Transcript
Page 1: Lecture #2 – Evolution of Populations

1

Lecture #2 – Evolution of Populations

Image of a population of penguins

Page 2: Lecture #2 – Evolution of Populations

2

Key Concepts:

• The Modern Synthesis• Populations and the Gene Pool• The Hardy-Weinberg Equilibrium• Micro-evolution• Sources of Genetic Variation• Natural Selection• Preservation of Genetic Variation

Page 3: Lecture #2 – Evolution of Populations

3

Images – species, population, community

Review definitions• Species – individual organisms capable of

mating and producing fertile offspring • Population – a group of individuals of a

single species• Community – a group of individuals of

different species

Page 4: Lecture #2 – Evolution of Populations

4

The Modern Synthesisintegrates our knowledge about

evolution• Darwin’s natural selection• Mendel’s hereditary patterns• Particulate transfer (chromosomes)• Structure of the DNA molecule

All explain how the genetic structure of populations changes over time

Page 5: Lecture #2 – Evolution of Populations

5

KEY POINT

Environmental factors act on the individual to control the genetic future of

the population

Individuals don’t evolve…..populations do

* * * * * * * * * * * * * * * * * * * * *** * * * * * *

Page 6: Lecture #2 – Evolution of Populations

6

Image – population of iris

Population = a +/- localized group of individuals of one species

Page 7: Lecture #2 – Evolution of Populations

7

Critical Thinking• How do we determine the boundaries of a

population???

Page 8: Lecture #2 – Evolution of Populations

8

Critical Thinking• How do we determine the boundaries of a

population???Boundaries are scale dependentSome sub-populations overlapSome are more isolatedWe can look at populations at many different

scales – micro to meta

Page 9: Lecture #2 – Evolution of Populations

9

Recall basic genetic principles:

• In a diploid species (most are), every individual has two copies of every geneOne copy came from each parent

• Most genes have different versions = alleles• Diploid individuals are either heterozygous

or homozygous for each geneHeterozygous = AaHomozygous = AA or aa

Page 10: Lecture #2 – Evolution of Populations

10

Recall basic genetic principles:

• The total number of alleles for any gene in a population is the number of individuals in the population x 2If the population has 10 individuals, there are

20 copies of the A gene – some “A” alleles and some “a” alleles

• All these alleles comprise the “gene pool”

Page 11: Lecture #2 – Evolution of Populations

11

Hardy-Weinberg Theorem

• Gene pool = all alleles in a population• All alleles have a frequency in the

populationThere is a percentage of “A” and a

percentage of “a” that adds up to 100%• Hardy-Weinberg Theorem demonstrates

that allele frequencies don’t change through meiosis and fertilization alone

Page 12: Lecture #2 – Evolution of Populations

12

Hardy-Weinberg Theorem

• A simple, mathematical model• Shows that repeated random meiosis and

fertilization events alone will not change the distribution of alleles in a populationEven over many generations

p2 + 2pq + q2 = 1

we will not focus on the math – you’ll work on this in lab

Page 13: Lecture #2 – Evolution of Populations

13

Hands On

• The equation: p2 + 2pq + q2 = 1 (Page 2)• p = the frequency of one allele• q = the frequency of the other allele• p + q MUST = 1 = 100% of the gene pool

Page 14: Lecture #2 – Evolution of Populations

14

Hands On

• If the allele frequencies are known, the HW equilibrium can be demonstrated by a Punnett square

Assume that we know there are

½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT Tt

t Tt tt

Page 15: Lecture #2 – Evolution of Populations

15

Hands On

• What are the frequencies of each allele in the F1 generation?

Assume that we know there are

½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT Tt

t Tt tt

Page 16: Lecture #2 – Evolution of Populations

16

Hands On – Results

• What are the frequencies of each allele in the F1 generation?

Assume that we know there are

½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT.5x.5=.25

Tt.5x.5=.25

t Tt.5x.5=.25

tt.5x.5=.25

Page 17: Lecture #2 – Evolution of Populations

17

Hands On – Resultsp2 + 2pq + q2 = 1

.52 + 2x.5x.5 + .52 = 125% TT : 50% Tt : 25% tt

50% T and 50% t = no changeAssume that we know there are

½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT.5x.5=.25

Tt.5x.5=.25

t Tt.5x.5=.25

tt.5x.5=.25

Page 18: Lecture #2 – Evolution of Populations

18

Hands On – Resultsp2 + 2pq + q2 = 1

25% TT : 50% Tt : 25% tt50% T and 50% t = no change

This goes on generation after generationThe phenotype remains 75% dominant and 25%

recessive

Assume that we know there are ½ T and ½ t alleles

Maternal Parent

T t

Paternal Parent

T TT.5x.5=.2

5

Tt.5x.5=.25

t Tt.5x.5=.2

5

tt.5x.5=.25

Page 19: Lecture #2 – Evolution of Populations

19

Hands On

• How do you determine the allele frequencies???

