois was 100
million but by 1995, the
population was reduced to
around 50 in Jasper County due
to over hunting and habitat
destruction which caused the
bottleneck to occur.
A comparison of the DNA from
the 1995 bird population
indicated the birds had lost most
of their genetic diversity.
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Bottleneck Example
• Additionally, less than 50%
of the eggs laid actually
hatched in 1993.
• Populations outside IL do
not experience the egg
hatching problem.
• Bottleneck populations
generally experience a
severe reduction in genetic
diversity within the
population.
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Bottleneck Example
Genetic drift in smaller populations produces changes in allele
frequency (evolution) whether it is due to a bottleneck or founder
effect.
A greater change of allele frequencies due to gene flow is evident in
smaller populations. As populations rebound in number, their genetic
diversity is still limited compared to the diversity that existed before
the bottleneck event.
Organism
Year/Population
Current Population
Northern Elephant Seal
1890/30
Thousands
Golden Hamster
1930/Single litter
Millions
American Bison
1890/750
360,000
Wisent European Bison
1900’s/12
3,000
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Founder Effect
• The founder effect is the loss of genetic
variation that occurs when a new
population is established by a very small
number of individuals from a larger
population and is a special case of
genetic drift.
• Founder effects are very hard to study!
17
Founder Effect
• Biologist got their chance after a hurricane wiped out all the
lizard species on certain islands in the Bahamas, scientists repopulated the small islands with two lizard pairs, one having
long limbs and one having short limbs.
18
Founder Effect
• Before the hurricane, these
islands supported populations
of a Caribbean lizard, the
brown anole, Anolis sagrei.
• After the hurricane, seven of
the islands were thoroughly
searched. No lizards were
found.
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Founder Effect
• In May 2005, the researchers
randomly selected one male
and one female brown anole
from lizards collected on a
nearby larger island to found
new anole populations on
seven small islands.
• They then sat back and
watched how those lizards
evolved to get an up-close
look at the Founder Effect.
20
Founder Effect
• During the next four years, the researchers repeatedly
sampled lizards from the source island, from the seven
experimental founder islands, and from 12 nearby
islands that served as a control.
• The team found that all lizard populations adapted to
their environment, yet retained characteristics from their
founders.
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A Human Founder Effect Example
• The Amish community was founded
by a small number of colonist.
• The founding group possessed the
gene for polydactyly (extra toes or
fingers).
• The Amish population has increased
in size but has remained genetically
isolated as few outsiders become a
part of the population.
• As a result polydactyly is much
more frequent in the Amish
community than it is in other
communities.
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Impact of Nonrandom Mating
• Nonrandom mating also changes allele frequency.
• Let’s revisit our adorable frogs and suppose that 4
frogs migrate to a pond some distance from the main
pond.
• It is likely that these 4 frogs will mate with one
another, leaving the rest of the population in the main
pond behind to also mate with one another.
• Nonrandom mating implies a choice of mates which is
more prevalent in animals.
23
Two Types of Sexual Selection
• Darwin wrote:
“The sexual struggle is of two kinds; in the one it is
between individuals of the same sex, generally the
males, in order to drive away or kill their rivals, the
females remaining passive; whilst in the other, the
struggle is likewise between the individuals of the same
sex, in order to excite or charm those of the opposite sex,
generally the females, which no longer remain passive,
but select the more agreeable partners.”
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Sexual Selection
• Sexual selection of mates also
affects allele frequency.
• The peacock provides a
particularly well known example
of intersexual selection, where
ornate males compete to be
chosen by females.
• The result is a stunning feathered
display, which is large and
unwieldy enough to pose a
significant survival disadvantage.
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Sexual Selection
• Female birds of many
species choose the male.
• Males that are “showier”
will better attract
females.
• These males have a
selective advantage even
though they are more
susceptible to predators.
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Sexual Selection
• Females that are drab, blend in to their
surroundings and as a result, avoid
predators which giving females a
survival advantage.
• This illustrates that the importance of
mating with the correct male
outweighs the importance of being
preyed upon.
• Sexual selection can lead to sexual
dimorphism where there is a distinct
difference between males and females.
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Hardy-Weinberg Equilibrium
So, when is there no change in the allele frequency? When the
population is said to be in Hardy-Weinberg Equilibrium, thus no evolution
is occurring.
FIVE Conditions of Hardy-Weinberg Equilibrium:
1. Population must be large so chance is not a factor. (No genetic drift).
2. Population must be isolated to prevent gene flow. (No immigration or
emigration)
3. No mutations occur.
4. Mating is completely random with respect to time and space.
5. Every offspring has an equal chance of survival without regard to
phenotypes. (No natural selection)
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Hardy-Weinberg Equilibrium
• Condition #1 can be met. It is important to have large
populations in order that the loss or addition of genes is not a
factor. By contrast, small populations experience genetic drift.
Additionally, if a small population moves to another area or
becomes isolated, the gene pool will be different from the
original gene pool. And the founder effect comes into play.
