Download Natural Selection

Chapter 23
The Evolution of Populations
族群 / 種群的演化
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve, in
the Darwinian sense, during their lifetimes
Ò 個體表現出的性狀差異並不能定義為演化
• Natural selection acts on individuals, but only
populations evolve Ò 實際上的演化係來自天擇對於種群的影響
• Genetic variations in populations contribute to
evolution Ò 遺傳變異使得種群具有價值----演化的發生
• Microevolution is a change in allele frequencies
in a population over generations
Ò 微演化發生於對偶基因 / 對位基因的頻度改變
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http://www.migeneticsconnection.org/genomics/Genetic%20Variation/DNA%20Molecule%20of%20Life.jpg
1
Fig. 23-1
http://www.dkimages.com/discover/previews/840/35004004.JPG
http://porpax.bio.miami.edu/~cmallery/150/unity/c1x17b-finches.jpg
Concept 23.1: Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
• Two processes,
1. mutation
Ò 兩個效應使得遺傳變異產生了演化的可能性
Ò 突變 ---- 產生差異
2. sexual reproduction
Ò 有性生殖 ---- 繼承親代卻融入改變
produce the variation in gene pools that
contributes to differences among individuals
Ò 突變與有性生殖造成了遺傳變異 / 遺傳歧異度
Ò 遺傳歧異度的具體表現便為個體差異
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http://www.teesdale.gov.uk/Image/Dog%20breeding.jpg
2
Concept 23.1: Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
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Genetic Variation
http://static.howstuffworks.com/gif/evolution-fly.gif
遺傳變異 / 遺傳歧異度
• Variation in individual genotype leads to
variation in individual phenotype
Ò 各體的基因型變異會導致表現型差異
• Not all phenotypic variation is heritable
Ò 並非所有的表型變異都具有遺傳性
• Natural selection can only act on variation with a
genetic component
Ò 天擇僅發生於具有改變性的遺傳特徵
phenotype
genotype
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http://www.juliantrubin.com/encyclopedia/genetics/genotype_phenotype_files/400px-Punnett_square_mendel_flowers.png
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Fig. 23-2
(a)
(b)
Variation Within a Population
種群內的變異
• Both discrete and quantitative characters
contribute to variation within a population
• Discrete characters can be classified on an
either-or basis Ò 明確特徵 polymorphism
• Quantitative characters vary along a continuum
within a population Ò 量化特性
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4
• Population geneticists measure polymorphisms in
a population by determining the amount of
heterozygosity at the gene and molecular levels
Ò 造成多型性的原因來自於異基因型合子的作用
• Average heterozygosity measures the average
percent of loci that are heterozygous in a
population Ò 異基因型合子比例
• Nucleotide variability is measured by comparing
the DNA sequences of pairs of individuals
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Variation Between Populations
種群間的變異
• Most species exhibit geographic variation,
differences between gene pools of separate
populations or population subgroups
Ò 種群出現地理性變異的原因來自於
1.
種群在彼此分離後的基因池 / 基因庫差異
2.
長時間分離所造成的亞群
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5
Fig. 23-3
1
2.4
3.14
5.18
6
7.15
8.11 9.12
10.1
6
13.17
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
Ò 地理漸變之生態群
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http://www.wsd1.org/southd/Activities/MS%20Paint/Bear%20World.bmp
6
• Some examples of geographic variation occur as a
cline, which is a graded change in a trait along a
geographic axis
Ò 地理漸變之生態群
1.
隨地理分布而逐漸改變
2.
