Transcript population

LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 23
The Evolution of Populations
棲群演化
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve
during their lifetimes 個體不會演化
• Natural selection acts on individuals, but only
populations evolve 天擇作用於個體 然棲群演化
• Consider, for example, a population of medium
ground finches on Daphne Major Island
– During a drought, large-beaked birds were
more likely to crack large seeds and survive
– The finch population evolved by natural
selection
© 2011 Pearson Education, Inc.
Figure 23.1
Average beak depth (mm)
Figure 23.2
10
9
8
0
1978
1976
(similar to the (after
prior 3 years) drought)
• Microevolution微演化 is a change in
allele frequencies in a population over
generations 棲群對偶頻度世代間之改變
• Three mechanisms cause allele
frequency change:
– Natural selection
– Genetic drift
– Gene flow
• Only natural selection causes adaptive
evolution 天擇引發適應性演化
© 2011 Pearson Education, Inc.
Concept 23.1: Genetic variation makes
evolution possible
• Variation in heritable traits is a
prerequisite for evolution 變異事先存在才
有演化事件
• Mendel’s work on pea plants provided
evidence of discrete heritable units
(genes)
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Genetic Variation
• Genetic variation among individuals is
caused by differences in genes or other
DNA segments
• Phenotype is the product of inherited
genotype and environmental influences
• Natural selection can only act on
variation with a genetic component天擇只
能作用於依因組成之變異
© 2011 Pearson Education, Inc.
Figure 23.3
(a)
(b)
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
棲群中的變異部分具可量化性質
© 2011 Pearson Education, Inc.
• Genetic variation can be measured as
gene variability or nucleotide variability
• For gene variability, average
heterozygosity measures the average
percent of loci that are heterozygous in a
population異源染色體基因座百分比
• Nucleotide variability is measured by
comparing the DNA sequences of pairs of
individuals 個體DNA序列變異度
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Variation Between Populations
• Most species exhibit geographic
variation地理變異, differences between
gene pools of separate populations
• For example, Madeira is home to several
isolated populations of mice
– Chromosomal variation among
populations is due to drift, not natural
selection 老鼠染色體變異乃基因漂變
而非天擇作用
© 2011 Pearson Education, Inc.
Figure 23.4
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
• For example, mummichog fish vary in a
cold-adaptive allele along a temperature
gradient
– This variation results from natural
selection
© 2011 Pearson Education, Inc.
Figure 23.5
Ldh-Bb allele frequency
1.0
0.8
0.6
0.4
0.2
0
46
44
42
Maine
Cold (6°C)
40
38
36
Latitude (ºN)
34
32
Georgia
Warm (21ºC)
30
Sources of Genetic Variation
• New genes and alleles can arise by
mutation or gene duplication
新基因或新對偶乃基因突變或複製
© 2011 Pearson Education, Inc.
Animation: Genetic Variation from Sexual Recombination
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Formation of New Alleles
• A mutation is a change in nucleotide
sequence of DNA
• Only mutations in cells that produce
gametes can be passed to offspring
• A point mutation點突變 is a change in
one base in a gene
© 2011 Pearson Education, Inc.
• The effects of point mutations can vary:
– Mutations in noncoding regions of
DNA are often harmless
– Mutations in a genes can be neutral
because of redundancy in the
genetic code
© 2011 Pearson Education, Inc.
• 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 be
beneficial
點突變可能無害、有害、有益
© 2011 Pearson Education, Inc.
Altering Gene Number or Position
• Chromosomal mutations that delete, disrupt,
or rearrange many loci are typically harmful
• Duplication of small pieces of DNA increases
genome size and is usually less harmful
• Duplicated genes can take on new functions
by further mutation
• An ancestral odor-detecting gene has been
duplicated many times: humans have 1,000
copies of the gene, mice have 1,300
© 2011 Pearson Education, Inc.
Rapid Reproduction
• 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
動植物基因突變速率遠低於原核生物或病毒
© 2011 Pearson Education, Inc.
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
有性生殖導致基因重組 增加適應性
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Gene Pools and Allele Frequencies
• A population is a localized group of
individuals capable of interbreeding and
producing fertile offspring
• A gene pool基因池 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
基因池油棲群所有個體基因總合
© 2011 Pearson Education, Inc.