• Condition #2 can only be met if the population is isolated. If
individuals immigrate or emigrate from the population, the allele
frequencies change and evolution occurs.
• Condition #3 cannot ever be met since mutations always occur.
Thus mutational equilibrium can never be met.
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Hardy-Weinberg Equilibrium
Condition #4 can never be met. Mating is never random. Pollen
from an apple tree in Ohio is more likely to pollinate a tree in
Ohio than one in Washington state.
Condition #5 can never be met. There will always be variation.
Variation can help organisms survive longer and/or reproduce
more effectively.
Since 3 out of the 5 H-W conditions can never be met, evolution
DOES occur and allele frequencies do indeed change.
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Applying the H-W Model
Here we go with our frogs again! Let’s suppose that in a population of 100 frogs,
36 were green (CGCG), 48 were blue (CGCR) and 16 were red (CRCR) and there
was total random mating.
Allele:
CG
CR
Green (36)
72
0
Blue (48)
48
48
Red (16)
0
32
120
80
Total:
Frequency:
p = 120 ÷ 200
p = 3/5 = 0.60
q = 80 ÷ 200
q = 2/5 = 0.40
Thus, it can be assumed that 60% of all the gametes (eggs and sperm) should
carry the CG allele and 40% of the gametes should carry the CR allele.
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Applying the H-W Model
CR 0.40
CG 0.60
CG 0.60 CGCG
0.36
CGCR
0.24
CR 0.40 CGCR
0.24
CRCR
0.16
A population Punnett square is shown above. It indicates that the next generation should
have the following offspring distribution: 36% green (CGCG), 48% blue(CGCR), 16% red
(CRCR). When the second generation gets ready to reproduce, the results will be the same as
before.
Allele:
CG
CR
Green (36)
72
0
Blue (48)
48
48
Red (16)
0
32
120
80
Total:
Frequency:
p = 120 ÷ 200
p = 3/5 = 0.60
q = 80 ÷ 200
q = 2/5 = 0.40
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Applying the H-W Model
So, the allele frequency remains at 0.40 CG and 0.60 CR thus no evolution is taking
place.
Let’s suppose that there is an environmental change that makes red frogs more
obvious to predators. How is the population affected and now the population
consists of 36 green, 48 blue, and 6 red frogs?
Allele:
CG
CR
Green (36)
72
0
Blue (48)
48
48
Red (6)
0
12
Total:
120
60
Frequency:
p = 120 ÷ 180
p = 2/3 = 0.66
q = 60 ÷ 180
q = 1/3 = 0.33
Now, allele frequencies are changing and there is an advantage to being green or
blue but NOT red. Evolution is indeed occurring.
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Deriving the H-W Model
CR 0.40
CG 0.60
CG 0.60 CGCG
0.36
CGCR
0.24
CR 0.40 CGCR
0.24
CRCR
0.16
Examine this Punnett square again. If p represents the allele frequency of CG (dominant)
and q represents the allele frequency of CR (recessive) then two equations for a
population in Hardy-Weinberg equilibrium can be derived where the following
genotypes are represented by:
CGCG = p2
CRCR = q2
CGCR = 2pq
Mathematically then p + q = 0.60 + 0.40 = 1 (1st H-W equation)
So, the Punnett square effectively crossed (p + q )  (p + q ) which gives
p2 + 2pq + q2 = 1 (2nd H-W equation)
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Natural Selection
Natural Selection is the only mechanism that consistently causes
adaptive evolution.
• Evolution by natural selection is a blend of chance and “sorting”.
– Chance in the context of mutations causing new genetic
variations
– Sorting in the context of natural selection favoring some alleles
over others
• This favoring process causes the outcome of natural selection to be
anything but random!
• Natural Selection consistently increases the frequencies of alleles
that provide reproductive advantage and thus leads to adaptive
evolution.
Relative Fitness
• There are animal species in which
individuals, usually males, lock
horns or otherwise compete
through combat for mating
privileges.
• Reproductive success is usually far
more subtle!
• Relative fitness is defined as the
contribution an individual makes to
the gene pool of the next
generation relative to the
contributions of other individuals.
Three Modes of Natural Selection
• Natural selection can alter the frequency distribution of
heritable traits in three ways depending on which
phenotype is favored:
– Directional Selection
– Disruptive Selection
– Stabilizing Selection
Directional Selection
• Directional selection occurs when conditions favor individuals
exhibiting one extreme of a phenotypic range.
• Commonly occurs when the population’s environment changes or
when members of a population migrate to a new (and different)
habitat.
Possible Effect of
Continual Directional Selection
If continued, the variance may decrease.
after
before
after
Phenotype (trait)
before
after
Frequency
Frequency
Frequency
before
Phenotype (trait)
Phenotype (trait)
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Disruptive or Diversifying Selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
• The “intermediates” in the population have lower
relative fitness.
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Disruptive or Diversifying Selection
• Disruptive selection occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
• The “intermediates” in the population have lower
relative fitness.