代表相鄰族群可在逐漸重疊區域進行交互生殖
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http://www.uni-graz.at/~sefck/Lake.jpg
Fig. 23-4
Ldh-B b allele frequency
1.0
0.8
0.6
0.4
0.2
0
46
44
Maine
Cold (6°C)
42
40
38
36
Latitude (°N)
34
32
30
Georgia
Warm (21°C)
7
Mutation
突變
• Mutations are changes in the nucleotide
sequence of DNA Ò 突變是DNA上核苷酸序列的改變
• Mutations cause new genes and alleles to arise
• Only mutations in cells that produce gametes can
be passed to offspring
Ò 與原本對偶基因不同
Ò 產生新的對偶基因予以取代
Ò 由配子傳遞新的突變
Ò 改變族群內的基因庫
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Point Mutations
http://www.accessexcellence.org/RC/VL/GG/images/mutation.gif
點突變
• A point mutation is a change in one base in a
gene
Ò 點突變僅為DNA單一鹼基的改變
http://www.intelihealth.com/i/P/PointMutationGEN.gif
http://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/mutation-xray.jpg
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8
• The effects of point mutations can vary:
– Mutations in noncoding regions of DNA are
often harmless Ò 發生於非編碼區的點突變通常為無害的
– Mutations in a gene might not affect protein
production because of redundancy in the
genetic code
Ò 發生於結構基因的點突變會因為遺傳密碼的贅飾性而不影響蛋白質構成
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• The effects of point mutations can vary:
– Mutations that result in a change in protein
production are often harmful
Ò 改變蛋白質產生或功能卻通常對機體本身有害
– Mutations that result in a change in protein
production can sometimes increase the fit
between organism and environment
Ò 但有時卻增加了個體對於環境的適應性
穩定環境
穩定環境
穩定環境
遽變環境
穩定環境
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9
Mutations That Alter Gene Number or Sequence
• Chromosomal mutations that delete,
delete disrupt,
disrupt or
rearrange many loci are typically harmful
Ò 發生於染色體上的基因座刪除、片斷與重新排列
• Duplication of large chromosome segments is
usually harmful Ò 明顯染色體節段的重覆
• 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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Mutations That Alter Gene Number or Sequence
http://www.geneticengineering.org/chemis/Chemis-NucleicAcid/Graphics/Replication.gif
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http://ghr.nlm.nih.gov/handbook/illustrations/duplication.jpg
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Mutation Rates
突變速率
• 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
Ò 自然的突變在動植物間皆是很緩慢的
Ò 平均於每一世代在105基因中約有一個突變發生
Ò 病毒的突變速率相對較快而原核生物則較緩慢
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Sexual Reproduction
http://micro.magnet.fsu.edu/cells/viruses/images/hivstructurefigure1.jpg
有性生殖
• 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
Ò 使對偶基因 / 對位基因有了重組表現的機會
Ò 對偶基因的表現在個體的適應上更勝於突變
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11
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
The Hardy-Weinberg Theorem
哈第--溫伯格定律 / 哈溫氏定律
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Gene Pools and Allele Frequencies
基因庫與對偶基因頻度
• A population is a localized group of individuals
capable of interbreeding and producing fertile
offspring Ò種群--特定地區的個體可藉由種內交配並產生具生殖能力子代
• A gene pool consists of all the alleles for all loci
in a population Ò種群內所有遺傳基因之總和
• A locus is fixed if all individuals in a population
are homozygous for the same allele
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12
Porcupine
herd range
T
ES S
H W RI E
R T TO
N O RRI
TE
Beaufort Sea
MAP
AREA
CANADA
Porcupine herd
ALASKA
Fig. 23-5
ALASKA
YUKON
Fortymile
herd range
Fortymile herd
• The frequency of an allele in a population can
be calculated Ò 對偶基因的出現頻率計算
– For diploid organisms,
organisms the total number of
alleles at a locus is the total number of
individuals x 2 Ò 具有兩套染色體之生物
– 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 Ò 同型合子與異型合子
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13
Diploid organisms
Ò 具有兩套染色體之生物
http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure19n.jpg
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• By convention, if there are 2 alleles at a locus,
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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14
The Hardy-Weinberg Principle
• The Hardy-Weinberg principle describes a
population that is not evolving Ò 解釋未有演化之種群
• If a population does not meet the criteria of the
