Presentation MEDIA: Genetics & Evolution Series
Population
Genetics
Set No. 6
Set 6: Population Genetics
Presentation MEDIA
© 1993-2001 Biozone International Ltd
ISBN 0-909031-45-2
Index to OHT Titles
OHT Title
OHT Title
1
Species
24
Genetic Drift Patterns
2
Populations
25
Population Bottlenecks 1
3
Gene Pool
26
Population Bottlenecks 2
4
Analysing a Gene Pool
27
Population Bottleneck in Cheetahs
5
Determining Allele Frequencies
28
The Founder Effect 1
6
Determining Genotype Frequencies
29
The Founder Effect 2
7
Changes in a Gene Pool 1
30
Natural Selection
8
Changes in a Gene Pool 2
31
Assortative Mating
9
Changes in a Gene Pool 3
32
Modes of Natural Selection
10
Hardy-Weinberg Equilibrium: An Introduction
33
Stabilising Selection 1
11
Conditions Required for Hardy-Weinberg
Equilibrium
34
Stabilising Selection 2
35
Stabilising Selection in Human Birth Weights
12
Derivation of the Hardy-Weinberg Equation
36
Directional Selection 1
13
The Hardy-Weinberg Equation
37
Directional Selection 2
14
How to Solve Hardy Weinberg Problems
38
Natural Selection in Peppered Moths 1
15
Hardy-Weinberg Problem: A Worked Example
39
Peppered Moths 2
16
Changing Allele Frequencies
40
Peppered Moths 3
17
Mutations
41
Disruptive Selection 1
18
Gene Flow
42
Disruptive Selection 2
19
The Effect of Population Size
43
Heterozygote Advantage
20
Genetic Drift
44
Sickle Cell and Malaria
21
Genetic Drift Example 1
45
Hybrid Vigor
22
Genetic Drift Example 2
46
Artificial Selection in Brassica
23
Genetic Drift Example 3
47
Artificial Selection in Dogs
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Species
A biological species is a grouping of organisms that can
interbreed and are reproductively isolated from (i.e.
cannot breed with) other interbreeding groups.
Species are usually recognised on the basis of their
morphology (size, shape, and appearance).
For example, there are over 200 species of turtles,
which are different in appearance and do not interbreed:
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OHT 1
Populations
From a population genetics viewpoint:
1. A population comprises the total number of a species
inhabiting a particular geographical area.
2. All members of a population have the potential to
interact with each other. This includes breeding.
Populations can be very large and occupy a large area,
with fairly continuous distribution.
Populations may also be limited in their distribution and
exist in isolated pockets or “islands”, cut off from other
populations of the same species.
Fragmented Distribution
Example: Some
frog species
Continuous Distribution
Example: Arctic tundra
plant species
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OHT 2
Gene Pool
Definition of a Gene Pool: The sum total of all the genes
present in a population at any one time.
Not all the individuals will be breeding at a given time.
The population may have a distinct geographical boundary.
Each individual is a carrier of part of the total genetic
complement of the population.
Geographic boundary
of gene pool
Individual with a
homozygous
recessive condition
Aa
Aa
aa
Individual with a
heterozygous
condition
AA
AA
AA
Aa
Aa
Aa
aa
Aa
aa
Individual with a
homozygous
dominant condition
AA
AA
aa
AA
A gene pool made up of 16 individual organisms
with gene A, and where gene A has two alleles
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OHT 3
Analysing a Gene Pool
By determining the frequency of allele types (e.g. A and a)
and genotypes (e.g. AA, Aa, and aa) it is possible to
determine the state of the gene pool.
The state of the gene pool will indicate if it is stable or
undergoing change for some reason – an important
indicator that evolutionary events may be taking place.
There are twice the number of alleles for each gene as
there are individuals, since each individual has two alleles.
Aa
Aa
aa
AA
AA
Aa
Aa
Aa
EXAMPLE
The small gene pool above consists of 8 individuals.
Each individual has 2 alleles for a single gene A, so
there are a total of 16 alleles in the population.
