The Evolution The Evolution of Populationsof Populations

BIOL BIOL 102: 102: General Biology IIGeneral Biology II

Chapter Chapter 2323

Rob Rob SwatskiSwatski Associate Professor Associate Professor of Biologyof Biology

HACCHACC--YorkYork

Overview of Overview of Natural Natural

SelectionSelection

Natural selection acts on individuals,

but only populations evolve

Evolution occurs through genetic

variations in populations

Ex: Medium ground finch &

beak size during drought

2 Medium ground finchMedium ground finch

MicroevolutionMicroevolution

Microevolution: changes in a

population’s allele frequencies over

generations

Three mechanisms cause allele frequency

change:

1) Natural selection, 2) Genetic drift, 3)

Gene flow

3

1976 (similar to the prior 3 years)

1978 (after

drought)

Ave

rage

be

ak d

ep

th (

mm

)

10

9

8

0

3

4

5

6

AllelesAlleles

7

Two main sources of Two main sources of gene pool variation:gene pool variation:

Mutation Sexual

Reproduction

8

9

10

Harold and Maude (1971)Harold and Maude (1971)

Genetic Genetic VariationVariation

Variation in individual genotype leads to variation in

individual phenotype

Natural selection can only act on variation

with a genetic component

Not all phenotypic variation is heritable

11

Moth caterpillars raised on oak flower diet resemble oak flowers

NonheritableNonheritable VariationVariation

12

Moth caterpillar siblings raised on oak leaves resemble oak twigs 13

Population variation is the Population variation is the result of:result of:

Discrete characters

Are classified as “either/or”

Quantitative characters

Vary along a continuum within

a population

Phenotype is often influenced

by 2 or more genes 14

Discrete Discrete CharactersCharacters

15

16

Quantitative CharactersQuantitative Characters

17

GenotypesGenotypes

Homozygous

Individual having 2 of the same

alleles for a given locus

Heterozygous

Individual having 2 different alleles for a given locus

18

Average Average HeterozygosityHeterozygosity

A measure of gene variability

Measures the average % of loci that are heterozygous in a

population

19

Nucleotide Nucleotide VariabilityVariability

Measured by comparing the differences between DNA sequences of pairs

of individuals

20

21

Drosophila Drosophila melanogastermelanogaster

Average heterozygosity

13,700 genes in genome

= 14% (1,920 loci)

Nucleotide variability

180 million nucleotides in genome

= 1% (1.8 million)

22

Geographic Geographic VariationVariation

Differences between gene pools of separate

populations or population subgroups

23

13.17 19 XX 10.16 9.12 8.11

1 2.4 3.14 5.18 6 7.15

9.10

1 2.19

11.12 13.17 15.18

3.8 4.16 5.14 6.7

XX

Geographic Variation in Isolated Mouse Geographic Variation in Isolated Mouse Populations on MadeiraPopulations on Madeira

Isolated populations have differences in fused chromosomes 24

karyotypes

ClineCline

A graded change in a trait along a

geographic axis

Ex: lactate dehydrogenase

frequency is higher in cold water (allows faster swimming in

fish)

25

1.0

0.8

0.6

0.4

0.2

0

46 44 42 40 38 36 34 32 30

Georgia Warm (21°C)

Latitude (Latitude (°°N)N) Maine Cold (6°C)

Mummichog

26

MutationMutation

A change in the nucleotide

sequence of DNA

Causes new genes & alleles to arise

Only mutations in gamete-producing cells can be passed

to offspring 27

Point Mutation:Point Mutation: a change in 1 base in a gene

28

Effects of Effects of Point Point

MutationsMutations

Mutations in noncoding regions of DNA are often

harmless due to redundancy

Mutations resulting in a change in protein

production are often harmful

Mutations may also be beneficial & increase an

organism’s fit into its environment

29

Types of MutationsTypes of Mutations

Deletions

More harmful

Disruptions

More harmful

Rearrangements

More harmful

Duplication

Less harmful

Genes can take on new

functions

30

31

Mutation Mutation RatesRates

Low in animals & plants: avg 1 mutation in every 100,000 genes

per generation

Often higher in prokaryotes & viruses

Prokaryotes & viruses have short generation times so mutations can

quickly produce genetic variation

32

Sexual Sexual ReproductionReproduction

Shuffles existing alleles into new combinations

Recombination

More important than mutation in

producing genetic differences …Why?

