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PopulationGenetics

Set No. 6

Presentation MEDIA: Genetics & Evolution Series

Set 6: Population Genetics

NEW ZEALAND:Biozone International LtdP.O. Box 13-034 HamiltonTelephone: +64 (7) 856-8104FAX: +64 (7) 856-9243E-mail: [email protected]

AUSTRALIA:Biozone Learning Media AustraliaP.O. Box 7523 GCMC 4217 QLDTelephone: +61 (7) 5575-4615FAX: +61 (7) 5572-0161E-mail: [email protected]

UNITED KINGDOM:Biozone Learning Media (UK)P.O. Box 16710, Glasgow G12 9WSTelephone: +44 (141) 337-3355FAX: +44 (141) 337-2266E-mail: [email protected]

© 1993-2001 Biozone International Ltd ISBN 0-909031-45-2

Presentation MEDIA

OHT Title OHT Title

1 Species

2 Populations

3 Gene Pool

4 Analysing a Gene Pool

5 Determining Allele Frequencies

6 Determining Genotype Frequencies

7 Changes in a Gene Pool 1



10 Hardy-Weinberg Equilibrium: An Introduction

11 Conditions Required for Hardy-WeinbergEquilibrium

12 Derivation of the Hardy-Weinberg Equation

13 The Hardy-Weinberg Equation

14 How to Solve Hardy Weinberg Problems

15 Hardy-Weinberg Problem: A Worked Example

16 Changing Allele Frequencies

17 Mutations

18 Gene Flow

19 The Effect of Population Size

20 Genetic Drift

21 Genetic Drift Example 1



24 Genetic Drift Patterns

25 Population Bottlenecks 1

26 Population Bottlenecks 2

27 Population Bottleneck in Cheetahs

28 The Founder Effect 1

29 The Founder Effect 2

30 Natural Selection

31 Assortative Mating

32 Modes of Natural Selection

33 Stabilising Selection 1

34 Stabilising Selection 2

35 Stabilising Selection in Human Birth Weights

36 Directional Selection 1

37 Directional Selection 2

38 Natural Selection in Peppered Moths 1

39 Peppered Moths 2

40 Peppered Moths 3

41 Disruptive Selection 1

42 Disruptive Selection 2

43 Heterozygote Advantage

44 Sickle Cell and Malaria

45 Hybrid Vigor

46 Artificial Selection in Brassica

47 Artificial Selection in Dogs

Index to OHT Titles

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BIOZONEINTERNATIONAL© 2001


Printing onto Paper Prohibited

These masters may only be used to generateOverhead Transparenc ies (OHTs) .

SpeciesA biological species is a grouping of organisms that caninterbreed and are reproductively isolated from (i.e.cannot breed with) other interbreeding groups.

Species are usually recognised on the basis of theirmorphology (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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Fragmented Distribution Continuous DistributionExample: Some

frog speciesExample: Arctic tundra

plant species

PopulationsFrom a population genetics viewpoint:

1. A population comprises the total number of a speciesinhabiting a particular geographical area.

2. All members of a population have the potential tointeract 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 andexist in isolated pockets or “islands”, cut off from otherpopulations of the same species.

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Individual with ahomozygousrecessive condition

Individual with aheterozygouscondition

Individual with ahomozygousdominant condition

Geographic boundaryof gene pool

A gene pool made up of 16 individual organismswith gene A, and where gene A has two alleles

Aa

AA

aaAA

Aa

aa

Aa

AA

Aa

aa AA

Aa

AA

aaAa

AA

Gene PoolDefinition of a Gene Pool: The sum total of all the genespresent 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 geneticcomplement of the population.

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AaAa Aaaa

AA Aa

AAAa

Analysing a Gene PoolBy determining the frequency of allele types (e.g. A and a)and genotypes (e.g. AA, Aa, and aa) it is possible todetermine the state of the gene pool.

The state of the gene pool will indicate if it is stable orundergoing change for some reason – an importantindicator that evolutionary events may be taking place.

There are twice the number of alleles for each gene asthere are individuals, since each individual has two alleles.

EXAMPLEThe small gene pool above consists of 8 individuals.