There are individuals with the following genotypes:
Homozygous dominant (AA)
Heterozygous (Aa)
Homozygous recessive (aa)
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OHT 4
Determining
Allele Frequencies
Count up the respective numbers of dominant and
recessive alleles in the total population, regardless of
the combinations in which they occur:
Convert these to percentages by:
No. of Dominant Alleles
Total No. of Alleles
Aa
Aa aa
AA
Aa
Aa aa
AA
Aa
AA
Aa
Aa
Aa
Frequency of Dominant Allele
There are 9 dominant alleles
out of a total of 16:
A = 9/16 x 100
= 56.25%
Aa
AA
X 100
Aa
Frequency of Recessive Allele
There are 7 recessive alleles
out of a total of 16:
a = 7/16 x 100
= 43.75%
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OHT 5
Determining
Genotype Frequencies
Count up the actual number of each genotype in the
population: homozygous dominant (AA), heterozygous
(Aa) and homozygous recessive (aa).
Aa
Aa aa
AA
Aa
Aa aa
AA
Aa
Aa aa
AA
Aa
AA
Aa
Aa
Aa
AA
Aa
Aa
Aa
AA = 2/8 x 100
= 25%
Heterozygous Frequency
There are 5 heterozygous genotypes
out of a total of 8:
Aa = 5/8 x 100
= 62.5%
Aa
AA
Homozygous Dominant Frequency
There are 2 homozygous dominant
genotypes out of a total of 8:
Aa
Homozygous Recessive Frequency
There is 1 homozygous recessive
genotype out of a total of 8:
AA = 1/8 x 100
= 12.5%
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OHT 6
Changes in a Gene Pool 1
Phase 1: Initial Gene Pool
– In the gene pool below there are 25 individuals, each
possessing two copies of a gene for a trait called A.
– This is the gene pool before changes occur:
Aa
aa
AA
Aa
Aa
Aa
AA
AA
Aa
Aa
Aa
Aa
Aa
Aa
aa
Aa
Aa
AA
AA
AA
Aa
AA
aa
aa
aa
A
a
AA
Aa
aa
No.
27
23
7
13
5
%
54
46
28
52
20
Allele Types
Allele Combinations
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OHT 7
Changes in a Gene Pool 2
Phase 2: Natural Selection
In the same gene pool, at a later time, two pale individuals
died due to the poor fitness of their phenotype:
Two pale individuals died and therefore their
alleles are removed from the gene pool
Aa
aa
AA
Aa
Aa
Aa
aa
Aa
AA
aa
Aa
Aa
AA
AA
Aa
Aa
aa
Aa
AA
Aa
aa
AA
AA
Aa
Aa
A
a
AA
Aa
aa
No.
27
19
7
13
3
%
58.7
41.3
Allele Types
30.4 56.5 13.0
Allele Combinations
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OHT 8
Changes in a Gene Pool 3
Phase 3: Immigration/Emigration
At a still later time, one beetle joins the gene pool
while another leaves:
This individual is entering
the population and will add
its alleles to the gene pool
AA
Aa
Aa
aa
AA
aa
This individual is leaving
the population, removing its
alleles from the gene pool
Aa
AA
Aa
Aa
AA
AA
aa
Aa
Aa
AA
Aa
AA
Aa
Aa
Aa
Aa
AA
Aa
A
a
AA
Aa
aa
No.
29
17
8
13
2
%
63
37
Allele Types
34.8 56.5
8.7
Allele Combinations
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OHT 9
Hardy-Weinberg Equilibrium:
An Introduction
Populations are often stable (no phenotypic change)
over many generations.
The stability of many populations over time was
explained by two scientists:
G. Hardy
an English mathematician
W. Weinberg a German physician
They formulated the Hardy-Weinberg law which
mathematically describes the frequency of alleles in a
sexually reproducing population.
It shows that no matter how many times alleles are
segregated by meiosis and recombined by fertilisation
the allele frequency remains constant generation
after generation – the alleles are just shuffled about
within the breeding population.
The population is in genetic equilibrium
and no change in allele frequencies occurs
over many generations.
The equilibrium is only maintained in the absence of
destabilising events.
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OHT 10
Conditions Required for
Hardy-Weinberg Equilibrium
The Hardy-Weinberg equilibrium is maintained if
all the following “stabilising” conditions are met:
1. Large Population: The population size is large.
2. Random Mating: Every individual of
reproductive age has an equal chance of finding
a mate.
3. No Emigration / Immigration: There is no
movement of individuals into or out of the
population – no gene flow.
4. No Selection Pressure: All genotypes have
an equal chance of reproductive success – no
selection pressure.
