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 populations So what do we study ID: 668858
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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 flowersSlide40Slide41
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.Slide134Slide135
DiversifyingSelection toward both extremes and against the norm.Ex: bill size in birdsSlide136Slide137
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!