33

FlowerFlower SymmetrySymmetry inin AntirrhinumAntirrhinum SpeciesSpecies

34

Recombination During MeiosisRecombination During Meiosis

35

Gene PoolGene Pool: all the alleles for all loci in a population

36

Porcupine herd

PorcupinePorcupine herd herd rangerange

Beaufort Sea

MAP AREA

FortymileFortymile herd herd rangerange

Fortymile herd

overlapoverlap

37

Calculate the Frequency of an Allele in a Calculate the Frequency of an Allele in a Population:Population:

Total # of alleles at a locus = total # of individuals x 2

38

Total # of Dominant or Total # of Dominant or Recessive Alleles at a LocusRecessive Alleles at a Locus

2 alleles for each homozygous

dominant or recessive individual plus…

1 allele for each heterozygous

individual

39

If there are 2 alleles at a locus, p & q are used to represent their frequencies

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

p + q = 1

p q

40

HardyHardy--Weinberg Weinberg PrinciplePrinciple

Describes a hypothetical population that is not

evolving

In real populations, allele & genotype

frequencies change over time

If a population does not meet H-W criteria, then

the population is evolving

41

HardyHardy--Weinberg Weinberg

EquilibriumEquilibrium

Allele & genotype frequencies in a population

remain constant from generation to generation

In a population where gametes randomly

contribute to the next generation, allele

frequencies will not change

Mendelian inheritance preserves genetic variation in

a population

42

CRCR

CWCW

CRCW

43

Frequencies of alleles

Alleles in the Alleles in the populationpopulation

Gametes produced

Each egg:

Each sperm:

80% chance

80% chance

20% chance

20% chance

q = frequency of

p = frequency of

CR allele = 0.8

CW allele = 0.2

equilibrium

random

Selecting Alleles at Random from a Selecting Alleles at Random from a Gene PoolGene Pool

44

p2 & q2 are the frequencies of the homozygous genotypes

2pq is the frequency of the heterozygous genotype

If p & q represent the relative frequencies of the only two possible alleles in a population at a

particular locus, then:

45

SpermSperm CR

(80%)

80% CR (p = 0.8)

CW (20%)

20% CW (q = 0.2)

16% (pq)

CRCW

4% (q2)

CW CW

64% (p2)

CRCR

16% (qp)

CRCW

Parent Generation:

F1:

46

Gametes of this generation:

64% CR + 16% CR = 80% CR = 0.8 = p

4% CW + 16% CW = 20% CW = 0.2 = q

Genotypes in the next generation:

With random mating, these gametes will result in the same mix of genotypes

64% CRCR, 32% CRCW, and 4% CWCW F1:

64% CRCR, 32% CRCW, and 4% CWCW plants F2:

47

No mutations Random mating

No natural selection

Extremely large

population size

No gene flow

The 5 H-W conditions for nonevolving populations are rarely met in nature:

48

3 Major Factors of 3 Major Factors of Evolutionary ChangeEvolutionary Change

Natural Selection

Genetic Drift

Gene Flow

49

Natural Natural SelectionSelection

Differential reproductive

success

Results in certain alleles being

passed to the next generation in

greater proportions than

other alleles

50

Genetic Genetic DriftDrift

Allele frequencies can fluctuate

unpredictably from one generation to

the next

Reduces genetic variation through

loss of alleles

The smaller a sample, the greater

the chance of deviation from a predicted result 51

52

53

Generation 1Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7 q (frequency of CW ) = 0.3