5. No Mutation: There is no mutation to introduce
new alleles.
Natural populations seldom meet all these requirements
and therefore allele frequencies will change.
A permanent change in the allele frequencies in a
population is termed microevolution.
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OHT 11
Derivation of the
Hardy-Weinberg Equation
The Hardy-Weinberg equation is applied to populations
with a simple genetic situation: dominant and recessive
alleles controlling a single trait.
The frequency of all of the dominant alleles (A) and
recessive alleles (a) equals the total genetic complement,
and adds up to 1 or 100% of the alleles present.
p represents the frequency of allele A while q represents
the frequency of allele a in the population.
Frequency of allele
combination AA in
the population
Punnett Square
p
q
p
pp
pq
q
qp
qq
Frequency of allele
combination aa in
the population
Frequency of allele combination Aa in the
population (add these together to get 2pq)
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OHT 12
The Hardy-Weinberg Equation
The Hardy-Weinberg equilibrium can be expressed
mathematically by giving the frequency of all the allele
types in the population:
Frequency of allele
combination: AA
(homozygous dominant)
Frequency of allele
combination: Aa
(heterozygous)
Frequency
of allele: a
Frequency of allele
combination: aa
(homozygous recessive)
Frequency
of allele: A
2
(p + q)
=
Frequency of
Allele Types
2
2
p + 2pq + q
= 1
Frequency of
Allele Combinations
The sum of all the allele types:
A and a = 1 (or 100%)
The sum of all the allele combinations:
AA, Aa and aa = 1 (or 100%)
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OHT 13
How to Solve
Hardy-Weinberg Problems
The following steps outline the procedure for solving a
Hardy-Weinberg problem:
1. Examine the question and determine what information you have
been given about the population.
In most cases, it is the percentage or frequency of the recessive
phenotype (q2) or the dominant phenotype (p2 2pq). These provide
the only visible means of gathering data about the gene pool.
2. The first objective is to find out the value of p or q. If this is achieved,
then every other value in the equation can be determined by simple
calculation. If necessary q2 can be obtained by calculating:
1 – frequency of the dominant phenotype
3. Take the square root of q2 to find q
4. Determine p by subtracting q from 1 (i.e. p = 1 – q)
5. Determine p2 by multiplying p by itself (i.e. p2 = p x p)
6. Determine 2pq by multiplying p times q times 2
7. Check your calculations by adding p2 + 2pq + q2
always equal 1.
–
these should
Remember that all calculations must be carried
out using decimal fractions – NOT PERCENTAGES!
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OHT 14
Hardy-Weinberg Problem:
A Worked Example
Approximately 70% of caucasian Americans can taste the
chemical called P.T.C. (the dominant phenotype) while 30%
are non-tasters (the recessive phenotype).
Data Provided:
Frequency of the dominant phenotype p = 70% or 0.7
Frequency of the recessive phenotype q = 30% or 0.3
Working
Recessive phenotype:
therefore:
therefore:
q2
q
p
= 0.30
= 0.5477 (square root of 0.30)
= 0.4522 (1 – q = p)
(1 – 0.5477 = 0.4522)
Now use p and q in the equation to solve any unknown:
Homozygous Dominant: p2 = 0.2045 (0.4522 x 0.4522)
Heterozygous:
2pq = 0.4953 (2 x 0.4522 x 0.5477)
The frequency of:
(a) Homozygous recessive phenotype = q2
(b) Dominant allele
=p
(c) Homozygous tasters
(d) Heterozygous tasters
= 30%
= 45.2%
= 20.5%
= p2
= 2pq = 49.5%
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OHT 15
Changing Allele Frequencies
Gene pools are subjected to a number of processes
that can alter the frequency of alleles for a gene:
1. Mutation
2. Gene flow (immigration and emigration)
3. Small population size and genetic drift
4. Natural selection
5. Non-random mating
Some of these processes cause random changes,
others may be directional (i.e. they favour some
alleles at the expense of others).
Mate
Selection
aa
Mutations
Immigration
Aa
Aa
AA
AA
Emigration
A'A
Aa
AA
aa
AA
Aa
Aa
AA
Aa
AA
Aa
aa
Aa
Aa
AA
Aa
aa
Boundary of
gene pool
Natural
Selection
Gene Flow
Geographical
Barrier
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OHT 16
Mutations
Mutations are the source of all new alleles.
