Download PPT Population Evolution Ch 23

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 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-1
 Two processes, mutation and sexual
reproduction, produce the variation in gene
pools that contributes to differences among
individuals
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 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-2
(a)
(b)
Fig. 23-2a
(a)
Fig. 23-2b
(b)
 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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 Most species exhibit geographic variation,
differences between gene pools of separate
populations or population subgroups
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Fig. 23-3
1
2.4
8.11
9.12
3.14
5.18
10.16 13.17
6
7.15
19
XX
1
2.19
3.8
4.16 5.14
9.10 11.12 13.17 15.18
6.7
XX
• Some examples of geographic variation occur as a
cline, which is a graded change in a trait along a
geographic axis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-4
1.0
0.8
0.6
0.4
0.2
0
46
Maine
Cold
(6°C)
44
42
40
38
36
Latitude
(°N)
34
32
30
Georgia
Warm
(21°C)
 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
Animation: Genetic Variation from Sexual Recombination
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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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 Both discrete and quantitative characters
contribute to variation within a population
 Discrete characters can be classified on an eitheror basis
 Quantitative characters vary along a continuum
within a population
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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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 The first step in testing whether evolution is
occurring in a population is to clarify what we mean
by a population
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-5
Porcupine herd
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
Fortymile herd
Fig. 23-5a
MAP
AREA
Beaufort Sea
Porcupine
herd range
Fortymile
herd range
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
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Fig. 23-6
Alleles in the population
Frequencies of alleles
p = frequency of
CR allele
= 0.8
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
Each sperm:
80%
20%
chance chance
80%
20%
chance chance
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-7-1
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CR
(80%)
CW
(20%)
64% (p2)
CRCR
16% (pq)
CRCW
16% (qp)
CRCW
4% (q2)
CW CW
Fig. 23-7-2
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW + 16% CW = 20% CW = 0.2 = q
Fig. 23-7-3
64% CRCR, 32% CRCW, and 4% CWCW
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:
64% CRCR, 32% CRCW, and 4% CWCW plants
Fig. 23-7-4
20% CW (q = 0.2)
80% CR ( p = 0.8)
Sperm
(80%)
CW
(20%)
64% ( p2)
CR CR
16% ( pq)
CR CW
CR
16% (qp)
CR CW
4% (q2)
CW CW
64% CR CR, 32% CR CW, and 4% CW CW
Gametes of this generation:
64% CR + 16% CR
= 80% CR = 0.8 = p
4% CW
= 20% CW = 0.2 = q
+ 16% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
 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
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 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
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 Three major factors alter allele frequencies and
bring about most evolutionary change:
 Natural selection
 Genetic drift
 Gene flow
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 Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-8-1
CR CR
CR CR
CR CW
CR CR
CW CW
CR CW
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
Fig. 23-8-2
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CW CW
CR CR
CR CW
CR CW
CR CR
CR CR
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
Fig. 23-8-3
CR CR
CR CR
CW CW
CR CW
CR CW
CR CR
CW CW
CR CR
CR CW
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
CW CW
CR CW
CR CR
CR CR
CR CR
CW CW
CR CR
CR CW
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CR
CR CW
Generation 2
p = 0.5
q = 0.5
CR CR
CR CR
Generation 3
p = 1.0
q = 0.0
 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 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
 Understanding the bottleneck effect can increase
understanding of how human activity affects other species
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-10a
Pre-bottleneck
(Illinois, 1820)
(a)
Range
of greater
prairie
chicken
Post-bottleneck
(Illinois, 1993)
Fig. 23-10b
Location
Population
size
Number
Percentage
of alleles of eggs
per locus hatched
Illinois
1,000–25,000
5.2
93
<50
3.7
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Minnesota, 1998
(no bottleneck)
4,000
5.3
85
1930–1960s
1993
(b)
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
1.
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 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
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 Gene flow can decrease the fitness of a
population
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 Gene flow can 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
 Natural selection brings about adaptive evolution by acting on
an organism’s phenotype
 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

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-13
Original population
Original
Evolved
population population
(a) Directional selection
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing
selection
Fig. 23-13a
Original population
Phenotypes (fur color)
Original population
Evolved population
(a) Directional selection
Fig. 23-13b
Original population
Phenotypes (fur color)
Evolved population
(b) Disruptive selection
Fig. 23-13c
Original population
Phenotypes (fur color)
Evolved population
(c) Stabilizing selection
 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
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Fig. 23-14a
(a) Color-changing ability in cuttlefish
Fig. 23-14b
Movable bones
(b) Movable jaw
bones in
snakes
 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
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 Sexual selection is natural selection for mating
success
 It can result in sexual dimorphism, marked
differences between the sexes in secondary
sexual characteristics
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Fig. 23-15
 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
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 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
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Fig. 23-16
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of Offspring of
SC father
LC father
Fitness of these half-sibling offspring compared
RESULTS
Fitness Measure
1995
1996
Larval growth
NSD
LC better
Larval survival
LC better
NSD
Time to metamorphosis
LC better
(shorter)
LC better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
 Diploidy maintains genetic variation in the form
of hidden recessive alleles
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 Balancing selection occurs when natural selection
maintains stable frequencies of two or more
phenotypic forms in a population
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Heterozygote Advantage
• 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
• The sickle-cell allele causes mutations in
hemoglobin but also confers malaria resistance
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Fig. 23-17
Frequencies of the
sickle-cell allele
0–2.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
 Neutral 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 fitness
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Selection can act only on existing variations
2. Evolution is limited by historical constraints
3. Adaptations are often compromises
4. Chance, natural selection, and the environment
interact
1.
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Fig. 23-19