Chapter 23 Evolution of Populations - PowerPoint Presentation

Slide1

Chapter 23

Evolution of PopulationsSlide2

Question? Is the unit of evolution the individual

or the

population?

Answer = while evolution effects individuals, it can only be tracked through time by looking at populationsSlide3

So what do we study? We need to study populations

, not individuals

We need a method to track the changes in populations over time

This is the area of Biology called ‘population genetics’Slide4

Population GeneticsThe study of genetic variation in populations.Represents the reconciliation of

Mendelism

and Darwinism

.

Modern Synthesis uses population genetics as the means to track and study evolution

Looks at the genetic basis of variation and natural selection Slide5

PopulationA localized group of individuals of the same species

.Slide6

SpeciesA group of similar organisms.A group of populations that could interbreed.Slide7

Gene PoolThe total aggregate of genes in a population.If evolution is occurring, then changes must

occur in the gene pool of the population over time.Slide8

MicroevolutionChanges in the relative frequencies of alleles in the gene pool.Slide9

OverviewA common misconception

is that organisms evolve, in the Darwinian sense, during their lifetimes

Natural selection acts on individuals, but only populations

evolve

Individuals are selected; populations evolve!Slide10

The Smallest Unit of EvolutionGenetic

variations in populations contribute to evolution

Microevolution

is a change in allele frequencies in a population over generationsSlide11

Is this finch evolving by natural selection?Slide12

Concept: Mutation and sexual reproduction produce the genetic variation that makes evolution possibleTwo processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individualsSlide13

Genetic Variation Variation in individual genotype leads to variation in individual phenotypeNot all phenotypic variation is heritable

Natural selection can only act on variation with a genetic componentSlide14

Nonheritable

variation?Slide15

Nonheritable

variation?Slide16

Variation Within a PopulationBoth discrete and quantitative characters contribute to variation within a populationDiscrete characters

can be classified on an either-or basis

Quantitative characters

vary along a continuum within a populationSlide17

Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levelsAverage

heterozygosity

measures the average percent of loci that are heterozygous in a population

Nucleotide variability is measured by comparing the DNA sequences of pairs of individualsSlide18

Variation Between PopulationsMost species exhibit

geographic variation

,

differences between gene pools of separate populations or population subgroups

Geographic variation in isolated mouse populations on MadeiraSlide19

Fig. 23-5

Porcupine herd

Porcupine

herd range

Beaufort Sea

NORTHWEST

TERRITORIES

MAP

AREA

ALASKA

CANADA

Fortymile

herd range

Fortymile herd

ALASKA

YUKONSlide20

Ldh

-B

b

allele frequency

1.0

0.8

0.6

0.4

0.2

0

46

44

42

40

38

36

34

32

30

Georgia

Warm (21°C)

Maine

Cold (6°C)

A cline determined by temperature

Latitude (°N)Slide21

MutationMutations are changes in the nucleotide sequence of DNA

Mutations cause new genes and alleles to arise

Only mutations in cells that produce gametes can be passed to offspringSlide22

Point mutationsA

point mutation

is a change in one base in a gene

The effects of point mutations can vary:

Mutations in

noncoding

regions of DNA are often harmless

Mutations in a gene might not affect protein production because of redundancy in the genetic codeSlide23

The effects of point mutations can vary:Mutations that result in a change in protein production are often harmfulMutations that result in a change in protein production can sometimes increase the fit between organism and environment

Point mutationsSlide24

Mutations That Alter Gene Number or SequenceChromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

Duplication of large chromosome segments is usually harmfulSlide25

Mutations That Alter Gene Number or SequenceDuplication of small pieces of DNA is sometimes less harmful and increases the genome size

Duplicated genes can take on new functions by further mutationSlide26

Hardy-WeinburgSlide27

Hardy-Weinberg TheoremDeveloped in 1908. Mathematical model of gene pool changes over time.Slide28

The frequency of an allele in a population can be calculatedFor diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive allelesSlide29

By convention, if there are 2 alleles at a locus, p and q

are used to represent their frequencies

The frequency of all alleles in a population will add up to 1

For example,

p

+

q

= 1Slide30

Basic Equationp + q = 1p = % dominant alleleq = % recessive alleleSlide31

Expanded Equationp + q = 1(p + q)2 = (1)2p2

+ 2pq + q

2

= 1Slide32

Genotypesp2 = Homozygous Dominants2pq = Heterozygousq

2

= Homozygous RecessivesSlide33

The Hardy-Weinberg principle describes a population that is not evolvingIf a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolvingSlide34