Mutations can therefore change the frequency of
existing alleles by competing with them.
Recurrent spontaneous mutations may become
common in a population if they are not harmful and
are not eliminated.
New recessive
allele
Aa
In the graph below, a mutation
creates a new recessive allele: a'
Aa
a'a
The frequency of this new allele
increases when environmental
conditions change, giving it a
competitive advantage over the
other recessive allele: a
AA
Aa
AA
aa
aa
Aa
Aa AA
aa
AA
Aa
aa
Aa
Aa
AA
AA
Aa
1.0
Environmental
conditions change
0.9
Allele Frequency
0.8
0.7
A
0.6
0.5
0.4
0.3
0.2
0.1
0
a'
Mutation causes the
formation of a new
recessive allele
5
0
a
10
15
20
Generations
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OHT 17
Gene Flow
Gene flow is the movement of genes into or out of a
population: immigration and emigration.
A population may gain or lose alleles through gene flow.
Gene flow tends to reduce the differences between
populations because the gene pools become more similar.
Barriers to gene flow
Aa
AA
Aa
Aa
AA
Aa
AA
AA
AA aa
aa
AA
AA Aa
Aa
AA
Aa
Aa
AA
Aa
AA
Aa AA
Aa
Aa
Aa
AA aa
Population B
Population C
Emigration out of
population B
AA
AA
Aa
Aa
Aa
AA
aa
AA
AA Aa
AA
aa
Aa
Aa
AA
Aa
aa AA
AA
Immigration into
population B
AA
Aa
Aa aa
Aa
Aa
Aa
No Gene Flow
Aa
AA
aa
aa
aa Aa
Aa Aa
Aa aa AA
aa
Aa
aa
Aa
AA
Population A
AA
Aa
aa
Aa
aa
Aa
AA
aa
Aa
aa
aa
Aa
AA
Aa AA
Aa
aa Aa
Aa Aa
Aa aa AA
aa
Aa
aa
Aa
Gene Flow
AA
Aa
AA
Population A
aa
Aa
Population B
Aa aa
Aa
aa AA
AA
aa
Aa
Population C
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OHT 18
The Effect of Population Size
Large populations have more stable allele frequencies
because they have a greater reservoir of variability and
are less affected by changes involving only a few
individuals.
Small populations have fewer alleles to begin with and
so the severity and speed of changes in the allele
frequencies are greater.
Endangered species with very low population numbers
may be subjected to severe and rapid allele changes.
Aa
aa
Aa
Aa
aa
Aa
aa Aa
Aa
Aa
AA
aa
Aa
Aa
Aa
Aa
Aa
aa
aa
Aa
Aa
Aa
Aa
Aa AA
Aa
Small Population
Aa
Aa
Aa Aa
Aa
Aa
aa Aa
AA
Aa
AA
aa Aa
Aa
Aa
AA
AA AA Aa
Aa AA
Aa
aa
aa
Large Population
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OHT 19
Genetic Drift
Not all individuals, for various reasons, will be able to
contribute their genes to the next generation.
Genetic drift refers to random changes in allele
frequencies that occur in all populations, although having
the most pronounced effects on small populations.
In small inbreeding populations, alleles may become:
1. Lost from the gene pool altogether
(frequency becomes 0%)
2. Fixed as the only allele present in the gene
pool (frequency becomes 100%)
Genetic drift is often a feature of small populations that
become isolated from the larger population gene pool (as
for example with island colonisers).
The next 3 OHTs show a hypothetical gene pool of a small
population over three generations:
– For various reasons, not all individuals are contributing
their alleles to the next generation.
– With the random loss of the alleles carried by these
individuals, the allele frequency changes from one
generation to the next.
– The change in frequency is ‘directionless’ as there is no
selecting pressure.
– The allele combinations for each successive generation are
determined by how many alleles of each type 'survive' and
are passed on from the preceding one.
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OHT 20
Genetic Drift Example 1
Generation 1: Two of the beetles fail to locate a mate
due to the sparse distribution of the population.
There was no other reason for them not to find a mate
and this factor alone prevented them from contributing
their genes to the next generation.
An example may be the sparsely distributed individuals
of the Siberian tiger population.
A = 16 (53%)
a = 14 (47%)
AA
Aa
Aa
Aa
aa
AA
Aa
Aa
Aa
aa
AA
AA
Aa
Aa
aa
Fail to locate a mate due
to low population density
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OHT 21
Genetic Drift Example 2
Generation 2: Another two beetles fail to breed because
they could not find a mate due to low population density.