Generation 2Generation 2

CR CW CR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5 q = 0.5

Generation 3Generation 3

p = 1.0 q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

54

Founder Founder EffectEffect

Occurs when a few individuals become

isolated from a larger population

Allele frequencies in the small founder population may differ from those in

the larger parent population

Ex: Amish

55

PolydactylyPolydactyly 56

Bottleneck Bottleneck EffectEffect

Occurs when population size is reduced due to a sudden change in the

environment

The resulting gene pool may no longer reflect the

original population’s gene pool

If the population remains small, it may be further affected by genetic drift

57

Original population

Bottlenecking event

Surviving population

58

Genetic Drift & Genetic Drift & the Greater the Greater

Prairie ChickenPrairie Chicken

Habitat loss caused a severe reduction in the population of greater

prairie chickens in Illinois

The surviving birds had low levels of genetic

variation

Only 50% of their eggs hatched

59

Range of greater prairie chicken

Pre-bottleneck (Illinois, 1820)

Post-bottleneck (Illinois, 1993)

60

Greater Prairie Greater Prairie Chicken Chicken

Research, cont.Research, cont. DNA from museum specimens used to compare genetic

variation before & after bottleneck

Results showed a loss of alleles at several loci

Introduced prairie chickens from other

states to increase gene pool diversity

Successfully introduced new alleles & increased egg hatch rate to 90%

61

NumberNumber of allelesof alleles per locusper locus

Minnesota, 1998Minnesota, 1998 (no bottleneck)

Nebraska, 1998Nebraska, 1998 (no bottleneck)

Kansas, 1998Kansas, 1998 (no bottleneck)

IllinoisIllinois

1930–1960s

1993

LocationLocation PopulationPopulation

sizesize

%% of eggsof eggs hatchedhatched

1,000–25,000

<50

750,000

75,000– 200,000

4,000

5.2

3.7

93

<50

5.8

5.8

5.3 85

96

99

62

Effects of Effects of Genetic Genetic

DriftDrift Significant in

small populations

Causes allele frequencies to

change at random

Can lead to a loss of genetic

variation within populations

May cause harmful alleles to

become fixed 63

Gene FlowGene Flow

Movement of alleles among populations

Transferred through movement of fertile

individuals or gametes

Usually reduces differences between

populations over time

More likely than mutation to directly

alter allele frequencies

64

65

66

Gene Flow & Gene Flow & Decreasing Decreasing

FitnessFitness

Ex: Bent grass

Alleles for copper tolerance are beneficial in populations

near copper mines, but harmful to those in other

soils

Windblown pollen moves alleles between populations

Movement of unfavorable alleles into a population

decreases the fitness between organism &

environment 67

NON- MINE SOIL

MINE SOIL

NON- MINE SOIL

Prevailing wind direction

Ind

ex o

f co

pp

er

tole

ran

ce

Distance from mine edge (meters)

70

60

50

40

30

20

10

0 20 0 20 0 20 40 60 80 100 120 140 160

68

Population in which the

surviving females

eventually bred

Central

Eastern

Surv

ival

rat

e (

%)

Surv

ival

rat

e (

%)

Females born

in central

population

Females born

in eastern

population

Parus major

60

50

40

30

20

10

0

Central

population

NORTH SEA Eastern

population Vlieland,

the Netherlands

2 km

69

Gene Flow & Gene Flow & Increasing Increasing

FitnessFitness Ex: Insecticide

resistance in mosquitoes

Insecticides have been used to kill mosquitoes

that carry West Nile virus & malaria

Alleles have evolved in some mosquito

populations that confer insecticide resistance

The flow of these resistance alleles into a population can increase

its fitness 70

71

72

73

Why are the phrases “survival of the fittest”

and “struggle for existence”

misleading?

74

Relative Relative FitnessFitness

Reproductive success is generally more subtle &

depends on many factors

The contribution an individual makes to the gene pool of the next

generation…

…relative to the contributions of other

individuals 75

76

3 Types of Selection3 Types of Selection

Directional Disruptive Stabilizing

77

Directional SelectionDirectional Selection

Favors individuals at one extreme of the phenotypic range

Original population

Phenotypes (fur color)

Evolved population

78

79

Disruptive SelectionDisruptive Selection

Favors individuals at both extremes of the phenotypic range

Original population

Phenotypes (fur color)

Evolved population

80

81

Stabilizing SelectionStabilizing Selection

Favors intermediate variants & acts against extreme phenotypes

Original population

Phenotypes (fur color)

Evolved population

82

83

Natural Natural Selection & Selection &

Adaptive Adaptive EvolutionEvolution

Natural selection increases the frequencies of alleles that enhance survival &

reproduction

Adaptive evolution occurs as the match between an

organism & its environment increases

Because the environment can change, adaptive

evolution is a continuous dynamic process

84

85

Bones shown in

green are movable.