Each individual has 2 alleles for a single gene A, sothere 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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AaAAaa

AA Aa

Aa

AaAa

AaAAaa

AA Aa

Aa

AaAa

DeterminingAllele Frequencies

Count up the respective numbers of dominant andrecessive alleles in the total population, regardless ofthe combinations in which they occur:

Convert these to percentages by:

Frequency of Dominant AlleleThere are 9 dominant alleles out of a total of 16:

A = 9/16 x 100= 56.25%

Frequency of Recessive AlleleThere are 7 recessive alleles out of a total of 16:

a = 7/16 x 100= 43.75%

No. of Dominant Alleles

Total No. of AllelesX 100

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AaAAaa

AA Aa

Aa

AaAa

AaAAaa

AA Aa

Aa

AaAa

AaAAaa

AA Aa

Aa

AaAa

DeterminingGenotype Frequencies

Count up the actual number of each genotype in thepopulation: homozygous dominant (AA), heterozygous(Aa) and homozygous recessive (aa).

Homozygous Dominant FrequencyThere are 2 homozygous dominantgenotypes out of a total of 8:

AA = 2/8 x 100= 25%

Heterozygous FrequencyThere are 5 heterozygous genotypesout of a total of 8:

Aa = 5/8 x 100= 62.5%

Homozygous Recessive FrequencyThere is 1 homozygous recessivegenotype out of a total of 8:

AA = 1/8 x 100= 12.5%

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Changes in a Gene Pool 1Phase 1: Initial Gene Pool

– In the gene pool below there are 25 individuals, eachpossessing two copies of a gene for a trait called A.

– This is the gene pool before changes occur:

AA

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AaAa

Aa

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Allele Types Allele Combinations

AA Aa aa

No.

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A a

27

54

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Changes in a Gene Pool 2Phase 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 theiralleles are removed from the gene pool

AA

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aaAA

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aa

AAAaAaaa

Aaaa

AA

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AA Aa aa

No.

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27

58.7

19

41.3

7

30.4

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56.5

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Changes in a Gene Pool 3Phase 3: Immigration/Emigration

At a still later time, one beetle joins the gene poolwhile another leaves:


AA Aa aa

No.

%

A a

29

63

17

37

8

34.8

13

56.5

2

8.7

This individual is enteringthe population and will addits alleles to the gene pool

This individual is leavingthe population, removing itsalleles from the gene pool

Aa

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Hardy-Weinberg Equilibrium:An Introduction

Populations are often stable (no phenotypic change)over many generations.

The stability of many populations over time wasexplained by two scientists:

G. Hardy an English mathematicianW. Weinberg a German physician

They formulated the Hardy-Weinberg law whichmathematically describes the frequency of alleles in asexually reproducing population.

It shows that no matter how many times alleles aresegregated by meiosis and recombined by fertilisationthe allele frequency remains constant generationafter generation – the alleles are just shuffled aboutwithin the breeding population.

The population is in genetic equilibriumand no change in allele frequencies occurs

over many generations.

The equilibrium is only maintained in the absence ofdestabilising events.

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Conditions Required forHardy-Weinberg Equilibrium

The Hardy-Weinberg equilibrium is maintained ifall the following “stabilising” conditions are met:

Natural populations seldom meet all these requirementsand therefore allele frequencies will change.

A permanent change in the allele frequencies in apopulation is termed microevolution.

1. Large Population: The population size is large.

2. Random Mating: Every individual ofreproductive age has an equal chance of findinga mate.

3. No Emigration / Immigration: There is nomovement of individuals into or out of thepopulation – no gene flow.

4. No Selection Pressure: All genotypes havean equal chance of reproductive success – noselection pressure.

5. No Mutation: There is no mutation to introducenew alleles.

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Derivation of theHardy-Weinberg EquationThe Hardy-Weinberg equation is applied to populationswith a simple genetic situation: dominant and recessivealleles controlling a single trait.