Two dark beetles were accidentally killed in a rock fall –
this could equally have killed any beetle, it was not a test
of the ‘fitness’ of the beetle’s phenotype.
The effect this had on the gene pool was to reduce the
dominant allele frequency from 53% to 50%:
A = 15 (50%)
a = 15 (50%)
AA
Aa
Aa
Aa
aa
AA
Aa
Aa
Aa
aa
AA
Aa
Aa
Aa
aa
Killed in a
rock fall
Fail to locate a mate due
to low population density
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OHT 22
Genetic Drift Example 3
Generation 3: A dark beetle was blown out to sea by
very strong winds during a cyclone and died. Again this
was not a true test of the ‘fitness’ of the phenotype but
was due to chance.
The effect on the gene pool was to further reduce the
dominant allele frequency from 50% to 43%:
A = 13 (43%)
a = 17 (57%)
AA
Aa
Aa
aa
aa
AA
Aa
Aa
aa
aa
AA
Aa
Aa
Aa
aa
Killed in a
cyclone
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OHT 23
Genetic Drift Patterns
Below are displayed the changes in allele frequencies in a
computer simulation showing random genetic drift.
Allele Frequency (%)
The breeding populations vary from 2,000 (top) to 20
(bottom) and each simulation was run for 140 generations.
100
Large Gene Pool
80
Breeding Population =
60
2,000
Fluctuations are minimal
because large numbers of
individuals buffer the population
against large changes in allele
frequencies
40
20
0
0
20
40
60
80
100
120
140
Allele Frequency (%)
Generations
100
Small Gene Pool
80
Breeding Population = 200
Fluctuations are more severe
because random changes in
a few alleles cause a greater
percentage change in allele
frequencies
60
40
20
0
0
20
40
60
80
100
120
140
Allele Frequency (%)
Generations
100
Very Small Gene Pool
80
Breeding Population = 20
Fluctuations are so extreme that
the allele may become fixed
(100%) or lost altogether (0%)
Allele lost from
the gene pool
60
40
20
0
0
20
40
60
80
100
120
140
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OHT 24
Population Bottlenecks 1
Populations may sometimes be reduced to low numbers
through periods of seasonal climatic change, heavy
predation or disease, or through catastrophic events such as
volcanic eruptions or other natural disasters.
As a result, only a small number of individuals remain in the
gene pool to contribute their genes to the next generation.
The small sample that survives will often not be representative
of the original, larger gene pool, and the resulting allele
frequencies may be severely altered.
In addition to this ‘bottleneck’ effect, the small surviving
population is often affected by inbreeding and genetic drift.
The original gene pool is made up of the offspring of
many lineages (family groups and sub-populations)
Lineage A
TIME
Extinction
Lineage B
Extinction
Extinction
Only 2 descendents
of lineage B survive
the extinction event
Lineage C
Extinction
Genetic
Bottleneck
Extinction
Extinction
Extinction event such
as a volcanic eruption
All present day descendents of the original gene pool trace
their ancestry back to lineage B and therefore retain only
a small sample of genes present in the original gene pool
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OHT 25
Large population with
plenty of genetic diversity
aa
AA
AA
Aa
Population crashes to a very
low number and has lost
most of its genetic diversity
Aa
AA
aa
AA
AA
Aa
AA
AA
AA
Aa
Population grows to a large
size again, but has lost
much of its genetic diversity
AA
AA
AA
AA
Aa
AA
AA
AA
Aa
AA
AA
AA
AA
AA
Aa
Aa
Aa
AA
OHT 26
Population Numbers
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Population Bottlenecks 2
AA
Aa
Population Bottleneck:
the population nearly
becomes extinct as
numbers plummet
Time
Population Bottleneck
in Cheetahs
The world population of cheetahs has declined in recent
years to fewer than 20,000.
Recent genetic analysis has found that the total cheetah
population has very little genetic diversity.
Cheetahs appear to have narrowly escaped extinction at the
end of the last ice age: 10 – 20,000 years ago.
All modern cheetahs may have arisen from a single
surviving litter – accounting for the lack of diversity.
At this time, 75% of all large mammals perished (including
mammoths, cave bears, and sabre-toothed tigers).
