Download - Chapter 23 - Population Genetics
Chapter 23 (or 7): Population Genetics: The Evolution of Populations
Oakton Community College - BIO 122Dr. Paul Gulezian
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Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but only populations evolve
• Genetic variations in populations contribute to evolution
• Microevolution is a change in allele frequencies in a population over generations
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• Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals
Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
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Genetic Variation
• Variation in individual genotype leads to variation in individual phenotype
• Not all phenotypic variation is heritable
• Natural selection can only act on variation with a genetic component for evolution to proceed
Fig. 23-2
(a) (b)
Non-Heritable Variation
Fig. 23-2a
(a)
Fig. 23-2b
(b)
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Variation Within a Population
• Both discrete and quantitative characters contribute to variation within a population
• Discrete characters can be classified on an either-or basis
• Quantitative characters vary along a continuum within a population
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• Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels
• Average 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 individuals
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Variation Between Populations
• Most species exhibit geographic variation, differences between gene pools of separate populations or population subgroups
Fig. 23-3
13.17 19 XX10.169.128.11
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
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• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis
Fig. 23-4
1.0
0.8
0.6
0.4
0.2
046 44 42 40 38 36 34 32 30
GeorgiaWarm (21°C)
Latitude (°N)
MaineCold (6°C)
Ldh
-B b
alle
le f
req
uen
cy
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Mutation
• Mutations 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 offspring.
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Point Mutations
• A point mutation is a change in one base in a gene
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• 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 code
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• The effects of point mutations can vary:
– Mutations that result in a change in protein production are often harmful
– Mutations that result in a change in protein production can sometimes increase the fit between organism and environment
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Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
• Duplication of large chromosome segments is usually harmful
• Duplication of small pieces of DNA is sometimes less harmful and increases the genome size
• Duplicated genes can take on new functions by further mutation
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Mutation Rates
• Mutation rates are low in animals and plants
• The average is about one mutation in every 100,000 genes per generation
• Mutations rates are often lower in prokaryotes and higher in viruses
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Sexual Reproduction
• Sexual reproduction can shuffle existing alleles into new combinations
• In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible
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Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
• The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population
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Gene Pools and Allele Frequencies
• A population is a localized group of individuals capable of interbreeding and producing fertile offspring
• A gene pool consists of all the alleles for all loci in a population
• A locus is fixed if all individuals in a population are homozygous for the same allele
Fig. 23-5Porcupine herd
Porcupineherd range
Beaufort Sea NO
RTH
WEST
TERR
ITOR
IES
MAPAREA
AL
AS
KA
CA
NA
DA
Fortymileherd range
Fortymile herd
AL
AS
KA
YU
KO
N
Fig. 23-5a
Porcupineherd range
Beaufort Sea NO
RTH
WEST
TERR
ITOR
IES
MAPAREA
AL
AS
KA
CA
NA
DA
Fortymileherd range
AL
AS
KA
YU
KO
N
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• The frequency of an allele in a population can be calculated
– For 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 alleles
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• 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 = 1
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The Hardy-Weinberg Principle
• The Hardy-Weinberg principle describes a population that is not evolving
• If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving
• Thus, the H-W principle is a null model.
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Hardy-Weinberg Equilibrium
• The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
• In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
• Mendelian inheritance preserves genetic variation in a population
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
CR allele = 0.8
CW allele = 0.2
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• Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
• If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then
– p2 + 2pq + q2 = 1
– where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
Fig. 23-7-1
SpermCR
(80%)
CW
(20 %
)
80% CR ( p = 0.8)
CW (20%)
20% CW (q = 0.2)
16% ( pq) CRCW
4% (q2) CW CW
CR
(80%
)
64% ( p2) CRCR
16% (qp) CRCW
Eg
gs
Fig. 23-7-2
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Fig. 23-7-3
Gametes of this generation:
64% CRCR, 32% CRCW, and 4% CWCW
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
64% CRCR, 32% CRCW, and 4% CWCW plants
Genotypes in the next generation:
Fig. 23-7-4
Gametes of this generation:
64% CR CR, 32% CR CW, and 4% CW CW
64% CR + 16% CR = 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
64% CR CR, 32% CR CW, and 4% CW CW plants
Genotypes in the next generation:
SpermCR
(80%)
CW
( 20%
)
80% CR ( p = 0.8)
CW (20%)
20% CW (q = 0.2)
16% ( pq) CR CW
4% (q2) CW CW
CR
(80%
)
64% ( p2) CR CR
16% (qp) CR CW
Eg
gs
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Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a hypothetical population
• In real populations, allele and genotype frequencies do change over time
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• The five conditions for nonevolving populations are rarely met in nature:
– No mutations
– Random mating
– No natural selection
– Extremely large population size
– No gene flow
• Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci
Applying the Hardy-Weinberg Principle• We 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 allele
– 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 frequencies
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• The occurrence of PKU is 1 per 10,000 births
– q2 = 0.0001
– q = 0.01
• The frequency of normal alleles is
– p = 1 – q = 1 – 0.01 = 0.99
• The frequency of carriers is
– 2pq = 2 x 0.99 x 0.01 = 0.0198
– or approximately 2% of the U.S. population
What is the frequency of carriers of the PKU allele?