The frequency of all of the dominant alleles (A) andrecessive 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 representsthe frequency of allele a in the population.

q

p

qp

pp

qq

pq

p qPunnett Square

Frequency of allelecombination AA inthe population

Frequency of allelecombination aa inthe population

Frequency of allele combination Aa in thepopulation (add these together to get 2pq)

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The Hardy-Weinberg EquationThe Hardy-Weinberg equilibrium can be expressedmathematically by giving the frequency of all the alleletypes in the population:

Frequency ofAllele Types

Frequency ofAllele Combinations

Frequencyof allele: A

Frequencyof allele: a

Frequency of allelecombination: AA(homozygous dominant)

Frequency of allelecombination: Aa(heterozygous)

Frequency of allelecombination: aa(homozygous recessive)

(p + q)2 = p

2 + 2pq + q

2 = 1

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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How to SolveHardy-Weinberg Problems

The following steps outline the procedure for solving aHardy-Weinberg problem:

1. Examine the question and determine what information you havebeen given about the population.

In most cases, it is the percentage or frequency of the recessivephenotype (q2) or the dominant phenotype (p2 2pq). These providethe 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 simplecalculation. 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 – these should

always equal 1.

Remember that all calculations must be carriedout using decimal fractions – NOT PERCENTAGES!

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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 0.7Frequency of the recessive phenotype q = 30% or 0.3

Working

Recessive phenotype: q2 = 0.30therefore: q = 0.5477 (square root of 0.30)therefore: p = 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 = 30%(b) Dominant allele = p = 45.2%

(c) Homozygous tasters = p2 = 20.5%(d) Heterozygous tasters = 2pq = 49.5%

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Changing Allele FrequenciesGene pools are subjected to a number of processesthat can alter the frequency of alleles for a gene:

1. Mutation2. Gene flow (immigration and emigration)3. Small population size and genetic drift4. Natural selection5. Non-random mating

Some of these processes cause random changes,others may be directional (i.e. they favour somealleles at the expense of others).

AA

aa

A'A

AA

AA

AA

AA

aa

Aa

Aa

AaAa

AaAa

AaAa

Aa

NaturalSelection

Emigration

Mutations

Immigration

GeographicalBarrier

MateSelection

Gene Flow

Boundary ofgene pool

AA

AA

aa

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aa

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MutationsMutations are the source of all new alleles.

Mutations can therefore change the frequency ofexisting alleles by competing with them.

Recurrent spontaneous mutations may becomecommon in a population if they are not harmful andare not eliminated.

AA

aa

a'a

Aa

Aa

Aa

Aa

aa

aa

aaAAAA

AA

AA

AAAaAa

Aa

Aa

Aa

In the graph below, a mutationcreates a new recessive allele: a'

The frequency of this new alleleincreases when environmentalconditions change, giving it acompetitive advantage over theother recessive allele: a

New recessiveallele

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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Generations

Mutation causes theformation of a new

recessive allele

Environmentalconditions change

0 10 205 15

A

a'

a

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Gene FlowGene flow is the movement of genes into or out of apopulation: immigration and emigration.

A population may gain or lose alleles through gene flow.

Gene flow tends to reduce the differences betweenpopulations because the gene pools become more similar.

AA

aa

Aa

AaAa

Aa

Aa

aa

aa

aaAA

aaAA

AA

AAAaAa

Aa

Aa

Aa

Gene Flow

AA

Aa

AA

aa

Aa

Aa

Aa

AaAa

Aa

Aa

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AA

AA AA

AA

AA

AA

aa

Aa

Aa

Aa

AaAa

Aa

Aa

AA

AA

aa

aa

aa

aa

aa

aa

Immigration intopopulation B Emigration out of

population B

Population A

Population B Population C

AA

aa

Aa

AaAa

Aa

Aa

aa

aa

aaAA

aaAA

AA

AAAaAa

Aa

Aa

Aa

AA

Aa

AA

aa

Aa

Aa

Aa

AaAa

Aa

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AA

AA AA

AA

AA

Barriers to gene flow

AA

aa

Aa

Aa

Aa

AaAa

Aa

Aa

AA

AA

aa

aa

aa

aa

aa

aa

No Gene Flow

Population A

Population B Population C

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The Effect of Population SizeLarge populations have more stable allele frequenciesbecause they have a greater reservoir of variability andare less affected by changes involving only a fewindividuals.