• How do you find p and q???• In this example, how do you know the

percentage of T and the percentage of t???

Page 20: Lecture #2 – Evolution of Populations

20

Hands On – Results

• Remember that the recessive phenotype is tt

• If you know the percentage of the population that expresses the recessive phenotype, then t (q) is the square root of that number

p2 + 2pq + q2 = 125% TT : 50% Tt : 25% tt

75% express dominant; 25% express recessive√.25 = .5; determine p by subtraction

Page 21: Lecture #2 – Evolution of Populations

21

Hands On

• Clasp your hands

Page 22: Lecture #2 – Evolution of Populations

22

Hands On

• Count right thumbs up vs. left thumbs up• Right thumb up is the recessive condition!• Determine the distribution of T and t in our

class population• Type up a summary of your results and

turn in tomorrow

Page 23: Lecture #2 – Evolution of Populations

24

Hardy-Weinberg Theorem

• Meiosis and fertilization randomly shuffle alleles, but they don't change proportionsLike repeatedly shuffling a deck of cardsThe laws of probability determine that the

proportion of alleles will not change from generation to generation

• This stable distribution of alleles is the Hardy-Weinberg equilibrium

Doesn’t happen in nature!!!

Page 24: Lecture #2 – Evolution of Populations

25

Conditions for H-W Equilibrium:

• No natural selection• Large population size• Isolated population• Random mating• No mutation

Doesn’t happen in nature!!!The violation of each assumption acts as

an agent of microevolution

Page 25: Lecture #2 – Evolution of Populations

26

The value of H-W???

• It provides a null hypothesis to compare to what actually happens in nature

• Allele frequencies DO change in nature• BUT, they change only under the conditions

of microevolutionIn nature, all the H-W assumptions are violated

• Result – populations DO evolve

Page 26: Lecture #2 – Evolution of Populations

27

Critical Thinking• What are the limitations of the Hardy-

Weinberg theorem???

Page 27: Lecture #2 – Evolution of Populations

28

Critical Thinking• What are the limitations of the Hardy-

Weinberg theorem???• The H-W model considers just one trait at a

time, and assumes that just one gene with 2 alleles (one completely dominant) controls that trait

• Recall your basic genetics – is this realistic???

Page 28: Lecture #2 – Evolution of Populations

29

Critical Thinking• Reality is much more complex for most traits

in most organismsIncomplete dominance or codominanceMore than 2 alleles for many genesPleiotropy – one gene affects multiple traitsPolygenic traits – multiple genes affect one traitEpistasis – one gene affects expression of

another geneEnvironmental effects on phenotypic expression

• Reproductive success depends on the way all genes and phenotypic traits interact

Page 29: Lecture #2 – Evolution of Populations

30

Individuals Do Not Evolve• Individuals vary, but populations evolve• Natural selection pressures make an

individual more or less likely to survive and reproduce

• But, it is the cumulative effects of selection on the genetic makeup of the whole population that results in changes to the species

The environment is a wall; natural selection is a gate

Page 30: Lecture #2 – Evolution of Populations

31

The environment is the wall; natural selection is the gate

* * * * * * * * * * * * * * * * * * * * *

** * * * * * *

***** *****

?

Page 31: Lecture #2 – Evolution of Populations

32

Image – natural variation in flower color; same image for all these summary slides

Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection• Genetic Drift• Gene Flow• Selective Mating• Mutation

Page 32: Lecture #2 – Evolution of Populations

33

Cartoon – beaver with chainsaw paws “natural selection does not grant organisms what they “need””

Natural Selection – the essence of Darwin’s theory

Mor

e on

this

late

r…. M

ore on this later….

