Transcript changing allele frequencies

MICROEVOLUTION:
CHANGING ALLELE FREQUENCIES
1
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.
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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 purple, 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
Purple (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 do4 the
frequencies change?
IMMIGRATION:
DETERMINING ALLELE FREQUENCY
Recall that currently: CG = 0.75 & CR = 0.25
Allele:
Green (15)
Purple (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)
Purple (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
purple (4)
4
4
purple (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
purple (2)
2
2
purple (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. 14
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!
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FOUNDER EFFECT
• Biologist got their chance after a hurricane wiped out
all the lizard species on certain islands in the
Bahamas, scientists re-populated the small islands
with two lizard pairs, one having long limbs and one
having short limbs.
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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.
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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
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change.
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 purple (CGCR) and 16 were
red (CRCR) and there was total random mating.
Allele:
CG
CR
Green (36)
72
0
purple (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 CR31
allele.
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
purple (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 purple, and 6 red frogs?
Allele:
CG
CR
Green (36)
72
0
purple (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
purple 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.
before
after
before
after
after
before
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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.
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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.
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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!
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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.
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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?
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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.
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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.
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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
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nonflying ancestors.
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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