The lack of genetic variation has led to:
1. Sperm abnormalities
2. Decreased fecundity
3. High cub mortality
4. Sensitivity to disease
Since the genetic bottleneck, there has been insufficient
time for random mutations to produce new genetic variation.
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OHT 27
The Founder Effect 1
Occasionally, a small number of individuals from a
population may migrate away, or become isolated, from
their original population.
This colonising or ‘founder’ population will have a small
and probably non-representative sample of alleles from
the parent population’s gene pool.
As a consequence of this founder effect, the
colonising population may evolve in a different direction
than the parent population.
Such small founder populations are subject to random
genetic drift.
Aa
Aa
AA
Aa
AA
Aa
Aa
Aa
AA
AA
Aa
Colonisation
aa
Aa
aa
Aa
Aa
AA
aa
Aa
AA
AA
Aa
aa
Aa
aa
Aa
AA
Aa
Aa
Aa
Aa
Aa
Mainland
Population
Aa
aa
Aa
aa
Aa
AA
Aa
Aa
Aa
Aa
AA
AA
AA
AA
Aa
Aa
AA
Aa
Aa
AA
Island
Population
The founder effect is typically seen in the populations of
islands which are colonised by individuals from mainland
populations - often these species have low mobility.
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OHT 28
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OHT 29
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The Founder Effect 2
In this hypothetical population of beetles, a small randomly selected group is blown
offshore to a neighbouring island where they establish a breeding population.
Aa
Aa
AA
Aa
Aa
Aa
AA
Some individuals from the mainland
population are carried at random to
the offshore island by natural forces
such as strong winds
AA
Aa
AA
Aa
aa
Aa
aa
Aa
Aa
AA
aa
Aa
AA
aa
Aa
aa
Aa
AA
Aa
AA
Aa
Aa
Aa
Aa
AA
AA
AA
Aa
Aa
Aa
Aa
AA
Aa
Aa
Aa
AA
Aa
AA
aa
Aa
aa
This population may
not have the same
allele frequencies as
the mainland
population
Aa
Aa
Aa
AA
Mainland
Population
Island
Population
Natural Selection
Populations of sexually reproducing organisms consist of
varied individuals, with some variants leaving more
offspring than others.
This differential success in reproduction is called natural
selection.
Natural selection acts on the phenotype of individuals in
the following way:
1. It eliminates or reduces the reproductive success of
individuals with poorly-suited phenotypes (their alleles
become less common in the gene pool).
2. It enhances the survival and reproductive success of
individuals with well-suited phenotypes (their alleles
become more common in the gene pool).
Natural selection therefore changes the composition of a
gene pool and increases the probability that favourable
alleles will come together in the same individual.
AA
aa
aa
Aa AA
Aa
Aa
AA
aa
Aa
aa
Aa
Aa
Aa
Aa
Aa Aa
AA
Selection pressures
may reduce certain
allele frequencies
AA
aa
Aa AA
Aa
Aa
AA
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OHT 30
Assortative Mating
Random (non-assortative) mating allows all genotypes to
have an equal chance of reproductive success.
Individuals may deviate from this by:
1. Mating more often with close neighbours than with
distant members of the population.
2. Choosing mates that are most like themselves.
The most extreme case is self-fertilisation in some
species of plant.
Sometimes individuals may show random mating for
some alleles but not others.
EXAMPLE: Humans exhibit assortative mating for racial
features, but mate randomly for blood types.
While assortative mating does not change the frequency
of alleles in the overall gene pool, it does cause the ratio
of genotypes to depart from that of random mating.
Aa
AA
aa
aa
AA
aa
aa
Aa
Aa
AA
Aa
AA
AA
aa
aa
Aa
aa
AA
AA
Aa
AA
aa
AA
Random Mating
AA
aa
aa
Assortative Mating
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OHT 31
Modes of Natural Selection
Natural selection changes allele frequencies in populations,
but it does not produce the “perfect organism”.
Rather than developing new phenotypes, it reduces the
frequency of phenotypes that are less suited to the prevailing
conditions.
Proportion of Population
Traits (e.g. skin colour, height) that are under polygenic
control show quantitative variation in the phenotype. Natural
selection acts on this variation.
Bell-shaped
curve
Variation in Phenotype
Natural selection may produce phenotypic change over time.