The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generationIn a given population where gametes contribute to the next generation randomly, allele frequencies will not change

Mendelian

inheritance preserves genetic variation in a populationSlide35

Fig. 23-6

Frequencies of alleles

Alleles in the population

Gametes produced

Each egg:

Each sperm:

80%

chance

80%

chance

20%

chance

20%

chance

q

= frequency of

p

= frequency of

C

R

allele = 0.8

C

W

allele = 0.2

Selecting alleles at random from a gene poolSlide36

Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene poolIf p

and

q

represent the relative frequencies of the only two possible alleles in a population at a particular locus, then

p

2

+ 2

pq

+

q

2

= 1

-where

p

2

and

q

2

represent the frequencies of the homozygous genotypes and 2

pq

represents the frequency of the heterozygous genotypeSlide37

Conditions for Hardy-Weinberg EquilibriumThe Hardy-Weinberg theorem describes a hypothetical population

In real populations, allele and genotype frequencies do change over time

Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other lociSlide38

The five conditions for nonevolving populations are rarely met in nature:No mutations

Random mating

No natural selection

Extremely large population size

No gene flowSlide39

Example CalculationLet’s look at a population where: A = red flowers a = white flowersSlide40
Slide41

Starting PopulationN = 500Red = 480 (320 AA+ 160 Aa)White = 20Total alleles = 2 x 500

=

1000Slide42

Dominant AlleleA = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80%Slide43

Recessive Allelea = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20%Slide44

A and a in HW equationCross: Aa X AaResult = AA + 2Aa + aaRemember: A = p, a = qSlide45

Substitute the values for A and ap2 + 2pq + q2 = 1(.8)2 + 2(.8)(.2) + (.2)

2

= 1

.64 + .32 + .04 = 1Slide46

Dominant AlleleA = p2 + pq = .64 + .16 = .80 = 80%

Slide47

Recessive Allelea = pq + q2 = .16 + .04 = .20 = 20%Slide48

Importance of Hardy-WeinbergYardstick to measure rates of evolution.Predicts that gene frequencies should NOT

change over time as long as the HW assumptions hold (no evolution should occur).

Way to calculate gene frequencies through time.Slide49

Example What is the frequency of the PKU allele?PKU is expressed only if the individual is homozygous recessive (aa).Slide50

Applying the Hardy-Weinberg PrincipleWe can assume the locus that causes phenylketonuria

(PKU) is in Hardy-Weinberg equilibrium given that:

The PKU gene mutation rate is low

Mate selection is random with respect to whether or not an individual is a carrier for the PKU alleleSlide51

Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions The population is large

Migration has no effect as many other populations have similar allele frequenciesSlide52

The occurrence of PKU is 1 per 10,000 birthsq2

= 0.0001

q

= 0.01

The frequency of normal alleles is

p

= 1 –

q

= 1 – 0.01 = 0.99

The frequency of carriers is

2

pq

= 2 x 0.99 x 0.01 = 0.0198

or approximately 2% of the U.S. populationSlide53

PKU FrequencyPKU is found at the rate of 1/10,000 births.PKU = aa = q2 q2 = .0001 q = .01Slide54

Dominant Allelep + q = 1 p = 1- q p = 1- .01 p = .99Slide55

Expanded Equationp2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2

= 1

.9801 + .0198 + .0001 = 1Slide56

Final ResultsNormals (AA) = 98.01%Carriers (Aa) = 1.98%PKU (aa) = .01%Slide57

ResultGene pool is in a state of equilibrium and has not changed because of sexual reproduction.No Evolution has occurred.Slide58

AP Problems Using Hardy-WeinbergSolve for q2 (% of total).Solve for q (equation).Solve for p (1- q).

H-W is

always

on the national AP Bio exam (but no calculators are allowed).Slide59

Remember Hardy-Weinberg Assumptions1. Large Population2. Isolation3. No Net Mutations

4. Random Mating

5. No Natural SelectionSlide60

If H-W assumptions hold true:The gene frequencies will not change over time.Evolution will not occur.But, how likely will natural populations hold to the H-W assumptions?Slide61

MicroevolutionCaused by violations of the 5 H-W assumptions.Slide62

Causes of Microevolution1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating

5. Natural SelectionSlide63

Genetic DriftChanges in the gene pool of a small population by chance.