Differential reproductive success is the only way to account for the accumulation of

favorable traits in a population

Page 33: Lecture #2 – Evolution of Populations

34

Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection• Genetic Drift• Gene Flow• Selective Mating• Mutation

Page 34: Lecture #2 – Evolution of Populations

35

• Reproductive events are samples of the parent population

Genetic Drift – random changes in allele frequency from generation to generation

1

2

1

2

Larger pop = ~29% blue Smaller pop = 100% blue

1

2

Parent pop = 10% blue

Larger samples are more representative than smaller samples (probability theory)

Page 35: Lecture #2 – Evolution of Populations

36

Genetic Drift – random changes in allele frequency from generation to generation• More pronounced in smaller and/or more

segregated populationsBottleneck effectFounder effect

1

2

1

2

Segregated pop = ~29% blue Segregated pop = 100% blue

1

2

Parent pop = 10% blue

Page 36: Lecture #2 – Evolution of Populations

37

Bottlenecking = extreme genetic drift

Diagram – bottlenecking

Page 37: Lecture #2 – Evolution of Populations

38

Critical Thinking• What events could cause a bottleneck???

Page 38: Lecture #2 – Evolution of Populations

39

Critical Thinking• What events could cause a bottleneck???

Bottlenecks occur when there is an extreme and indiscriminate reduction in the reproducing populationDiseaseHerbivoryMalnutritionMajor disturbance (flood, fire)Human intervention

Page 39: Lecture #2 – Evolution of Populations

40

Image – cheetah

Conservation implications – cheetahs are a bottlenecked species

Page 40: Lecture #2 – Evolution of Populations

41

Maps – historic and current range of cheetahs

Extreme range reduction due to

habitat destruction and poaching

+Cheetahs were

naturally bottlenecked about 10,000 years

ago by the last major ice age (kinked tail)

The species is at risk of extinction

Page 41: Lecture #2 – Evolution of Populations

42

Images – bottlenecked and now endangered species

Australian Flame Robin, California Condor, Mauritian Kestrel

…..and many more, all driven nearly to extinction…..

Some colorful results of a quick web search on “bottlenecked species”

Page 42: Lecture #2 – Evolution of Populations

43

Founder Effect = extreme genetic drift

• Occurs when a single individual, or small group of individuals, breaks off from a larger population to colonize a new habitatIslandsOther side of mountainOther side of a river…

• This small group may not represent the allele distribution of the parent population

Page 43: Lecture #2 – Evolution of Populations

44Founder Effect

Page 44: Lecture #2 – Evolution of Populations

45

Page 45: Lecture #2 – Evolution of Populations

46

Page 46: Lecture #2 – Evolution of Populations

47

Image – a founding population of seeds; possibly also the bird if it’s a gravid female

Long distance dispersal events can lead to the founder effect

Page 47: Lecture #2 – Evolution of Populations

48

Critical Thinking

• What do you think follows long distance dispersal to a new ecosystem???

Page 48: Lecture #2 – Evolution of Populations

49

Critical Thinking

• What do you think follows long distance dispersal to a new ecosystem???

• Adaptive radiation frequently leads to many new, closely related species as the organisms adapt to new habitat zones in their new home

FoundingPopulation

1

2

3

4

Page 49: Lecture #2 – Evolution of Populations

50

Hands On

• Genetic drift is random• Some drift is expected with every generation

Genetic drift is not necessarily extreme• Use the beads to explore this idea (Page 4)

Count out 50 beads each of 2 colorsEach bead represents an allele in the gene poolSince B and b are in equal proportion, what is

the phenotypic makeup of the diploid population???

p2 + 2pq + q2 = 1

Page 50: Lecture #2 – Evolution of Populations

51

Hands On – Results

• Genetic drift is random• Some drift is expected with every generation

Genetic drift is not necessarily extreme• Use the beads to explore this idea

Count out 50 beads each of 2 colorsEach bead represents an allele in the gene poolSince B and b are in equal proportion, what is

the phenotypic makeup of the diploid population???75% express dominant; 25% express recessive

Page 51: Lecture #2 – Evolution of Populations

52

Hands On

• Now simulate random mating by shaking up the beads and pulling out 2 beads at a time, with your eyes closedBe sure to return the beads to the “gene pool”We are sampling with replacement – random

• Record each offspring allele structureBe sure to assign a dominant and recessive

color• Repeat 50 times

Shake the pool each time to maintain random

Page 52: Lecture #2 – Evolution of Populations

53

Hands On

• Count and record the allele structure of your second generation

• Make a new gene pool of 100 beads that reflects this new allele structure

• Repeat the bead selection for a 3rd generation

• Repeat for a total of 5 generations

Page 53: Lecture #2 – Evolution of Populations

54

Hands On

• Is your 5th generation allele structure the same as your 1st generation???