Ligament

Movable jaw bones in snakes 86

Sexual Sexual SelectionSelection

Natural selection for mating success

May result in sexual dimorphism

Can lead to significant differences between

secondary sexual traits 87

88

89

90

Types of Sexual Types of Sexual SelectionSelection

Intrasexual selection

Intersexual selection

(Mate choice)

91

92

Competition between individuals of one sex (often males) for mates of the opposite sex

IntrasexualIntrasexual SelectionSelection

93

94

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

Intersexual Selection (Mate Choice)Intersexual Selection (Mate Choice)

Sooty Sooty Grouse Grouse

mating ritualmating ritual

Male showiness can increase his chances of attracting a female, but also decrease his overall chances of survival 95

Good Genes Good Genes HypothesisHypothesis

One explanation for the evolution of female

preference

If a trait is related to male health, selection should favor both the

male trait & the female preference for that trait

Ex: Gray tree frog mating call

96

Significance of Significance of Call Duration on Call Duration on

Mate ChoiceMate Choice

Long-Calling (LC) & Short-Calling (SC)

Does call duration indicate the male’s

overall genetic quality?

Do females choose mates based upon this

trait?

97

SC male

Female gray tree frog

LC male

SC sperm Eggs LC sperm

Offspring of LC father

Offspring of SC father

Fitness of these half-sibling offspring compared

EXPERIMENTEXPERIMENT

98

RESULTSRESULTS

Time to metamorphosis

Larval survival

Larval growth

NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

Offspring Performance 1995 1996

LC better NSD

NSD

LC better (shorter)

LC better (shorter)

LC better

99

The Preservation of Genetic VariationThe Preservation of Genetic Variation

Diploidy Balancing selection

Heterozygote advantage

Frequency-dependent selection

Neutral variation

100

DiploidyDiploidy Maintains genetic variation in the form of hidden recessive alleles

101

Biston betularia morpha typica Biston betularia morpha carbonaria

Balancing Balancing SelectionSelection

Natural selection maintains stable frequencies of 2 or more phenotypic

forms in a population

102

Heterozygote Heterozygote AdvantageAdvantage

Heterozygotes have a higher fitness than both

homozygotes

Natural selection will tend to maintain 2 or more

alleles at that locus

The sickle-cell allele causes mutations in hemoglobin, but also provides malaria

resistance 103

Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote)

Key

Frequencies of the sickle-cell allele

0–2.5%

2.5–5.0%

5.0–7.5%

7.5–10.0%

10.0–12.5%

>12.5%

104

FrequencyFrequency--Dependent Dependent SelectionSelection

The fitness of a phenotype decreases if it becomes

too common in the population

Selection can favor the least common phenotype

in a population

Ex: scale-eating fish (Perissodus)

105

“Left-mouthed”

P. microlepis

“Right-mouthed”

P. microlepis

1.0

0.5

0 1981

Sample year ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90

Fre

qu

en

cy o

f

“le

ft-m

ou

thed

” i

nd

ivid

uals

Neutral Neutral VariationVariation

Genetic variation that appears to provide no selective advantage or

disadvantage

Ex: Variations in noncoding regions of DNA

Ex: Variations in proteins that have little effect on function or reproductive

fitness 106

Selection can act only on existing

variations

Evolution is limited by historical constraints

Adaptations are often compromises

Chance, natural selection, & the

environment interact

Why Natural Selection Cannot Fashion Why Natural Selection Cannot Fashion “Perfect” Organisms“Perfect” Organisms

107

108

109

CreditsCredits by Rob Swatski, 2013

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