Types:

1. Bottleneck Effect

2. Founder's Effect

The smaller a sample, the greater the chance of deviation from a predicted resultSlide64

Genetic DriftGenetic drift describes how allele frequencies fluctuate unpredictably from one generation to the nextGenetic drift tends to reduce genetic variation through losses of allelesSlide65

Fig. 23-8-1

Generation 1

p

(frequency of

C

R

) = 0.7

q

(frequency of

C

W

) = 0.3

C

W

C

W

C

R

C

R

C

R

C

W

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

W

C

R

C

W

C

R

C

W

Genetic DriftSlide66

Fig. 23-8-2

Generation 1

p

(frequency of

C

R

) = 0.7

q

(frequency of

C

W

) = 0.3

Generation 2

p

= 0.5

q

= 0.5

C

W

C

W

C

R

C

R

C

R

C

W

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

W

C

R

C

W

C

R

C

W

C

R

C

W

C

R

C

W

C

R

C

W

C

R

C

W

C

W

C

W

C

W

C

W

C

W

C

W

C

R

C

R

C

R

C

R

C

R

C

R

Genetic DriftSlide67

Fig. 23-8-3

Generation 1

C

W

C

W

C

R

C

R

C

R

C

W

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

W

C

R

C

W

C

R

C

W

p

(frequency of

C

R

) = 0.7

q

(frequency of

C

W

) = 0.3

Generation 2

C

R

C

W

C

R

C

W

C

R

C

W

C

R

C

W

C

W

C

W

C

W

C

W

C

W

C

W

C

R

C

R

C

R

C

R

C

R

C

R

p

= 0.5

q

= 0.5

Generation 3

p

= 1.0

q

= 0.0

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

C

R

Genetic DriftSlide68

Genetic Drift By ChanceSlide69

Bottleneck Effect

Loss of most of the population by disasters.Slide70

Bottleneck EffectThe bottleneck effect is a sudden reduction in population size due to a disaster or a change in the environment

Surviving population may have a different gene pool than the original population, it may no longer be reflective of the original population’s gene poolSlide71

If the population remains small, it may be further affected by genetic driftUnderstanding the bottleneck effect can increase understanding of how human activity affects other speciesSlide72

Case Study: Impact of Genetic Drift on the Greater Prairie ChickenLoss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois

The surviving birds had low levels of genetic variation, and only 50% of their eggs hatchedSlide73

Fig. 23-10a

Range

of greater

prairie

chicken

Pre-

bottleneck

(Illinois, 1820)

Post-bottleneck

(Illinois, 1993)

(a)Slide74

Fig. 23-10b

Number

of alleles

per locus

Minnesota, 1998

   (no bottleneck)

Nebraska, 1998

   (no bottleneck)

Kansas, 1998

   (no bottleneck)

Illinois

1930–1960s

1993

Location

Population

size

Percentage

of eggs

hatched

1000–25,000

<50

750,000

75,000–

200,000

4,000

5.2

3.7

93

<50

5.8

5.8

5.3

85

96

99

(b)Slide75

Prairie ChickenResearchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneckThe results showed a loss of alleles at several loci

Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%Slide76

ImportanceReduction of population size may reduce gene pool for evolution to work with. Ex: CheetahsSlide77

Founder's EffectThe founder effect occurs when a few individuals become isolated from a larger populationAllele frequencies in the small founder population can be different from those in the larger parent populationSlide78

Founder's EffectGenetic drift in a new colony that separates from a parent population.Examples:

Old-Order

Amish

A butterfly that gets blown to an island and lays her eggsSlide79

Result of Bottleneck and Founder’s effectsGenetic variation reduced.Some alleles increase in frequency while others are lost (as compared to the parent population).

Very common in islands and other groups that don't interbreedSlide80

Effects of Genetic Drift: A SummaryGenetic drift is significant in small populations

Genetic drift causes allele frequencies to change at random

Genetic drift can lead to a loss of genetic variation within populations

Genetic drift can cause harmful alleles to become fixedSlide81

Gene FlowGene flow consists of the movement of alleles among populations (in or out of a population)ImmigrationEmigration

Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)Slide82

Gene FlowGene flow tends to reduce differences between populations over timeGene flow is more likely than mutation to alter allele frequencies directlySlide83

Fig. 23-11Slide84

Gene flow can decrease the fitness of a populationIn Bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soilsWindblown pollen moves these alleles between populations