Small populations have fewer alleles to begin with andso the severity and speed of changes in the allelefrequencies are greater.

Endangered species with very low population numbersmay be subjected to severe and rapid allele changes.

Small Population

Large Population

AA

aa

Aa

Aa

Aa

AaAa

aa

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AA

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Genetic DriftNot all individuals, for various reasons, will be able tocontribute their genes to the next generation.

Genetic drift refers to random changes in allelefrequencies that occur in all populations, although havingthe 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 thatbecome isolated from the larger population gene pool (asfor example with island colonisers).

The next 3 OHTs show a hypothetical gene pool of a smallpopulation over three generations:

– For various reasons, not all individuals are contributingtheir alleles to the next generation.

– With the random loss of the alleles carried by theseindividuals, the allele frequency changes from onegeneration to the next.

– The change in frequency is ‘directionless’ as there is noselecting pressure.

– The allele combinations for each successive generation aredetermined by how many alleles of each type 'survive' andare passed on from the preceding one.

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Genetic Drift Example 1Generation 1: Two of the beetles fail to locate a matedue to the sparse distribution of the population.

There was no other reason for them not to find a mateand this factor alone prevented them from contributingtheir genes to the next generation.

An example may be the sparsely distributed individualsof the Siberian tiger population.

Fail to locate a mate dueto low population density

A = 16 (53%)a = 14 (47%)

AA

AA

AA AA

Aa

Aa

Aa

AaAa

Aa

Aa aa

aa

aa

Aa

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Genetic Drift Example 2Generation 2: Another two beetles fail to breed becausethey 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 testof the ‘fitness’ of the beetle’s phenotype.

The effect this had on the gene pool was to reduce thedominant allele frequency from 53% to 50%:

AA

AA

AA Aa

AaAa

Aa

Aa Aa

Aa

Aa

Aa aa

aa

aa

Fail to locate a mate dueto low population density

Killed in arock fall

A = 15 (50%)a = 15 (50%)

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Genetic Drift Example 3Generation 3: A dark beetle was blown out to sea byvery strong winds during a cyclone and died. Again thiswas not a true test of the ‘fitness’ of the phenotype butwas due to chance.

The effect on the gene pool was to further reduce thedominant allele frequency from 50% to 43%:

Killed in acyclone

A = 13 (43%)a = 17 (57%)

AA

AA

AA Aa

AaAa

Aa

Aa Aa

Aa

aa

aa

aa

aa

aa

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Genetic Drift PatternsBelow are displayed the changes in allele frequencies in acomputer simulation showing random genetic drift.

The breeding populations vary from 2,000 (top) to 20(bottom) and each simulation was run for 140 generations.

Large Gene Pool

Breeding Population = 2,000Fluctuations are minimalbecause large numbers ofindividuals buffer the populationagainst large changes in allelefrequencies

Small Gene Pool

Breeding Population = 200Fluctuations are more severebecause random changes ina few alleles cause a greaterpercentage change in allelefrequencies

Very Small Gene Pool

Breeding Population = 20Fluctuations are so extreme thatthe allele may become fixed(100%) or lost altogether (0%)

0

20

40

60

80

100

Alle

le F

req

uen

cy (

%)

0 20 40 60 80 100 120 140

Generations

0

20

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100

Alle

le F

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cy (

%)

0 20 40 60 80 100 120 140

Generations

0

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40

60

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100

Alle

le F

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uen

cy (

%)

0 20 40 60 80 100 120 140

Allele lost fromthe gene pool

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Population Bottlenecks 1Populations may sometimes be reduced to low numbersthrough periods of seasonal climatic change, heavypredation or disease, or through catastrophic events such asvolcanic eruptions or other natural disasters.

As a result, only a small number of individuals remain in thegene pool to contribute their genes to the next generation.

The small sample that survives will often not be representativeof the original, larger gene pool, and the resulting allelefrequencies may be severely altered.

In addition to this ‘bottleneck’ effect, the small survivingpopulation is often affected by inbreeding and genetic drift.