• What is the phenotypic distribution of your 5th generation?

• What are your conclusions?• Use your lab notebook to record

observations

Page 54: Lecture #2 – Evolution of Populations

55

Hands On

• Now simulate a bottleneck• Shake up the beads and pull out 2 beads

at a time, with your eyes closedBe sure to return the beads to the “gene pool”We are sampling with replacement – random

• Record each offspring allele structureBe sure to assign a dominant and recessive

color• Repeat 5 times

Shake the pool each time to maintain random

Page 55: Lecture #2 – Evolution of Populations

56

Hands On

• Count and record the allele structure of your second generation

• Make a new gene pool of 100 beads that reflects this new allele structure

• Repeat the bead selection for a 3rd generation

• Repeat for a total of 5 generations

Page 56: Lecture #2 – Evolution of Populations

57

Hands On

• Is your 5th generation allele structure the same as your 1st generation???

• What is the phenotypic distribution of your 5th generation?

• What are your conclusions?• Compare 50 vs. 5 reproductive “events”• Use your lab notebook to record

observations, and type up a summary to turn in tomorrow

Page 57: Lecture #2 – Evolution of Populations

58

Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection• Genetic Drift• Gene Flow• Selective Mating• Mutation

Page 58: Lecture #2 – Evolution of Populations

59

Gene Flow

• Mixes alleles between populationsImmigrationEmigration

• Most populations are NOT completely isolated

Page 59: Lecture #2 – Evolution of Populations

60

Critical Thinking

• Will gene flow tend to increase or decrease speciation???

Page 60: Lecture #2 – Evolution of Populations

61

Critical Thinking

• Will gene flow tend to increase or decrease speciation???

• Gene flow tends to preserve species by shuffling alleles between all sub-populations

Page 61: Lecture #2 – Evolution of Populations

62Gene Flow

Page 62: Lecture #2 – Evolution of Populations

63

Hands On

• How could we demonstrate gene flow with our beads???

Page 63: Lecture #2 – Evolution of Populations

64

Hands On – Results

• How could we demonstrate gene flow with our beads???Have bead pairs migrate between teams

• Would beads be more likely to migrate within a bench or between benches???What are the implications for speciation?

Page 64: Lecture #2 – Evolution of Populations

65

Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection• Genetic Drift• Gene Flow• Selective Mating• Mutation

Page 65: Lecture #2 – Evolution of Populations

66

Image – male peacock with mating display

Selective Breeding

Page 66: Lecture #2 – Evolution of Populations

67

Critical Thinking

• Animal behaviors are obvious examples• Can you think of others???

Page 67: Lecture #2 – Evolution of Populations

68

Image – fungi spores

Critical Thinking

• Animal behaviors are obvious examples• Can you think of others???• Proximity is important even in species that

do not have mating behaviorsMany plants and fungi are randomly fertilized

or pollinated…..but generally the exchange is between closer neighbors

Page 68: Lecture #2 – Evolution of Populations

69

Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection• Genetic Drift• Gene Flow• Selective Mating• Mutation

Page 69: Lecture #2 – Evolution of Populations

70

Diagram – mutations in DNA strand

Cartoon - jackalope

Mutations• Random, rare, but

regular events• The only source of

completely new traits

just for fun…..

Page 70: Lecture #2 – Evolution of Populations

71

Evolution = random events

x“the gate”

* * * * * * * * * * * * * * * * * * * * *** * * * * * *

Page 71: Lecture #2 – Evolution of Populations

72

Review: Micro-evolution:population-scale changes in allele

frequencies

• Natural Selection• Genetic Drift• Gene Flow• Selective Mating• Mutation

Page 72: Lecture #2 – Evolution of Populations

73

Sources of Genetic Variation

• Natural selection acts on natural variation• Where does this variation come from???