The movement of unfavorable alleles into a population results in a decrease in fit between organism and environmentSlide85

Fig. 23-12a

NON-

MINE

SOIL

MINE

SOIL

NON-

MINE

SOIL

Prevailing wind direction

Index of copper tolerance

Distance from mine edge (meters)

70

60

50

40

30

20

10

0

20

0

20

0

20

40

60

80

100

120

140

160Slide86

Fig. 23-12b

Bent grassSlide87

Gene flow can increase the fitness of a populationInsecticides have been used to target mosquitoes that carry West Nile virus and malariaAlleles have evolved in some populations that confer insecticide resistance to these mosquitoes

The flow of insecticide resistance alleles into a population can cause an increase in fitnessSlide88

ResultChanges in gene frequencies within a population.Immigration often brings new alleles into populations increasing genetic diversity.Slide89

MutationsInherited changes in a gene.Slide90

Result of mutationsMay change gene frequencies (small population).Source of new alleles for selection.Often lost by genetic drift.Slide91

Nonrandom MatingFailure to choose mates at random from the population.Slide92

Inbreeding within the same “neighborhood”.Assortative mating (like with like).Results in increases in the number of homozygous loci.Does not in itself alter the overall gene frequencies in the population.Slide93

Sexual Mate selectionSexual selection is natural selection for mating successMay not be adaptive to the environment, but increases reproduction success of the individual

.Slide94

Sexual Mate selectionThis is a VERY important selection type for species.It can result in

sexual dimorphism

, marked differences between the sexes in secondary sexual characteristicsSlide95

Fig. 23-15

Sexual dimorphismSlide96

Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sexIntersexual selection, often called mate choice,

occurs when individuals of one sex (usually females) are choosy in selecting their mates

Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survivalSlide97

How do female preferences evolve?The good genes hypothesis suggests that if a trait is related to male health, both the male trait and female preference for that trait should be selected forSlide98

Fig. 23-16a

SC male gray

tree frog

Female gray

tree frog

LC male gray

tree frog

SC sperm 

Eggs



LC sperm

Offspring of

LC father

Offspring of

SC father

Fitness of these half-sibling offspring compared

EXPERIMENTSlide99

Fig. 23-16b

RESULTS

1995

Fitness Measure

1996

Larval growth

Larval survival

Time to metamorphosis

LC better

NSD

LC better

(shorter)

LC better

(shorter)

NSD

LC better

NSD = no significant difference; LC better = offspring of LC males

superior to offspring of SC males.Slide100

ResultSexual dimorphism.Secondary sexual features for attracting mates.Slide101

CommentFemales may drive sexual selection and dimorphism since they often "choose" the mate.Slide102

Natural SelectionDifferential success in survival and reproduction.Differential success in reproduction results in certain alleles being passed to the next generation in greater proportionsSlide103

Natural SelectionOnly natural selection consistently results in adaptive evolutionNatural selection brings about adaptive evolution by acting on an organism’s phenotype

Result - Shifts in gene frequencies.Slide104

CommentAs the Environment changes, so does Natural Selection and Gene Frequencies.Slide105

ResultIf the environment is "patchy", the population may have many different local populations.Slide106

The Key Role of Natural Selection in Adaptive EvolutionNatural selection increases the frequencies of alleles that enhance survival and reproductionAdaptive evolution occurs as the match between an organism and its environment increasesSlide107

Fig. 23-14a

Color-changing ability in cuttlefishSlide108

Fig. 23-14b

Movable

jaw

bones in

snakes

Movable bonesSlide109

Because the environment can change, adaptive evolution is a continuous processGenetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environmentSlide110

Genetic Basis of Variation1. Discrete Characters – Mendelian traits with clear phenotypes.2. Quantitative Characters –

Multigene

traits with overlapping phenotypes.Slide111

PolymorphismThe existence of several contrasting forms of the species in a population.Usually inherited as Discrete Characteristics.Slide112

Examples of Polymorphism Garter Snakes

GaillardiaSlide113

Human ExampleABO Blood GroupsMorphs = A, B, AB, OSlide114

Other examplesSlide115

Quantitative CharactersAllow continuous variation in the population.Result – Geographical VariationClines: a change along a geographical axisSlide116

Yarrow and AltitudeSlide117

Sources of Genetic VariationMutations.Recombination though sexual reproduction.Crossing-overRandom fertilizationSlide118

The Preservation of Genetic VariationVarious mechanisms help to preserve genetic variation in a population

1.

