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Figure 23.1 CAMPBELL BIOLOGY Figure 23.1a The Smallest Unit of Evolution TENTH EDITION Reece • Urry • Cain • Wasserman • Minorsky • Jackson ! A common misconception is that organisms evolve during their lifetimes 23 ! Natural selection acts on individuals, but only populations evolve The Evolution of Populations ! 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 Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick 2 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 3 4 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Concept 23.1: Genetic variation makes evolution possible Genetic Variation Average beak depth (mm) Figure 23.2 10 9 ! Microevolution is a change in allele frequencies in a population over generations ! Variation in heritable traits is a prerequisite for evolution ! Genetic variation among individuals is caused by differences in genes or other DNA segments ! Three mechanisms cause allele frequency change ! Mendel’s work on pea plants provided evidence of discrete heritable units (genes) ! Phenotype is the product of inherited genotype and environmental influences ! Natural selection 8 ! Natural selection can only act on variation with a genetic component ! Genetic drift ! Gene flow 0 1976 1978 (similar to the (after prior 3 years) drought) ! Only natural selection causes adaptive evolution 5 © 2014 Pearson Education, Inc. 6 © 2014 Pearson Education, Inc. 7 © 2014 Pearson Education, Inc. 8 © 2014 Pearson Education, Inc. Figure 23.3 Animation: Genetic Variation from Sexual Recombination ! Some phenotypic differences are determined by a single gene and can be classified on an either-or basis ! 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 ! Other phenotypic differences are determined by the influence of two or more genes and vary along a continuum within a population ! Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals ! Nucleotide variation rarely results in phenotypic variation 9 © 2014 Pearson Education, Inc. 10 © 2014 Pearson Education, Inc. Figure 23.4 Insertion sites 1 ! Some phenotypic variation does not result from genetic differences among individuals, but rather from environmental influences (a) (b) ! Only genetically determined variation can have evolutionary consequences Substitution resulting in translation of different amino acid 1,500 Figure 23.5a 1,000 500 Intron Exon (a) 12 © 2014 Pearson Education, Inc. Figure 23.5 Base-pair substitutions Deletion 2,000 2,500 13 © 2014 Pearson Education, Inc. 11 © 2014 Pearson Education, Inc. 14 © 2014 Pearson Education, Inc. 15 © 2014 Pearson Education, Inc. 16 © 2014 Pearson Education, Inc. Figure 23.5b Sources of Genetic Variation (b) Formation of New Alleles ! New genes and alleles can arise by mutation or gene duplication ! A mutation is a random change in nucleotide sequence of DNA ! The effects of point mutations can vary ! Sexual reproduction can result in genetic variation by recombining existing alleles ! Only mutations in cells that produce gametes can be passed to offspring ! A point mutation is a change in one base in a gene ! Mutations that result in a change in protein production are often harmful ! Harmful mutations can be hidden from selection in recessive alleles ! Mutations that result in a change in protein production can sometimes be beneficial 17 © 2014 Pearson Education, Inc. ! The effects of point mutations can vary 18 © 2014 Pearson Education, Inc. Altering Gene Number or Position Rapid Reproduction Sexual Reproduction ! Mutation rates are low in animals and plants ! Mutations accumulate quickly in prokaryotes and viruses because they have short generation times ! An ancestral odor-detecting gene has been duplicated many times: humans have 350 copies of the gene, mice have 1,000 21 22 © 2014 Pearson Education, Inc. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving Gene Pools and Allele Frequencies ! In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible ! Mutation rates are often lower in prokaryotes and higher in viruses ! Duplicated genes can take on new functions by further mutation © 2014 Pearson Education, Inc. ! Sexual reproduction can shuffle existing alleles into new combinations ! The average is about one mutation in every 100,000 genes per generation ! Duplication of small pieces of DNA increases genome size and is usually less harmful ! Mutations to genes can be neutral because of redundancy in the genetic code 23 © 2014 Pearson Education, Inc. ! A locus is fixed if all individuals in a population are homozygous for the same allele Porcupine herd 25 Fortymile herd range CANADA Fortymile herd 26 © 2014 Pearson Education, Inc. MAP AREA Porcupine herd range ! A gene pool consists of all the alleles for all loci in a population © 2014 Pearson Education, Inc. Figure 23.6a ALASKA ! A population is a localized group of individuals capable of interbreeding and producing fertile offspring Porcupine herd 27 © 2014 Pearson Education, Inc. Figure 23.6b 28 © 2014 Pearson Education, Inc. Figure 23.UN01 ! If there are two or more alleles for a locus, diploid individuals may be either homozygous or heterozygous ! The frequency of an allele in a population can be calculated CR CR ! For diploid organisms, the total number of alleles at a locus is the total number of individuals times 2 CW CW ! The total number of dominant alleles at a locus is two alleles for each homozygous dominant individual plus one allele for each heterozygous individual; the same logic applies for recessive alleles CR CW Fortymile herd 29 © 2014 Pearson Education, Inc. 24 © 2014 Pearson Education, Inc. Figure 23.6 ! The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population 20 © 2014 Pearson Education, Inc. ! Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful ! Point mutations in noncoding regions generally result in neutral variation, conferring no selective advantage or disadvantage 19 © 2014 Pearson Education, Inc. 30 © 2014 Pearson Education, Inc. 31 © 2014 Pearson Education, Inc. 32 © 2014 Pearson Education, Inc. The Hardy-Weinberg Equation ! By convention, if there are two 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 ! For example, consider a population of wildflowers that is incompletely dominant for color ! 320 red flowers (CRCR) ! The Hardy-Weinberg equation describes the genetic makeup we expect for a population that is not evolving at a particular locus ! p = freq CR = 800 / (800 + 200) = 0.8 ! q = freq CW = 200 / (800 + 200) = 0.2 ! 160 pink flowers (CRCW) ! 20 white flowers ! To calculate the frequency of each allele ! If the observed genetic makeup of the population differs from expectations under Hardy-Weinberg, it suggests that the population may be evolving ! The sum of alleles is always 1 (CWCW) ! 0.8 + 0.2 = 1 ! Calculate the number of copies of each allele ! CR = (320 × 2) + 160 = 800 ! CW = (20 × 2) + 160 = 200 33 © 2014 Pearson Education, Inc. 34 © 2014 Pearson Education, Inc. Figure 23.7 35 © 2014 Pearson Education, Inc. 36 © 2014 Pearson Education, Inc. Frequencies of alleles Hardy-Weinberg Equilibrium ! In a population where gametes contribute to the next generation randomly and Mendelian inheritance occurs, allele and genotype frequencies remain constant from generation to generation p = frequency of CR allele = 0.8 q = frequency of CW allele = 0.2 ! Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool Alleles in the population ! The frequency of genotypes can be calculated ! CRCR = p2 = (0.8)2 = 0.64 ! Consider, for example, the same population of 500 wildflowers and 1,000 alleles where ! CWCW = q2 = (0.2)2 = 0.04 ! p = freq CR = 0.8 ! Such a population is in Hardy-Weinberg equilibrium ! CRCW = 2pq = 2(0.8)(0.2) = 0.32 ! The frequency of genotypes can be confirmed using a Punnett square ! q = freq CW = 0.2 Gametes produced Each sperm: Each egg: 80% 20% chance chance 37 © 2014 Pearson Education, Inc. Figure 23.8 80% 20% chance chance 38 © 2014 Pearson Education, Inc. 80% CR (p = 0.8) Figure 23.8a 20% CW (q = 0.2) 39 © 2014 Pearson Education, Inc. Figure 23.8b 20% CW (q = 0.2) 80% CR (p = 0.8) 64% CRCR, 32% CRCW, and 4% CWCW Sperm CR p = 0.8 CW q = 0.2 CR Sperm CR p = 0.8 CW q = 0.2 p = 0.8 0.64 (p2) C RC R Eggs CW 0.16 (pq) C RC W 0.16 (qp) C RC W q = 0.2 0.04 (q2) CWCW Eggs R + 16% C R W = 80% CR = 0.8 = p (from C C plants) W 4% CW + 16% C = 20% CW = 0.2 = q (from CWCW plants) (from CRCW plants) W 4% + 16% C = 20% CW = 0.2 = q (from CWCW plants) (from CRCW plants) 0.64 (p2) C RC R q = 0.2 With random mating, these gametes will result in the same mix of genotypes in the next generation: 0.16 (pq) C RC W 0.16 (qp) C RC W 41 © 2014 Pearson Education, Inc. R + 16% C = 80% CR = 0.8 = p (from CRCW plants) CW CW With random mating, these gametes will result in the same mix of genotypes in the next generation: 64% CRCR, 32% CRCW, and 4% CWCW plants 64% CR (from CRCR plants) p = 0.8 Gametes of this generation: 64% CR (from CRCR plants) Gametes of this generation: CR 64% CRCR, 32% CRCW, and 4% CWCW 40 © 2014 Pearson Education, Inc. 0.04 (q2) CWCW ! 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 64% CRCR, 32% CRCW, and 4% CWCW plants 42 © 2014 Pearson Education, Inc. 43 © 2014 Pearson Education, Inc. 44 © 2014 Pearson Education, Inc. Conditions for Hardy-Weinberg Equilibrium Applying the Hardy-Weinberg Equation ! The Hardy-Weinberg theorem describes a hypothetical population that is not evolving ! The five conditions for nonevolving populations are rarely met in nature ! Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci 1. No mutations ! In real populations, allele and genotype frequencies do change over time ! We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that 1. The PKU gene mutation rate is low 2. Random mating 2. Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele 3. No natural selection 4. Extremely large population size 5. No gene flow 45 © 2014 Pearson Education, Inc. 46 © 2014 Pearson Education, Inc. 47 © 2014 Pearson Education, Inc. 48 © 2014 Pearson Education, Inc. Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population ! The occurrence of PKU is 1 per 10,000 births 3. Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions 5. Migration has no effect as many other populations have similar allele frequencies ! Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions ! Three major factors alter allele frequencies and bring about most evolutionary change ! q2 = 0.0001 ! q = 0.01 ! For example, an allele that confers resistance to DDT in fruit flies increased in frequency after DDT was used widely in agriculture ! Natural selection ! The frequency of normal alleles is 4. The population is large Natural Selection ! Genetic drift ! p = 1 - q = 1 - 0.01 = 0.99 ! Gene flow ! The frequency of carriers is ! 2pq = 2 × 0.99 × 0.01 = 0.0198 49 © 2014 Pearson Education, Inc. ! or approximately 2% of the U.S. population 50 © 2014 Pearson Education, Inc. 51 © 2014 Pearson Education, Inc. 52 © 2014 Pearson Education, Inc. Figure 23.9–1 Figure 23.9–2 Genetic Drift ! Natural selection can cause adaptive evolution, an improvement in the match between organisms and their environment ! The smaller a sample, the greater the chance of random deviation from a predicted result C RC R ! Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next C RC W C RC R 53 © 2014 Pearson Education, Inc. C RC R C RC R C RC R C RC R CWCW C RC W CWCW CWCW C RC W C RC R C RC W C RC R C RC W C RC W ! Genetic drift tends to reduce genetic variation through losses of alleles C RC W CWCW C RC W C RC R C RC R CWCW 5 plants leave offspring C RC R C RC W C RC R C RC R C RC W C RC W Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 C RC W Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 54 Generation 2 p = 0.5 q = 0.5 55 56 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. BioFlix: Mechanisms of Evolution Animation: Causes of Evolutionary Change The Founder Effect Figure 23.9–3 5 plants leave offspring C RC R CWCW C RC W C RC R C RC R C RC R C RC R CWCW 2 plants leave offspring C RC W C RC R C RC W C RC R C RC R C RC R C RC R C RC W C RC R C RC R C RC W Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 ! Allele frequencies in the small founder population can be different from those in the larger parent population C RC R CWCW CWCW C RC W C RC R C RC W C RC R ! The founder effect occurs when a few individuals become isolated from a larger population C RC R C RC W Generation 2 p = 0.5 q = 0.5 C RC R C RC R Generation 3 p = 1.0 q = 0.0 57 © 2014 Pearson Education, Inc. 58 © 2014 Pearson Education, Inc. 59 © 2014 Pearson Education, Inc. Figure 23.10–1 60 © 2014 Pearson Education, Inc. Figure 23.10–2 Figure 23.10–3 The Bottleneck Effect ! The bottleneck effect is a sudden reduction in population size due to a change in the environment ! The resulting gene pool may no longer be reflective of the original population’s gene pool ! If the population remains small, it may be further affected by genetic drift Original population 61 © 2014 Pearson Education, Inc. Original population 62 © 2014 Pearson Education, Inc. Bottlenecking event Original population 63 © 2014 Pearson Education, Inc. Bottlenecking event Surviving population 64 © 2014 Pearson Education, Inc. Figure 23.11 Pre-bottleneck (Illinois, 1820) Case Study: Impact of Genetic Drift on the Greater Prairie Chicken ! Understanding the bottleneck effect can increase understanding of how human activity affects other species ! Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois Greater prairie chicken (a) ! The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Number of alleles per locus Percentage of eggs 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 Illinois 1930–1960s 1993 65 © 2014 Pearson Education, Inc. 66 © 2014 Pearson Education, Inc. Figure 23.11b Pre-bottleneck Post-bottleneck (Illinois, 1820) (Illinois, 1993) Range of greater prairie chicken Population size Location Figure 23.11a Post-bottleneck (Illinois, 1993) Greater prairie chicken (a) Range of greater prairie chicken 67 (b) © 2014 Pearson Education, Inc. 68 © 2014 Pearson Education, Inc. Figure 23.11c Effects of Genetic Drift: A Summary Population size Number of alleles per locus Percentage of eggs 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 Location Illinois 1930–1960s 1993 ! 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 populations in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90% Greater prairie chicken 1. Genetic drift is significant in small populations 2. Genetic drift can cause 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 (b) 69 © 2014 Pearson Education, Inc. 70 © 2014 Pearson Education, Inc. 71 © 2014 Pearson Education, Inc. 72 © 2014 Pearson Education, Inc. Figure 23.12 Figure 23.12a Gene Flow Central population NORTH SEA N ! Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland ! Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Parus major Survival rate (%) ! Immigration from the mainland introduces alleles that decrease fitness on the island ! Natural selection removes alleles that decrease fitness 73 © 2014 Pearson Education, Inc. ! Birds born in the central region with high immigration have a lower fitness; birds born in the east with low immigration have a higher fitness 20 10 0 10 74 50 Population in which the surviving females eventually bred 40 Central Eastern 30 20 0 © 2014 Pearson Education, Inc. 2 km Population in which the surviving females eventually bred 40 Central Eastern 30 50 ! Mating causes gene flow between the central and eastern populations ! Gene flow tends to reduce variation among populations over time Eastern population Vlieland, the Netherlands Survival rate (%) ! Gene flow can decrease the fitness of a population ! Gene flow consists of the movement of alleles among populations Females born in Females born in central population eastern population Females born in Females born in central population eastern population 75 © 2014 Pearson Education, Inc. 76 © 2014 Pearson Education, Inc. Figure 23.12b ! Gene flow can increase the fitness of a population ! Consider, for example, the spread of alleles for resistance to insecticides Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution ! Gene flow is an important agent of evolutionary change in modern human populations ! Evolution by natural selection involves both chance and “sorting” ! New genetic variations arise by chance ! Insecticides have been used to target mosquitoes that carry West Nile virus and malaria ! