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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) © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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: © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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% © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. • 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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 © 2011 Pearson Education, Inc. 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