The direction of this change will depend on the nature of the
selection pressure. Selection may be:
❑ Stabilising
❑ Directional
❑ Disruptive
These can be depicted with graphs that show how the
frequencies of different phenotypes change over time.
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OHT 32
Stabilising Selection 1
Probably the most common trend in natural populations.
Stabilising selection favours the most common
phenotype as the best adapted.
Selection reduces variation by selecting against the
extremes at each end of the range – the resulting bell
shaped curve is narrower, about the same mean.
EXAMPLE: Human birth weights are maintained in the
3-4 kg range by selection pressures at the extremes.
Frequency
Retained
Eliminated
Eliminated
Variation in Phenotype
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OHT 33
Stabilising Selection 2
Before selection (below) there is a broad range of
variation in the population:
Frequency
Retained
Eliminated
Eliminated
Frequency
After selection (below) and some generations later
there is a reduction in the amount of variation:
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OHT 34
Selection in
Human Birth Weights
Stabilising selection against extremes in the birth
weight range results in most births being between 3-4 kg.
Percent of Births Sampled
Modern medical intervention is reducing this selection.
40
80
35
70
30
60
25
50
20
40
15
30
10
20
5
10
0
0
0
1
2
3
4
5
6
Birth Weight (kg)
Selection against low birth
weight (small babies) with
poor organ development
Selection against high birth
weight (large babies) due
to childbirth complications
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OHT 35
Directional Selection 1
Most common during periods of environmental change.
Directional selection favours the phenotypes at one
extreme of a phenotypic range.
Selection reduces variation at one extreme of the range
while favouring variants at the other end – the resulting
bell shaped curve shifts in the direction of selection.
EXAMPLE: Fossil evidence shows that the average size
of black bears in Europe increased with each ice age,
only to decrease again during the interglacials.
Frequency
Retained
Eliminated
Variation in Phenotype
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OHT 36
Directional Selection 2
Before selection (below) there is a broad range of
variation in the population:
Frequency
Retained
Eliminated
Frequency
After selection (below) and some generations later
there is a reduction in variation at one extreme of the
range while favouring variants at the other end.
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OHT 37
Natural Selection
in Peppered Moths 1
The peppered moth, Biston betularia, occurs in two
forms (or morphs):
The mottled grey form is well camouflaged against the
lichen- covered bark of trees in unpolluted regions,
making detection by predators more difficult.
The dark melanic forms are disadvantaged in such
environments as their body shape stands out against
the background.
Grey or mottled form of
the peppered moth Biston betularia
Melanic or carbonaria form of
the peppered moth Biston betularia
With the onset of the Industrial Revolution in England,
the air quality declined, killing off lichen and resulting in
a marked increase in the frequency of the dark moths.
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OHT 38
Peppered Moths 2
In the 1940s and 1950s, coal burning was still intense
around the industrial centres of Manchester and Liverpool.
During this time, melanic forms were still very dominant.
In the rural areas further south and west of these industrial
centres, the grey forms increased dramatically.
Industrial areas
Non-industrial areas
Scale
60 km
60 miles
Frequency of
Peppered Moth
forms in 1950
Glasgow
Newcastle
Belfast
Middlesbrough
Leeds
Manchester
Liverpool
Hull
Sheffield
Nottingham
Key to Frequency Graphs
Birmingham
Grey or
speckled form
Cardiff
Melanic or
carbonaria form
Leicester
Coventry
London
Bristol
Portsmouth
Plymouth
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OHT 39
Peppered Moths 3
With the decline of coal burning factories and the Clean
Air Acts in cities, the air quality improved between 1960
and 1980.
Sulphur dioxide and smoke levels dropped to a fraction
of their previous levels.
This caused the relative numbers of melanic peppered
moths to plummet.
Frequency of Melanic Peppered Moth Related
to Reduced Air Pollution
75
Melanic Biston betularia
150
80
50
Summer
smoke
70
60
25
Winter
sulphur
dioxide
50
0
40
1960
1965
1970
1975
1980
Summer smoke (µg/m3)
Frequency of melanic form
of Biston betularia (%)
90
100
50
Winter sulphur dioxide (µg/m3)
100
0
1985
Year
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OHT 40
Disruptive Selection 1
Occurs when environmental conditions are varied or when
the environmental range of an organism is large.
Disruptive selection favours phenotypes at both
extremes of a phenotypic range over intermediate
variants. The bell shaped curve acquires two peaks (i.e.
becomes bimodal).