MAP
AREA
CANADA
ALASKA
Figure 23.6
Beaufort Sea
Porcupine
herd range
Porcupine herd
Fortymile
herd range
Fortymile herd
• 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 times 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
© 2011 Pearson Education, Inc.
• 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
© 2011 Pearson Education, Inc.
• For example, consider a population of
wildflowers that is incompletely
dominant for color:
– 320 red flowers (CRCR)
– 160 pink flowers (CRCW)
– 20 white flowers (CWCW)
• Calculate the number of copies of each
allele:
– CR  (320  2)  160  800
– CW  (20  2)  160  200
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• To calculate the frequency of each allele:
– p  freq CR  800 / (800  200)  0.8
– q  freq CW  200 / (800  200)  0.2
• The sum of alleles is always 1
– 0.8  0.2  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
對偶頻度不變 棲群不會演化
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Figure 23.7
Alleles in the population
Gametes produced
Frequencies of alleles
p = frequency of
CR allele
= 0.8
Each egg:
Each sperm:
q = frequency of
CW allele
= 0.2
20%
80%
chance chance
20%
80%
chance chance
• Hardy-Weinberg equilibrium describes
the constant frequency of alleles in such
a gene pool
• Consider, for example, the same
population of 500 wildflowers and 100
alleles where
– p  freq CR  0.8
– q  freq CW  0.2
© 2011 Pearson Education, Inc.
• The frequency of genotypes can be
calculated
– CRCR  p2  (0.8)2  0.64
– CRCW  2pq  2(0.8)(0.2)  0.32
– CWCW  q2  (0.2)2  0.04
• The frequency of genotypes can be
confirmed using a Punnett square
© 2011 Pearson Education, Inc.
Figure 23.8
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
CW (20%)
CR (80%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRCW
4% (q2)
CWCW
16% (qp)
CRCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
Figure 23.8a
80% CR (p = 0.8)
20% CW (q = 0.2)
CR
Sperm
(80%)
CW (20%)
CR
(80%)
Eggs
CW
(20%)
64% (p2)
CRCR
16% (qp)
CRCW
16% (pq)
CRCW
4% (q2)
CWCW
Figure 23.8b
Sperm
CR (80%)
CW (20%)
CR
(80%)
64% (p2)
CRCR
Eggs
CW
16% (pq)
CRCW
4% (q2)
CWCW
16% (qp)
CRCW
(20%)
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR
(from CRCR plants)
R
+ 16% C R W
(from C C plants)
= 80% CR = 0.8 = p
4% CW
(from CWCW plants)
W
+ 16% C R W
(from C C plants)
= 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
• 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
© 2011 Pearson Education, Inc.
Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population that is not evolving
• In real populations, allele and genotype
frequencies do change over time
© 2011 Pearson Education, Inc.
• The five conditions for nonevolving
populations are rarely met in nature:
1. No mutations 沒有突變
2. Random mating 逢機交配
3. No natural selection 沒有天擇
4. Extremely large population size 大棲群
5. No gene flow 沒有基因流
© 2011 Pearson Education, Inc.
• Natural populations can evolve at some
loci, while being in Hardy-Weinberg
equilibrium at other loci
© 2011 Pearson Education, Inc.
Applying the Hardy-Weinberg Principle
• We can assume the locus that causes
phenylketonuria (PKU)苯酮尿 is in HardyWeinberg equilibrium given that:
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1. The PKU gene mutation rate is low
2. Mate selection is random with respect to
whether or not an individual is a carrier for
the PKU allele
3. Natural selection can only act on rare
homozygous individuals who do not
follow dietary restrictions
4. The population is large
5. 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  0.99  0.01  0.0198
– or approximately 2% of the U.S. population
© 2011 Pearson Education, Inc.
Concept 23.3: Natural selection, genetic drift,
and gene flow can alter allele frequencies in
a population
• Three major factors alter allele frequencies
and bring about most evolutionary change:
– Natural selection
– Genetic drift
– Gene flow
© 2011 Pearson Education, Inc.