Extinction Extinction Extinction Extinction Extinction Extinction

Only 2 descendentsof lineage B survivethe extinction event

The original gene pool is made up of the offspring ofmany lineages (family groups and sub-populations)

All present day descendents of the original gene pool tracetheir ancestry back to lineage B and therefore retain onlya small sample of genes present in the original gene pool

GeneticBottleneck

Lineage A Lineage CLineage B

Extinction event suchas a volcanic eruption

TIM

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Population Bottlenecks 2

Pop

ulat

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Num

bers

Population Bottleneck:the population nearlybecomes extinct asnumbers plummet

Time

Large population withplenty of genetic diversity

Population grows to a largesize again, but has lostmuch of its genetic diversity

Population crashes to a verylow number and has lostmost of its genetic diversity

AA

AA

AA

Aa

AA

AA

AA

Aa

Aa

AA

AA

AA AA

AA

AA

AaAA

AA AA

AaAAaa AaAA

AaAAaa AaAA

AaAA AaAAAA

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Population Bottleneckin Cheetahs

The world population of cheetahs has declined in recentyears to fewer than 20,000.

Recent genetic analysis has found that the total cheetahpopulation has very little genetic diversity.

Cheetahs appear to have narrowly escaped extinction at theend of the last ice age: 10 – 20,000 years ago.

All modern cheetahs may have arisen from a singlesurviving litter – accounting for the lack of diversity.

At this time, 75% of all large mammals perished (includingmammoths, 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 insufficienttime for random mutations to produce new genetic variation.

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The Founder Effect 1Occasionally, a small number of individuals from apopulation may migrate away, or become isolated, fromtheir original population.

This colonising or ‘founder’ population will have a smalland probably non-representative sample of alleles fromthe parent population’s gene pool.

As a consequence of this founder effect, thecolonising population may evolve in a different directionthan the parent population.

Such small founder populations are subject to randomgenetic drift.

AA

AA

MainlandPopulation

IslandPopulation

Colonisation

Aa

AaAa

Aa

AA

AA

AA

Aa Aa

Aa

Aa AA

AA

AA Aa

Aa

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Aa

The founder effect is typically seen in the populations ofislands which are colonised by individuals from mainlandpopulations - often these species have low mobility.

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The Founder Effect 2In this hypothetical population of beetles, a small randomly selected group is blownoffshore to a neighbouring island where they establish a breeding population.

Some individuals from the mainlandpopulation are carried at random tothe offshore island by natural forcessuch as strong winds

This population maynot have the sameallele frequencies asthe mainlandpopulation

MainlandPopulation

IslandPopulation

AA

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AaAa

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Natural SelectionPopulations of sexually reproducing organisms consist ofvaried individuals, with some variants leaving moreoffspring than others.

This differential success in reproduction is called naturalselection.

Natural selection acts on the phenotype of individuals inthe 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 agene pool and increases the probability that favourablealleles will come together in the same individual.

AA

aa

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aaaa

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AA

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AaAa

Selection pressuresmay reduce certainallele frequencies

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Assortative MatingRandom (non-assortative) mating allows all genotypes tohave 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 somespecies of plant.

Sometimes individuals may show random mating forsome alleles but not others.EXAMPLE: Humans exhibit assortative mating for racialfeatures, but mate randomly for blood types.

While assortative mating does not change the frequencyof alleles in the overall gene pool, it does cause the ratioof genotypes to depart from that of random mating.

Random Mating Assortative Mating

aaaa

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Modes of Natural SelectionNatural selection changes allele frequencies in populations,but it does not produce the “perfect organism”.

Rather than developing new phenotypes, it reduces thefrequency of phenotypes that are less suited to the prevailingconditions.

Traits (e.g. skin colour, height) that are under polygeniccontrol show quantitative variation in the phenotype. Naturalselection acts on this variation.

Natural selection may produce phenotypic change over time.The direction of this change will depend on the nature of theselection pressure. Selection may be:❑ Stabilising ❑ Directional ❑ Disruptive

These can be depicted with graphs that show how thefrequencies of different phenotypes change over time.