MeiosisMutation

• Additional mechanisms help preserve variation (later)

Page 73: Lecture #2 – Evolution of Populations

74

Diagram – meiosis I

Meiosis = key source of variation

Page 74: Lecture #2 – Evolution of Populations

75

Diagram – meiosis II

Page 75: Lecture #2 – Evolution of Populations

76

Diagram – results of meiosis with n=2

Random, Independent Assortment of Homologous Chromosomes

n = 2

Page 76: Lecture #2 – Evolution of Populations

77

Probability theory reveals that for random, independent events:

• If each event has 2 possible outcomesIn this case, one side of the plate or the other

• The possible number of distribution combinations = 2n, where n = the number of eventsIn this case, the distribution event is the

distribution of chromosomes to the gametesn = the haploid number of chromosomes

• If n is 2, then combinations are 22 = 4

Page 77: Lecture #2 – Evolution of Populations

78

Diagram – results of meiosis with n=2

Random, Independent Assortment of Homologous Chromosomes

n = 2

Four possible

distributions

Page 78: Lecture #2 – Evolution of Populations

79

Probability theory reveals that for random, independent events:

• If each event has 2 possible outcomesIn this case, one side of the plate or the other

• The possible number of distribution combinations = 2n, where n = the number of eventsIn this case, distribution refers to the distribution

of chromosomes to the gametesn = the haploid number of chromosomes

• If n is 23, then combinations are 223 = 8.4 million!

Page 79: Lecture #2 – Evolution of Populations

80

Probability is Multiplicative:

8.4 million x 8.4 million > 70 trillion!!!

That is the number of possible combinations of maternal and paternal chromosomes in the offspring of a randomly mating pair of

humans

Page 80: Lecture #2 – Evolution of Populations

81

Diagram – recombinationRecombination increases the

potential variation to

infinity

Page 81: Lecture #2 – Evolution of Populations

82

Critical Thinking

• Can meiosis produce totally new traits???

Page 82: Lecture #2 – Evolution of Populations

83

Critical Thinking

• Can meiosis produce totally new traits???• No – remember, normal meiosis just

shuffles the alleles• Only mutation can make entirely new

alleles

Page 83: Lecture #2 – Evolution of Populations

84

Natural Selection as a Mechanism of Evolutionary Adaptation

• Natural selection acts on the variation produced by meiosis and mutation

• Selection increases the “fitness” of a population in a given environment

• Fitness = ???

Page 84: Lecture #2 – Evolution of Populations

85

Natural Selection as a Mechanism of Evolutionary Adaptation

• Natural selection acts on the variation produced by meiosis and mutation

• Selection increases the “fitness” of a population in a given environment

• Fitness = reproductive success NOT big, NOT smart, NOT strongThe production of successful offspring is the

key

Page 85: Lecture #2 – Evolution of Populations

86

Natural selection has limits• Individuals vary in fitness

Natural selection promotes the most fit• Selection acts on the phenotype – the

whole, complex organismResults from the combination of many different

genes for any organismThese genes are expressed in the whole,

complex environment • Selection is always constrained by the

whole, complex evolutionary history of the species

Page 86: Lecture #2 – Evolution of Populations

87

Critical Thinking

• Can evolution respond to “needs”???

Beaver cartoon again

Page 87: Lecture #2 – Evolution of Populations

88

Critical Thinking

• Can evolution respond to “needs”???• NO!!!• Evolution is a combination of random

events + successful reproduction in a given environment

• The environment is the wall; natural selection is the gate!!!!If the phenotype “works”, the genotype

passes through the gate

Page 88: Lecture #2 – Evolution of Populations

89

Hands On

• Calculate the allele distribution to the F1 with the dominant phenotype resulting in a 20% decline in the reproductive success rate (Page 3, with a twist)

• The twist – start with a 50/50 distribution of dominant and recessive alleles in the gene pool

Page 89: Lecture #2 – Evolution of Populations

90

Hands On – Results

• p2 + 2pq + q2 = 1• 50% T; 50% t• 25% TT : 50% Tt : 25% tt• 75% dominate phenotype; 25% recessive• F1 would include .75 x .80 = 60% TT or Tt• By subtraction = 40% tt

The frequency of t is thus .63 (the √.4)• By subtraction, the frequency of T is now .37

Page 90: Lecture #2 – Evolution of Populations

91

Hands On

• Calculate the allele distribution to the F1 with the recessive phenotype resulting in 100% mortality (Page 3, with a twist)

• The twist – start with a 50/50 distribution of dominant and recessive alleles in the gene pool

Page 91: Lecture #2 – Evolution of Populations

92

Hands On – Results

• p2 + 2pq + q2 = 1• 50% T; 50% t• 25% TT : 50% Tt : 25% tt• 75% dominate phenotype; 25% recessive• F1 would include zero tt

The t allele only present in heterozygotesThe frequency of t is thus (½ of 2pq) / (p2 + 2pq)

• The frequency of t = .5(.5) / (.25 + .5) = .33• By subtraction, the frequency of T is now .67

Page 92: Lecture #2 – Evolution of Populations

93

Hands On

• In either case would either the T or t allele become extinct?