Natural Selection 天擇
• Differential success in reproduction
results in certain alleles being passed to
the next generation in greater
proportions
• For example, an allele that confers
resistance to DDT increased in
frequency after DDT was used widely in
agriculture
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Genetic Drift 基因漂變
• 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
© 2011 Pearson Education, Inc.
Animation: Causes of Evolutionary Change
Right-click slide / select “Play”
© 2011 Pearson Education, Inc.
Figure 23.9-1
CRCR
CRCR
CRCW
CWCW
CRCR
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
Figure 23.9-2
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
Figure 23.9-3
CRCR
CRCR
CRCW
CWCW
5
plants
leave
offspring
CRCR
CWCW
CRCW
CRCR
CWCW
CRCR
CRCW
CRCW
CRCR
CRCR
CRCW
CRCW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW) = 0.3
CWCW
CRCW
2
plants
leave
offspring
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCR
CRCW
Generation 2
p = 0.5
q = 0.5
CRCR
CRCR
CRCR
CRCR
Generation 3
p = 1.0
q = 0.0
The Founder Effect 奠基者效應
• 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 瓶頸效應
• 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
© 2011 Pearson Education, Inc.
Figure 23.10-1
Original
population
Figure 23.10-2
Original
population
Bottlenecking
event
Figure 23.10-3
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect can
increase understanding of how human
activity affects other species
人類經常對其他生物做出瓶頸效應作為
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Figure 23.11
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
松雞
Greater prairie chicken
Range
of greater
prairie
chicken
(a)
Location
Illinois
1930–1960s
1993
Population
size
Percentage
Number
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
Figure 23.11a
Pre-bottleneck
(Illinois, 1820)
Greater prairie chicken
(a)
Range
of greater
prairie
chicken
Post-bottleneck
(Illinois, 1993)
Figure 23.11b
Location
Illinois
1930–1960s
1993
Population
size
Number Percentage
of alleles of eggs
per locus hatched
1,000–25,000
<50
5.2
3.7
93
<50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
(b)
• 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 基因漂變效應在小棲群容易凸顯
2. Genetic drift causes allele frequencies to
change at random基因漂變導致對偶頻度改變
3. Genetic drift can lead to a loss of genetic
variation within populations基因漂變造成遺
傳變異減少
4. Genetic drift can cause harmful alleles to
become fixed 基因漂變可能固定有害基因
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Gene Flow 基因流
• 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 variation among
populations over time 基因流降低棲群間的
遺傳差異
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• Gene flow can decrease the fitness of a
population
• Consider, for example, the great tit 大山雀
(Parus major) on the Dutch island of Vlieland
– Mating causes gene flow between the
central and eastern populations
– Immigration from the mainland introduces
alleles that decrease fitness
– Natural selection selects for alleles that
increase fitness
– Birds in the central region with high
immigration have a lower fitness; birds in
the east with low immigration have a higher
fitness
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Figure 23.12
60
Survival rate (%)
50
Population in which the
surviving females
eventually bred
Central
Eastern
Central
population
NORTH SEA
Eastern
population
Vlieland,
the Netherlands
40
2 km
30
20
10
0
Females born
in central
population
Females born
in eastern
population
Parus major
• Gene flow can increase the fitness of a
population
• Consider, for example, the spread of alleles
for resistance to insecticides 殺蟲劑
– Insecticides have been used to target
mosquitoes that carry West Nile virus西尼羅病
毒 and malaria瘧疾
– 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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• Gene flow is an important agent of
evolutionary change in human populations
基因流在人類棲群演化變異扮演重要角色
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Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
• Evolution by natural selection involves both
change and “sorting”
– New genetic variations arise by chance
– Beneficial alleles are “sorted” and favored by
natural selection
• Only natural selection consistently results in
adaptive evolution 如果有適應性演化則是
天擇的結果
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A Closer Look at Natural Selection
• Natural selection brings about adaptive
evolution by acting on an organism’s
phenotype
天擇作用於個體之表型
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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 生存競爭或生存適應不意旨個
體直接競爭
• Reproductive success is generally more
subtle and depends on many factors
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• Relative fitness相對適度存 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方向性選汰 favors
individuals at one end of the phenotypic
range
– Disruptive selection分散性選汰 favors
individuals at both extremes of the
phenotypic range
– Stabilizing selection穩定性選汰 favors
intermediate variants and acts against
extreme phenotypes
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Frequency of
individuals
Figure 23.13
Original
population
Evolved
population
(a) Directional selection
Original population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
Figure 23.13a
Original
population
Evolved
population
(a) Directional selection
Figure 23.13b
Original
population
Evolved
population
(b) Disruptive selection
Figure 23.13c
Original
population
Evolved
population
(c) Stabilizing selection
The Key Role of Natural Selection in
Adaptive Evolution
• Striking adaptation have arisen by
natural selection
– For example, cuttlefish can change
color rapidly for camouflage
– For example, the jaws of snakes
allow them to swallow prey larger
than their heads
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Figure 23.14
Bones shown in
green are movable.