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Variation in Phenotype

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Stabilising Selection 1Probably the most common trend in natural populations.

Stabilising selection favours the most commonphenotype as the best adapted.

Selection reduces variation by selecting against theextremes at each end of the range – the resulting bellshaped curve is narrower, about the same mean.

EXAMPLE: Human birth weights are maintained in the3-4 kg range by selection pressures at the extremes.

Fre

quen

cy

Retained

Eliminated Eliminated


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Stabilising Selection 2Before selection (below) there is a broad range ofvariation in the population:

Fre

quen

cy

Retained

Eliminated Eliminated

Fre

quen

cy

After selection (below) and some generations laterthere is a reduction in the amount of variation:

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Selection inHuman Birth Weights

Stabilising selection against extremes in the birthweight range results in most births being between 3-4 kg.

Modern medical intervention is reducing this selection.

Per

cen

t o

f B

irth

s S

amp

led

Birth Weight (kg)0 1 2 3 4 5 6

0

10

20

30

40

50

60

70

80

0

5

10

15

20

25

30

35

40

Selection against low birthweight (small babies) withpoor organ development

Selection against high birthweight (large babies) dueto childbirth complications

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Directional Selection 1Most common during periods of environmental change.

Directional selection favours the phenotypes at oneextreme of a phenotypic range.

Selection reduces variation at one extreme of the rangewhile favouring variants at the other end – the resultingbell shaped curve shifts in the direction of selection.

EXAMPLE: Fossil evidence shows that the average sizeof black bears in Europe increased with each ice age,only to decrease again during the interglacials.

Fre

quen

cy

Retained

Eliminated


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Directional Selection 2Before selection (below) there is a broad range ofvariation in the population:

Fre

quen

cy

Retained

Eliminated

Fre

quen

cy

After selection (below) and some generations laterthere is a reduction in variation at one extreme of therange while favouring variants at the other end.

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Natural Selectionin Peppered Moths 1

The peppered moth, Biston betularia, occurs in twoforms (or morphs):

The mottled grey form is well camouflaged against thelichen- covered bark of trees in unpolluted regions,making detection by predators more difficult.

The dark melanic forms are disadvantaged in suchenvironments as their body shape stands out againstthe background.

Melanic or carbonaria form ofthe peppered moth Biston betularia

Grey or mottled form ofthe peppered moth Biston betularia

With the onset of the Industrial Revolution in England,the air quality declined, killing off lichen and resulting ina marked increase in the frequency of the dark moths.

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Peppered Moths 2In the 1940s and 1950s, coal burning was still intensearound 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 industrialcentres, the grey forms increased dramatically.

Scale

60 km

60 miles

Industrial areas

Non-industrial areas

LondonCardiff

Plymouth

Bristol

Portsmouth

CoventryLeicester

Nottingham

Sheffield

HullLeedsManchester

Birmingham

Liverpool

Middlesbrough

Newcastle

Belfast

Glasgow

Grey orspeckled form

Melanic orcarbonaria form

Key to Frequency Graphs

Frequency ofPeppered Mothforms in 1950

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Peppered Moths 3With the decline of coal burning factories and the CleanAir Acts in cities, the air quality improved between 1960and 1980.

Sulphur dioxide and smoke levels dropped to a fractionof their previous levels.

This caused the relative numbers of melanic pepperedmoths to plummet.

Frequency of Melanic Peppered Moth Relatedto Reduced Air Pollution

1960 1965 1970 1975 1980 1985Year

100

90

80

70

60

50

75

50

25

0

150

100

50

0

Su

mm

er s

mo

ke (

µg/

m3 )

Win

ter

sulp

hu

r d

ioxi

de

(µg/

m3 )

Freq

uenc

y of

mel

anic

form

of

Bis

ton

bet

ula

ria

(%)

Summersmoke

Wintersulphurdioxide

Melanic Biston betularia

40

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Disruptive Selection 1Occurs when environmental conditions are varied or whenthe environmental range of an organism is large.

Disruptive selection favours phenotypes at bothextremes of a phenotypic range over intermediatevariants. The bell shaped curve acquires two peaks (i.e.becomes bimodal).