Hardy-Weinberg principle, it can be concluded
that the population is evolving
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Hardy-Weinberg Equilibrium
哈溫氏平衡
• The Hardy-Weinberg principle states that
frequencies of alleles and genotypes in a
population remain constant from generation
to generation Ò 對偶基因頻度與基因型在族群的世代傳遞中維持恆定
• 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
= 0.8
CR allele
q = frequency of
CW allele
= 0.2
Gametes produced
Each egg:
80%
chance
20%
chance
Each sperm:
80%
chance
20%
chance
• Hardy-Weinberg equilibrium describes the
constant frequency of alleles in such a gene
pool Ò 哈溫氏平衡解釋在基因庫中對偶基因頻度的穩定
• 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
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16
Fig. 23-7-1
80% CR (p = 0.8)
20% CW (q = 0.2)
Sperm
(80%)
CW
(20%)
64% (p2)
CRCR
16% (pq)
CRCW
(20%)
Eggs
CR
(80%)
CR
CW
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
4% CW
= 80% CR = 0.8 = p
+ 16% CW = 20% CW = 0.2 = q
17
Fig. 23-7-3
64% CRCR, 32% CRCW, and 4% CWCW
Gametes of this generation:
64% CR + 16% CR
4% CW
= 80% CR = 0.8 = p
+ 16% CW = 20% CW = 0.2 = q
Genotypes in the next generation:
64% CRCR, 32% CRCW, and 4% CWCW plants
20% CW (q = 0.2)
80% CR ( p = 0.8)
CW
(20%)
Eggs
CR
(80%)
Sperm
CR
(80%)
64% ( p2)
CR CR
CW
(20%)
Fig. 23-7-4
16% ( pq)
CR CW
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
+ 16% CW
= 20% CW = 0.2 = q
4% CW
Genotypes in the next generation:
64% CR CR, 32% CR CW, and 4% CW CW plants
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Conditions for Hardy-Weinberg Equilibrium
• The Hardy-Weinberg theorem describes a
hypothetical population Ò 哈溫氏定律解釋的是個假想的種群
• In real populations, allele and genotype
frequencies do change over time
Ò 在實際的種群中，對偶基因與基因型頻度皆會發生改變
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哈溫氏定律的前提
• The five conditions for nonevolving
populations are rarely met in nature:
– No mutations
Ò 沒有突變發生
– Random mating
Ò 可自由 / 隨機的交配
– No natural selection
Ò 不受天擇作用影響
– Extremely large population size Ò 具有一定規模之種群
– No gene flow
Ò 沒有基因流動
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19
• Natural populations can evolve at some loci,
while being in Hardy-Weinberg equilibrium at
other loci
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Applying the Hardy-Weinberg Principle
• We can assume the locus that causes
phenylketonuria (PKU) is in Hardy-Weinberg
equilibrium given that: Ò 苯酮尿症
– 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
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http://www.newbornscreening.info/tools/GraphicsLib/phenylketonuria.jpg
20
– 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
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21
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:
Ò 引起對偶基因頻率與演化改變的三大要素
– Natural selection
– Genetic drift
– Gene flow
Ò 天擇
Ò 遺傳漂變
Ò 基因流動
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Natural Selection
天擇
• Differential success in reproduction results in
certain alleles being passed to the next
generation in greater proportions
Ò 因為繁殖結果造成個別性差異
Ò 會經過天擇衡量而顯出優劣
Ò 改變之後世代的表現比率
http://evolution.berkeley.edu/evolibrary/images/interviews/naturalselection1.gif
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22
Genetic Drift
遺傳漂變
• The smaller a sample, the greater the chance of
deviation from a predicted result Ò 小型族群易顯其感受性
• 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
The Founder Effect 創造者效應
The Bottleneck Effect 瓶頸效應
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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) =
q0.7
(frequency of CW ) = 0.3
23
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 CW
CR CR
CR CR
CW CW
CR CR
CR CW
CR CW
Generation 1
p (frequency of CR) = 0.7
q (frequency of CW ) = 0.3
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
24
The Founder Effect
創造者效應
• The founder effect occurs when a few individuals
become isolated from a larger population
Ò 少數個體自種群中分離
• Allele frequencies in the small founder
population can be different from those in the
larger parent population
Ò 被分離的個體在基因庫中無法充分代表原本母群基因庫中的對偶基因頻度
http://www.geonet.org.nz/images/volcano/our-volcanoes/White-Island-28766-20-lge.jpg