Diploidy

- preserves recessives as

heterozygotes

.

2.

Balanced Polymorphisms

- preservation of diversity by natural selection.Slide119

DiploidyDiploidy maintains genetic variation in the form of hidden recessive allelesSlide120

Heterozygote Advantage - When the heterozygote or hybrid survives better (have a higher fitness) than the homozygotes. Also called Hybrid vigor.Natural selection will tend to maintain two or more alleles at that locus

Heterozygote AdvantageSlide121

Heterozygote AdvantageCan't bred "true“ and the diversity of the population is maintained.Example of hybrid vigor – Sickle Cell Anemia

The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistanceSlide122

Fig. 23-17

0–2.5%

Distribution of

malaria caused by

Plasmodium falciparum

(a parasitic unicellular eukaryote)

Frequencies of the

sickle-cell allele

2.5–5.0%

7.5–10.0%

5.0–7.5%

>12.5%

10.0–12.5%Slide123

In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the populationSelection can favor whichever phenotype is less common in a population

Frequency-Dependent SelectionSlide124

Fig. 23-18a

“Right-mouthed”

“Left-mouthed”Slide125

Fig. 23-18

“Right-mouthed”

1981

“Left-mouthed”

Frequency of

“left-mouthed” individuals

Sample year

1.0

0.5

0

’82

’83

’84

’85

’86

’87

’88

’89

’90Slide126

CommentPopulation geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.Ex – Sickle Cell and Malaria, Tay

-Sachs and TuberculosisSlide127

Fitness - DarwinianThe relative contribution an individual makes to the gene pool of the next generation.Slide128

Relative FitnessRelative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individualsSelection favors certain genotypes by acting on the phenotypes of certain organismsSlide129

Relative FitnessContribution of one genotype to the next generation compared to other genotypes.The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals

Reproductive success is generally more subtle and depends on many factorsSlide130

Rate of SelectionDiffers between dominant and recessive alleles.Selection pressure by the environment.Slide131

Modes of SelectionThree modes of selection:Directional selection favors individuals at one end of the phenotypic range

Disruptive selection

favors individuals at both extremes of the phenotypic range

Stabilizing selection

favors intermediate variants and acts against extreme phenotypesSlide132

StabilizingSelection toward the average and against the extremes.Ex: birth weight in humansSlide133

Directional SelectionSelection toward one extreme.Ex: running speeds in race animals.Ex. Galapagos Finch beak size and food source.Slide134
Slide135

DiversifyingSelection toward both extremes and against the norm.Ex: bill size in birdsSlide136
Slide137

CommentDiversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.Slide138

Fig. 23-13

Original population

(c) Stabilizing selection

(b) Disruptive selection

(a) Directional selection

Phenotypes (fur color)

Frequency of individuals

Original

population

Evolved

populationSlide139

Balancing SelectionBalancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a populationSlide140

Neutral VariationNeutral variation is genetic variation that appears to confer no selective advantage or disadvantage

For example,

Variation in

noncoding

regions of DNA

Variation in proteins that have little effect on protein function or reproductive fitnessSlide141

Why Natural Selection Cannot Fashion Perfect OrganismsSelection can act only on existing variations

Evolution is limited by historical constraints

Adaptations are often compromises

Chance, natural selection, and the environment interactSlide142

Fig. 23-19Slide143

QuestionDoes evolution result in perfect organisms?Slide144

Answer - No1. Historical Constraints2. Compromises

3. Non-adaptive Evolution (chance)

4. Available variations – most come from using a current gene in a new way.Slide145

SummaryKnow the difference between a species and a population.Know that the unit of evolution is the population and not the individual.Slide146

SummaryKnow the H-W equations and how to use them in calculations.Know the H-W assumptions and what happens if each is violated.Slide147

SummaryIdentify various means to introduce genetic variation into populations.Know the various types of natural selection.Slide148

You should now be able to:Explain why the majority of point mutations are harmless

Explain how sexual recombination generates genetic variability

Define the terms population, species, gene pool, relative fitness, and neutral variation

List the five conditions of Hardy-Weinberg equilibriumSlide149

Apply the Hardy-Weinberg equation to a population genetics problemExplain why natural selection is the only mechanism that consistently produces adaptive changeExplain the role of population size in genetic driftSlide150

Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selectionList four reasons why natural selection cannot produce perfect organismsSlide151

End of Chapter 23!