This type of selection can lead to the formation of clinesor ecotypes (organisms of the same species that areslightly different in appearance), and polymorphism.

EXAMPLE: African swallow tail butterfly has developedthree different populations that are Batesian mimics ofother local distasteful butterfly species.

Fre

quen

cy

Retained

Eliminated

Retained


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Disruptive Selection 2Before selection (below) there is a broad range ofvariation in the population:

Fre

quen

cy

Retained

Eliminated

Retained

Fre

quen

cy

Two peaks

After selection (below) and some generations laterindividuals at both extremes of a phenotypic range arefavoured over intermediate variants (2 peaks).

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Heterozygote AdvantageOften individuals that are heterozygous for a particulargene (Aa) are more common in the population than wouldbe predicted using Hardy-Weinberg laws.

This is because the heterozygous condition can have agreater fitness than either homozygote (AA or aa).

This is termed heterozygote advantage.Example: Sickle cell gene

The mutant allele for sickle cell disease producesabnormal haemoglobin. This causes deformation of thered blood cells, so that they are destroyed.

Heterozygotes (HbS,Hb) carry alleles for both sickle celland normal haemoglobin. This results in greater malariaresistance but only mild anaemia.

Homozygotes for normal haemoglobin (Hb,Hb) havegreater susceptibility to malaria – in regions where malariais prevalent, heterozygotes occur in greater numbers.

There is a good correlation between the incidence ofmalaria and regions where there is a high frequency of thesickle cell gene.

Hb, HbAll red blood cells

are normal

HbS, HbSAll red blood cellsare sickle shaped

HbS, HbMixture of normal andsickle red blood cells

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Sickle Cell Disease and Malaria

Frequency of Sickle Cell Gene (HbS)

10% - 20%

5% - 10%

1% - 5%

Areas affected byfalciparum malaria

Incidence of Falciparum Malaria

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Hybrid VigourIndividuals that are heterozygous for a number ofdifferent genes often demonstrate hybrid vigour.

The hybrid offspring of two inbred organisms oftenshow greater fitness (better growth and productivity forexample) than either parent.

The reasons for hybrid vigour are not always clear:

Genes may affect more than one trait andheterozygotes may benefit from the effects of a numberof different interactions expressed in the phenotype.

EXAMPLE: Hybrid corn is valued for its highproductivity. It is produced by crossing inbred parentalstrains with a high degree of homozygosity.

Parental Strain A Parental Strain B

Homozygousfor many genesXHomozygous

for many genes

Hybrid

Heterozygousfor many genesAaBbCcDdEeFf

AAbbCCddEEff aaBBccDDeeFF

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Artificial Selection in BrassicaDifferent parts of this plant have been developed byhuman selection to produce 6 separate vegetables withenormous visible differences.

All these vegetables form a single species and willinterbreed if allowed to flower (e.g. the new broccoflower).

Cauliflower(Flower)

Brussels Sprout(Lateral Buds)

Kohlrabi(Stem)

Broccoli(Inflorescence)

Cabbage(Terminal

Buds)

Kale(Leaf)

Wild FormBrassica oleracea

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Artificial Selection in DogsThe dog was probably first domesticated at least 14,000years ago from a grey wolf ancestor.

Some 400 breeds have developed from this single wildspecies as a result of selective breeding by humans.

Grey WolfCanis lupus pallipes

Small wolf. Once commonthroughout Europe, Asia and,

North America which displayeda wide variety of coat coloration.

SheepdogCanis familiaris metris optimae

Originating in Europe, thisbreed has been used to guard

flocks from predators forthousands of years.

Wolf-likeCanis familiaris palustris

Found in snow covered habitatsin northern Europe, Asia (Siberia)

and North America (Alaska).

Pointer-TypeCanis familiaris intermediusProbably derived from thegreyhound breed for the

purpose of hunting small game.

GreyhoundCanis familiaris leineri

Drawings of this type on potterydated 8,000 years ago in theMiddle East, make this one of

the oldest.

Mastif-TypeCanis familiaris inostranzeviOriginally from Tibet, the firstrecords of this breed of doggo back to Neolithic times.

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