41
Stabilizing Selection
• Stabilizing selection removes extreme variants from
the population and preserves intermediate types.
• This reduces variation and tends to maintain the
status quo for a particular phenotypic character.
42
Sexual Selection
• A form of selection in which individuals with certain
inherited characteristics are more likely than other
individuals to obtain mates.
• Can result in sexual dimorphism which is a difference
between the two sexes with regard to secondary sexual
characteristics.
43
Intrasexual vs. Intersexual Selection
• How does sexual selection operate?
• Intrasexual—selection within the same sex, individuals
of one sex compete directly for mates of the opposite
sex. Males are famous for this!
44
Intrasexual vs. Intersexual Selection
• Intersexual selection (mate choice)—individuals of one
sex are choosy.
• Often these are females that select mates based on their
showiness.
45
Preserving Genetic Variation
• Some of the genetic variation is populations represents
neutral variation, differences in DNA sequence that do
not confer a selective advantage or disadvantage.
• There are several mechanisms that counter the tendency
for directional and stabilizing selection to reduce
variation:
–
–
–
–
Diploidy
Balancing Selection
Hererzygote Advantage
Frequency-Dependent Selection
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Diploidy
• In diploid eukaryotes each organism has two copies
of every gene and a considerable amount of genetic
variation is hidden from selection in the form of
recessive alleles.
• Often alleles are recessive and less favorable than
their dominant counterparts.
• By contrast, haploid organisms express every gene
that is in their genome. What you see is what you
get. It reduces genetic variability.
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Diploidy
• Recessive alleles persist by propagation in
heterozygous individuals.
• This latent variation is exposed to natural selection
only when both parents carry the same recessive
allele and two copies end up in the same zygote.
• As you might expect, this happens rarely if the
allelic frequency of the recessive allele is very low.
• Why is heterozygote protection of potentially
negative recessive alleles important to species
survival?
48
Balancing Selection
• Balancing selection occurs when natural selection
maintains two or more forms in a population.
• This type of selection includes heterozygote advantage
and frequency-dependent selection.
• Heterozygote advantage involves an individual who is
heterozygous at a particular gene locus thus has a
greater fitness than a homozygous individual.
49
Heterozygote Advantage
• A well-studied case is that of sickle
cell anemia in humans, a hereditary
disease that damages red blood cells.
• Sickle cell anemia is caused by the
inheritance of a variant hemoglobin
gene (HgbS) from both parents.
• In these individuals, hemoglobin in
red blood cells is extremely sensitive
to oxygen deprivation, and this
causes shorter life expectancy.
50
Heterozygote Advantage
• A person who inherits the
sickle cell gene from one
parent, and a normal
hemoglobin gene (HgbA)
from the other, has a normal
life expectancy.
• However, these heterozygote
individuals, known as carriers
of the sickle cell trait, may
suffer problems from time to
time.
51
Heterozygote Advantage
• The heterozygote is resistant to
the malarial parasite which
kills a large number of people
each year in Africa.
• There exists a balancing
selection between fierce
selection against homozygous
sickle-cell sufferers, and
selection against the standard
HgbA homozygotes by malaria.
• The heterozygote has a
permanent advantage (a higher
fitness) wherever malaria
exists.
52
Heterozygote Advantage
53
Frequency-Dependent Selection
• The fitness of a phenotype depends on how common it
is in the population.
• In positive frequency-dependent selection the fitness
of a phenotype increases as it becomes more common.
• In negative frequency-dependent selection the fitness
of a phenotype increases as it becomes less common.
• For example in prey switching, rare morphs of prey are
actually fitter due to predators concentrating on the
more frequent morphs.
54
Balanced Polymorphism
Balanced polymorphism occurs in a given
population when two distinct types (or
morphs) exists and the allele frequencies do
not change. This may be due to
• Variation in the environment where one
morph may be favored over another.
• One morph may be better adapted to a
certain time of the year over the other.
The lady bird beetle has 2 morphs. The red
variety is more abundant in the spring and
winter, whereas the black morph is more
abundant in the summer and fall.
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Why Natural Selection Cannot Fashion
Perfect Organisms
1. Selection can act only on existing variations.
• Natural selection favors only the fittest phenotypes among
those in the population, which may not be the ideal traits.
New advantageous alleles do not arise on demand.
2. Evolution is limited by historical constraints.
• Each species has a legacy of descent with modification
from ancestral forms. Evolution does not scrap the
ancestral anatomy. For example in birds and bats, an
existing pair of limbs took on new functions for flight as
these organisms evolved from nonflying ancestors.
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Why Natural Selection Cannot Fashion
Perfect Organisms
3. Adaptations are often compromises.
• The loud call that enables a frog to attract mates also
attracts predators.
4. Chance, natural selection and the environment
interact.
• Chance can affect the subsequent evolutionary history of
populations. A storm can blow birds hundreds of
kilometers over an ocean to an island, the wind does not
necessarily transport those individuals that are best suited
to the environment!
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Created by:
Carol Leibl
Director of Science Content
National Math and Science
Dallas, TX