• Why or why not?

Page 93: Lecture #2 – Evolution of Populations

94

Hands On – Results

• In either case would either the T or t allele become extinct?

• Why or why not?I think the T allele can become extinct since

the phenotype is deleterious (I only did the math out one more generation)

I think the t allele will always remain hidden in heterozygous individuals

• Homework – carry both questions out to 5 generations, or extinction (print clearly!)

Page 94: Lecture #2 – Evolution of Populations

95

Diagram – patterns of natural selection

Patterns of Change by Natural Selection

• Directional Selection• Diversifying Selection (AKA disruptive)• Stabilizing Selection

Page 95: Lecture #2 – Evolution of Populations

96

Diagram – patterns of natural selection

Remember, all populations exhibit a range of natural variation

Page 96: Lecture #2 – Evolution of Populations

97

Diagram – directional selection

Directional Selection

• Phenotypes at one extreme of the range are most successfulColorPatternFormMetabolic processes

• The population shifts to favor the successful phenotype

Page 97: Lecture #2 – Evolution of Populations

98

Diagram – diversifying selection

Diversifying Selection

• Multiple, but not all, phenotypes are successfulPatchy environmentsSub-populations migrate to new habitats

• The population begins to fragment and new species begin to diverge

Page 98: Lecture #2 – Evolution of Populations

99

Diagram – stabilizing selection

Stabilizing Selection

• The intermediate phenotypes are most successfulHomogenous environmentsStable conditions

• The range of variation within the population is reduced

Page 99: Lecture #2 – Evolution of Populations

100

Critical Thinking

• Which selection mode will most quickly lead to the development of diversity???

Page 100: Lecture #2 – Evolution of Populations

101

Critical Thinking

• Which selection mode will most quickly lead to the development of diversity???

• Diversifying selection tends to produce multiple species, and the parent species may also persist

Page 101: Lecture #2 – Evolution of Populations

102

Critical Thinking

• Can you think of a real-life example of an adaptive phenotype???

Page 102: Lecture #2 – Evolution of Populations

103

Critical Thinking

• Can you think of a real-life example of an adaptive phenotype???

• Everything!Variation is randomSelection is adaptive

Page 103: Lecture #2 – Evolution of Populations

104

Images – natural variation in flower color

Preservation of Natural Variation

• Diploidy• Balanced Polymorphism• Neutral Variation

Page 104: Lecture #2 – Evolution of Populations

105

Diploidy – 2 alleles for every gene

• Recessive alleles retained in heterozygotesNot expressedNot eliminated, even if the recessive trait is aa may be eliminated, while Aa is preserved in

the population• Recessive alleles function as latent

variation that may prove helpful if environment changes

Page 105: Lecture #2 – Evolution of Populations

106

Balanced Polymorphism

• Heterozygote advantage• Frequency dependent selection• Phenotypic variation

Page 106: Lecture #2 – Evolution of Populations

107

Map – global distribution of sickle cell allele

Images – normal and sickled red blood cells

Balanced Polymorphism – heterozygote advantage

Sickle-cell Anemia

a mutation in the gene that codes for hemoglobin causes a single amino acid substitution in the protein, RBC shape changes from round to sickle shape

Page 107: Lecture #2 – Evolution of Populations

108

Graph – frequency dependent selection results

Balanced Polymorphisms – Frequency Dependent Selection

rare clone is less infected

Page 108: Lecture #2 – Evolution of Populations

109

Images – balanced polymorphisms in asters and snakes

Balanced Polymorphisms – Phenotypic Variationmultiple morphotypes are favored by heterogeneous

(patchy) environment

Page 109: Lecture #2 – Evolution of Populations

110

Neutral Variation

• Genetic variation that has no apparent effect on fitness

• Not affected by natural selection• May provide an important base for future

selection, if environmental conditions change

Page 110: Lecture #2 – Evolution of Populations

111

Key Concepts: QUESTIONS???

• The Modern Synthesis• Populations and the Gene Pool• The Hardy-Weinberg Equilibrium• Micro-evolution• Sources of Genetic Variation• Natural Selection• Preservation of Genetic Variation


Top Related