Ligament
• Natural selection increases the frequencies
of alleles that enhance survival and
reproduction 天擇增加適合生存之對偶頻度
• Adaptive evolution occurs as the match
between an organism and its environment
increases
• Because the environment can change,
adaptive evolution is a continuous process
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• 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
• Sexual selection 性擇 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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Figure 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
雄性炫耀增加擇偶機會 然降低生存機會
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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 increase in frequency
© 2011 Pearson Education, Inc.
Figure 23.16
EXPERIMENT
Recording of LC
male’s call
Recording of SC
male’s call
Female gray
tree frog
LC male gray
tree frog
SC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
SC father
Offspring of
LC father
Survival and growth of these half-sibling offspring compared
RESULTS
Offspring Performance
1995
1996
Larval survival
LC better
NSD
Larval growth
NSD
LC better
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.
Figure 23.16a
EXPERIMENT
Recording of SC
male’s call
Recording of LC
male’s call
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm  Eggs  LC sperm
Offspring of
SC father
Offspring of
LC father
Survival and growth of these half-sibling offspring compared
Figure 23.16b
RESULTS
Offspring Performance
1995
1996
Larval survival
LC better
NSD
Larval growth
NSD
LC better
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.
The Preservation of Genetic Variation
• Neutral variation中性變異 is genetic
variation that does not confer a selective
advantage or disadvantage
• Various mechanisms help to preserve
genetic variation in a population
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Diploidy
• Diploidy maintains genetic variation in the
form of hidden recessive alleles
• Heterozygotes can carry recessive
alleles that are hidden from the effects of
selection
雙套染色體的設計可掩護隱性基因
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Balancing Selection
• Balancing selection平衡選汰 occurs when
natural selection maintains stable
frequencies of two or more phenotypic forms
in a population
• Balancing selection includes
– Heterozygote advantage
– Frequency-dependent selection
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Heterozygote Advantage
• Heterozygote advantage occurs when
heterozygotes have a higher fitness than
do both homozygotes異質基因優勢
• 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
© 2011 Pearson Education, Inc.
Figure 23.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%
Frequency-Dependent Selection
• In frequency-dependent selection頻度
相關選汰, 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
• For example, frequency-dependent
selection selects for approximately equal
numbers of “right-mouthed” and “leftmouthed” scale-eating fish
© 2011 Pearson Education, Inc.
Figure 23.18
“Left-mouthed”
P. microlepis
Frequency of
“left-mouthed” individuals
1.0
“Right-mouthed”
P. microlepis
0.5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
Why Natural Selection Cannot Fashion
Perfect Organisms
1. 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
© 2011 Pearson Education, Inc.
Figure 23.19
Figure 23.UN01
CRCR
CWCW
CRCW
Figure 23.UN02
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Stabilizing
selection
Figure 23.UN03
Sampling sites
(1–8 represent
pairs of sites)
2
1
3
4
5
6
7
8
9
10 11
2
Allele
frequencies
lap94 alleles
Other lap alleles
Data from R. K. Koehn and T. J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).
Salinity increases toward the open ocean
1
Long Island 2 3
Sound
7 8
6
4 5
9
10
11
Atlantic
Ocean