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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 Ò 發生於突然被削弱的種群大小
Ò 多肇因於環境的突來巨變
• 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
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25
Fig. 23-9
Original
population
Bottlenecking
event
Surviving
population
• Understanding the bottleneck effect can increase
understanding of how human activity affects
other species
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26
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
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Fig. 23-10
Pre-bottleneck Post-bottleneck
(Illinois, 1820) (Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Percentage
of eggs
hatched
Population
size
Number
of alleles
per locus
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
Location
Illinois
1930–1960s
1993
(b)
27
Fig. 23-10a
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Fig. 23-10b
Location
Population
size
Number
Percentage
of alleles of eggs
per locus hatched
Illinois
1930–1960s
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
1993
(b)
28
• 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
Ò 遺傳漂變在小規模種群中分外明顯
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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29
Gene Flow
基因流動
• Gene flow consists of the movement of alleles
among populations Ò 種群中對偶基因的移動
• Alleles can be transferred through the movement
of fertile individuals or gametes (for example,
pollen) Ò 可能的來源有 1. 具生殖能力的個體 2. 配子
• 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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Fig. 23-11
30
• 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
Fig. 23-12
Index of copper tolerance
70
MINE
SOIL
NON-
60 MINE
NONMINE
SOIL
SOIL
50
Prevailing wind direction
40
30
20
10
0
20
0
20
0
20
40
60
80
10
Distance from mine edge (meters) 0
12
0
14
0
16
0
31
• 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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Concept 23.4: Natural selection is the only
mechanism that consistently causes adaptive
evolution
• Only natural selection consistently results in
adaptive evolution
天擇是適應演化的機制
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32
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
相對適應度
• The phrases “struggle for existence”
existence and
“survival of the fittest”
fittest are misleading as they
imply direct competition among individuals
Ò 為生存而競爭? / 最適者生存?
• Reproductive success is generally more subtle
and depends on many factors
Ò個體間的競爭目的為何?
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33
• Relative fitness is the contribution an individual
makes to the gene pool of the next generation,
relative to the contributions of other individuals
Ò 個體貢獻給下一世代基因庫的相對改變程度
• 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 Ò 方向性天擇
– Disruptive selection favors individuals at
both extremes of the phenotypic range Ò 分歧性天擇
– Stabilizing selection favors intermediate
variants and acts against extreme phenotypes
Ò 穩定性天擇
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34
Frequency of individuals
Fig. 23-13
Original
population
Evolved
population
(a) Directional selection
Original population
Phenotypes (fur color)
(b) Disruptive selection
(c) Stabilizing selection
Frequency of individuals
Fig. 23-13a
Original population
Ò 最適的種群位置
Ò 個體表現的頻度
Phenotypes (fur color)
Ò 表現型—皮毛的顏色
Original population
Evolved population
(a) Directional selection
35
Frequency of individuals
Fig. 23-13b
Original population
Phenotypes (fur color)
Evolved population
(b) Disruptive selection
Frequency of individuals
Fig. 23-13c
Original population
Phenotypes (fur color)
Evolved population
(c) Stabilizing selection
36
The Key Role of Natural Selection in Adaptive
Evolution
• Natural selection increases the frequencies of
alleles that enhance survival and reproduction
Ò 天擇增強了基因頻度的表現，以致提高物種了活存與繁殖率
• Adaptive evolution occurs as the match between
an organism and its environment increases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-14
(a) Color-changing ability in cuttlefish
Movable bones
(b) Movable jaw
bones in
snakes
37
• Because the environment can change, adaptive
evolution is a continuous process
Ò 因為環境不斷演化，而適應演化是一個持續發展的過程