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• Three major factors alter allele frequencies and bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
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Natural Selection
• Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
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Genetic Drift
• The smaller a sample, the greater the chance of deviation from a predicted result
• Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next
• Genetic drift tends to reduce genetic variation through losses of alleles
Fig. 23-8-1
Generation 1p (frequency of CR) = 0.7q (frequency of CW
) = 0.3
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
Fig. 23-8-2
Generation 1p (frequency of CR) = 0.7q (frequency of CW
) = 0.3
Generation 2p = 0.5q = 0.5
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CW
CR CW
CR CW
CR CWCR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
Fig. 23-8-3
Generation 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) = 0.7q (frequency of CW
) = 0.3
Generation 2
CR CWCR CW
CR CW
CR CW
CW CW
CW CW
CW CW
CR CR
CR CR
CR CR
p = 0.5q = 0.5
Generation 3p = 1.0q = 0.0
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR CR CR
CR CR
CR CR CR CR
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The Founder Effect
• The founder effect occurs when a few individuals become isolated from a larger population
• Allele frequencies in the small founder population can be different from those in the larger parent population
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The Bottleneck Effect
• The bottleneck effect is a sudden reduction in population size due to a change in the environment
• The resulting gene pool may no longer be reflective of the original population’s gene pool
• If the population remains small, it may be further affected by genetic drift
Fig. 23-9
Originalpopulation
Bottleneckingevent
Survivingpopulation
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• Understanding the bottleneck effect can increase understanding of how human activity affects other species
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Case Study: Impact of Genetic Drift on the Greater Prairie Chicken
• Loss 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 hatched
Fig. 23-10
Numberof allelesper locus
Rangeof greaterprairiechicken
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
Minnesota, 1998 (no bottleneck)
Nebraska, 1998 (no bottleneck)
Kansas, 1998 (no bottleneck)
Illinois
1930–1960s
1993
Location Populationsize
Percentageof eggshatched
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
(a)
(b)
Fig. 23-10a
Rangeof greaterprairiechicken
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
(a)
Fig. 23-10b
Numberof allelesper locus
Minnesota, 1998 (no bottleneck)
Nebraska, 1998 (no bottleneck)
Kansas, 1998 (no bottleneck)
Illinois
1930–1960s
1993
Location Populationsize
Percentageof eggshatched
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
(b)
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• Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck
• The 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%
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Effects of Genetic Drift: A Summary
1. Genetic drift is significant in small populations
2. Genetic drift causes allele frequencies to change at random
3. Genetic drift can lead to a loss of genetic variation within populations
4. Genetic drift can cause harmful alleles to become fixed
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Gene Flow
• Gene flow consists of the movement of alleles among populations
• Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
• Gene flow tends to reduce differences between populations over time
• Gene flow is more likely than mutation to alter allele frequencies directly
Fig. 23-11
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• Gene flow can decrease the fitness of a population, particularly if it is adapted to specific environmental conditions
• In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils
• Windblown pollen moves these alleles between populations
• The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment
Fig. 23-12
NON-MINESOIL
MINESOIL
NON-MINESOIL
Prevailing wind direction
Ind
ex o
f co
pp
er t
ole
ran
ce
Distance from mine edge (meters)
70
60
50
40
30
20
10
020 0 20 0 20 40 60 80 100 120 140 160
Fig. 23-12a
NON-MINESOIL
MINESOIL
NON-MINESOIL
Prevailing wind direction
Ind
ex o
f co
pp
er t
ole
ran
ce
Distance from mine edge (meters)
70
60
50
40
30
20
10
020 0 20 0 20 40 60 80 100 120 140 160
Fig. 23-12b
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• Gene flow can also increase the fitness of a population
• Insecticides have been used to target mosquitoes that carry West Nile virus and malaria
• Alleles 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 fitness
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• Only natural selection consistently results in adaptive evolution
Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
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A Closer Look at Natural Selection
• Natural selection brings about adaptive evolution by acting on an organism’s phenotype
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Relative Fitness
• 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 factors
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• Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
• Selection favors certain genotypes by acting on the phenotypes of certain organisms
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Directional, Disruptive, and Stabilizing Selection
• Three 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 phenotypes
Fig. 23-13
Original population
(c) Stabilizing selection(b) Disruptive selection(a) Directional selection
Phenotypes (fur color)F
req
uen
cy o
f in
div
idu
als
Originalpopulation
Evolvedpopulation
Fig. 23-13a
Original population
(a) Directional selection
Phenotypes (fur color)
Fre
qu
enc
y o
f in
div
idu
als
Original population
Evolved population
Fig. 23-13b
Original population
(b) Disruptive selection
Phenotypes (fur color)
Fre
qu
enc
y o
f in
div
idu
als
Evolved population
Fig. 23-13c
Original population
(c) Stabilizing selection
Phenotypes (fur color)
Fre
qu
enc
y o
f in
div
idu
als
Evolved population
What kind of selection does this situation describe?