This type of selection can lead to the formation of clines
or ecotypes (organisms of the same species that are
slightly different in appearance), and polymorphism.
EXAMPLE: African swallow tail butterfly has developed
three different populations that are Batesian mimics of
other local distasteful butterfly species.
Frequency
Eliminated
Retained
Retained
Variation in Phenotype
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OHT 41
Disruptive Selection 2
Before selection (below) there is a broad range of
variation in the population:
Frequency
Eliminated
Retained
Retained
Frequency
After selection (below) and some generations later
individuals at both extremes of a phenotypic range are
favoured over intermediate variants (2 peaks).
Two peaks
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OHT 42
Heterozygote Advantage
Often individuals that are heterozygous for a particular
gene (Aa) are more common in the population than would
be predicted using Hardy-Weinberg laws.
This is because the heterozygous condition can have a
greater fitness than either homozygote (AA or aa).
This is termed heterozygote advantage.
Example: Sickle cell gene
The mutant allele for sickle cell disease produces
abnormal haemoglobin. This causes deformation of the
red blood cells, so that they are destroyed.
Heterozygotes (HbS,Hb) carry alleles for both sickle cell
and normal haemoglobin. This results in greater malaria
resistance but only mild anaemia.
Homozygotes for normal haemoglobin (Hb,Hb) have
greater susceptibility to malaria – in regions where malaria
is prevalent, heterozygotes occur in greater numbers.
Hb, Hb
HbS, Hb
HbS, HbS
All red blood cells
are normal
Mixture of normal and
sickle red blood cells
All red blood cells
are sickle shaped
There is a good correlation between the incidence of
malaria and regions where there is a high frequency of the
sickle cell gene.
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OHT 43
Sickle Cell Disease and Malaria
Areas affected by
falciparum malaria
Incidence of Falciparum Malaria
1% - 5%
5% - 10%
10% - 20%
Frequency of Sickle Cell Gene (HbS)
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OHT 44
Hybrid Vigour
Individuals that are heterozygous for a number of
different genes often demonstrate hybrid vigour.
The hybrid offspring of two inbred organisms often
show greater fitness (better growth and productivity for
example) than either parent.
The reasons for hybrid vigour are not always clear:
Genes may affect more than one trait and
heterozygotes may benefit from the effects of a number
of different interactions expressed in the phenotype.
EXAMPLE: Hybrid corn is valued for its high
productivity. It is produced by crossing inbred parental
strains with a high degree of homozygosity.
Parental Strain A
Homozygous
for many genes
Parental Strain B
Homozygous
for many genes
X
AAbbCCddEEff
aaBBccDDeeFF
AaBbCcDdEeFf
Heterozygous
for many genes
Hybrid
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OHT 45
Artificial Selection in Brassica
Different parts of this plant have been developed by
human selection to produce 6 separate vegetables with
enormous visible differences.
All these vegetables form a single species and will
interbreed if allowed to flower (e.g. the new broccoflower).
Cauliflower
(Flower)
Broccoli
(Inflorescence)
Cabbage
(Terminal
Buds)
Brussels Sprout
(Lateral Buds)
Kale
(Leaf)
Kohlrabi
(Stem)
Wild Form
Brassica oleracea
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OHT 46
Artificial Selection in Dogs
The dog was probably first domesticated at least 14,000
years ago from a grey wolf ancestor.
Some 400 breeds have developed from this single wild
species as a result of selective breeding by humans.
Grey Wolf
Sheepdog
Greyhound
Canis lupus pallipes
Small wolf. Once common
throughout Europe, Asia and,
North America which displayed
a wide variety of coat coloration.
Canis familiaris metris optimae
Originating in Europe, this
breed has been used to guard
flocks from predators for
thousands of years.
Canis familiaris leineri
Drawings of this type on pottery
dated 8,000 years ago in the
Middle East, make this one of
the oldest.
Wolf-like
Mastif-Type
Pointer-Type
Canis familiaris palustris
Found in snow covered habitats
in northern Europe, Asia (Siberia)
and North America (Alaska).
Canis familiaris inostranzevi
Originally from Tibet, the first
records of this breed of dog
go back to Neolithic times.
Canis familiaris intermedius
Probably derived from the
greyhound breed for the
purpose of hunting small game.
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OHT 47