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sexual Selection
性擇
• Sexual selection is natural selection for mating
success Ò 能否決定交配成功的天擇過程
• It can result in sexual dimorphism,
dimorphism marked
differences between the sexes in secondary
sexual characteristics
Ò 性別二型性 ---- 因個體進入生殖成熟階段顯出第二性徵而呈現的狀態
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
38
Fig. 23-15
• Intrasexual selection is competition among
individuals of one sex (often males) for mates of
the opposite sex Ò 同性選汰 ---- 同性個體直接競爭
http://www.arkive.org/media/88/885B5A19-F0A0-4B8B-AA4C-EAACC94373F5/Presentation.Large/photo.jpg
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39
• Intersexual selection, often called mate choice,
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
Ò 異性選汰 ---- 配偶選擇
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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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
40
Fig. 23-16
EXPERIMENT
Female gray
tree frog
SC male gray
tree frog
LC male gray
tree frog
SC sperm
×
Eggs
Offspring of
SC father
×
LC sperm
Offspring of
LC father
Fitness of these half-sibling offspring compared
RESULTS
Fitness Measure
1995
NSD
Larval
growth
Larval
survival
Time to
metamorphosis
LC
better
LC
better
(shorter)
1996
LC
better
NSD
LC
better
(shorter)
NSD = no significant difference; LC better = offspring of LC males
superior to offspring of SC males.
Fig. 23-16a
EXPERIMENT
Female gray
tree frog
LC male gray
tree frog
SC male gray
tree frog
SC sperm ×
Eggs ×
Offspring of
SC father
LC sperm
Offspring of
LC father
Fitness of these half-sibling offspring compared
41
Fig. 23-16b
RESULTS
Fitness Measure
Larval
growth
Larval
survival
Time to
metamorphosis
1995
1996
NSD
LC better
LC better
NSD
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
• Various mechanisms help to preserve genetic
variation in a population
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42
Diploidy
二倍體
• Diploidy maintains genetic variation in the form
of hidden recessive alleles
Bb
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Balancing Selection
• Balancing selection occurs when natural
selection maintains stable frequencies of two or
more phenotypic forms in a population
1. Heterozygote Advantage
2. Frequency-Dependent Selection
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43
Heterozygote Advantage
異型合子優勢
• Heterozygote advantage occurs when
heterozygotes have a higher fitness than do both
homozygotes Ò 異型合子的保護效果
• 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
Ò 顯性同組合型 ---- 對瘧疾具有感受性
Ò 隱性同組合型 ---- 形成鐮形細胞 (鐮形細胞貧血症)
Ò 顯性異組合型 ---- 對瘧疾具有抵抗性
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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%
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Frequency-Dependent Selection
頻率結合性天擇
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 23-18
“Right-mouthed”
Frequency of
“left-mouthed” individuals
1.0
“Left-mouthed”
0.
5
0
1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Sample year
45
Fig. 23-18a
“Right-mouthed”
“Leftmouthed”
Fig. 23-18b
Frequency of
“left-mouthed” individuals
1.0
0.5
0
1981 ’8
2
’8
3
’8 ’8 ’8 ’8
4 Sample
5 6year7
’8
8
’8
9
’9
0
46
Neutral Variation
中性變異
• Neutral variation is genetic variation that
appears to confer no selective advantage or
disadvantage Ò 中性遺傳變異指的是部分遺傳變異對其生殖成功
• For example,
的影響並不重要
– Variation in noncoding regions of DNA
– Variation in proteins that have little effect on
protein function or reproductive fitness
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
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47
Fig. 23-19
Fig. 23-UN1
Original
population
Evolved
population
Directional
selection
Disruptive
selection
Stabilizing
selection
48
Fig. 23-UN2
Sampling sites
(1–8 represent
pairs of sites)
1
2
3
4
5
6
7
9
8
10
11
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
Sound
N
W
8
6 7
4 5
9
10
E
S
2 3
11
Atlantic
Ocean
Fig. 23-UN3
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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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50
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
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51