What kind of selection does this situation describe?
Stabilizing selection….intermediates are favored.
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The Key Role of Natural Selection in Adaptive Evolution
• Natural selection increases the frequencies of alleles that enhance survival and reproduction
• Adaptive evolution occurs as the match between an organism and its environment increases
Fig. 23-14
(a) Color-changing ability in cuttlefish
(b) Movable jaw bones in snakes
Movable bones
Fig. 23-14a
(a) Color-changing ability in cuttlefish
Fig. 23-14b
(b) Movable jaw bones in snakes
Movable bones
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• Because the environment can change, adaptive evolution is a continuous process
• Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment
Sexual Selection
• Sexual selection is natural selection for mating success
• It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
Fig. 23-15
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• Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
• Intersexual 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 survival (an evolutionary tradeoff)
• 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 for
Sexual Selection: Female Mate Choice
Fig. 23-16
SC male graytree frog
Female graytree frog
LC male graytree frog
EXPERIMENT
SC sperm Eggs LC sperm
Offspring ofLC father
Offspring ofSC father
Fitness of these half-sibling offspring compared
RESULTS
1995Fitness 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 malessuperior to offspring of SC males.
Fig. 23-16a
SC male graytree frog
Female graytree frog
LC male graytree frog
SC sperm Eggs LC sperm
Offspring ofLC father
Offspring ofSC father
Fitness of these half-sibling offspring compared
EXPERIMENT
Fig. 23-16b
RESULTS
1995Fitness 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 malessuperior to offspring of SC males.
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The Preservation of Genetic Variation
• Various mechanisms help to preserve genetic variation in a population
– Diploidy
– Balancing Selection
• Heterozygote Advantage
• Frequency-dependent Selection
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Diploidy
• Diploidy maintains genetic variation in the form of hidden recessive alleles that selection does not act upon because they are not manifest in the phenotype.
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Balancing Selection
• Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
– Heterozygote Advantage
– Frequency-dependent selection
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• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
• Natural selection will tend to maintain two or more alleles at that locus
• Example: The sickle-cell allele causes mutations in hemoglobin (bad) but also confers malaria resistance (good)
Heterozygote Advantage
Fig. 23-17
0–2.5%
Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)
Frequencies of thesickle-cell allele
2.5–5.0%
7.5–10.0%
5.0–7.5%
>12.5%
10.0–12.5%
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• In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population
• Selection can favor whichever phenotype is less common in a population
• Example: prey-switching behavior in many predators leads to multiple forms of prey coexisting in populations
Frequency-Dependent Selection
Fig. 23-18
“Right-mouthed”
1981
“Left-mouthed”
Fre
qu
ency
of
“lef
t-m
ou
thed
” in
div
idu
als
Sample year
1.0
0.5
0’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Fig. 23-18a
“Right-mouthed”
“Left-mouthed”
Fig. 23-18b
1981
Fre
qu
en
cy
of
“le
ft-m
ou
the
d”
ind
ivid
ua
ls
Sample year
1.0
0.5
0’82
’83
’84
’85
’86
’87
’88
’89
’90
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Neutral Variation
• Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage
• For example,
– Variation in noncoding regions of DNA (although many of these regions are now known to be “swtiches” that control the expression of other genes)
– Variation in proteins that have little effect on protein function or reproductive fitness
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Why Natural Selection Cannot Fashion Perfect Organisms
1. Selection can act only on existing variations, it is not the source of new alleles (mutations are)
2. Evolution is limited by historical constraints (developmental patterns, for example)
3. Adaptations are often compromises between different organismal needs (mate attraction, predator avoidance)
4. Chance events, natural selection, and the environment interact and conidtions constantly change so “perfection” is a moving target
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You should now be able to:
1. Explain why the majority of point mutations are harmless
2. Explain how sexual recombination generates genetic variability
3. Define the terms population, species, gene pool, relative fitness, and neutral variation
4. List the five conditions of Hardy-Weinberg equilibrium
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5. Apply the Hardy-Weinberg equation to a population genetics problem
6. Explain why natural selection is the only mechanism that consistently produces adaptive change
7. Explain the role of population size in genetic drift
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8. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection
9. List four reasons why natural selection cannot produce perfect organisms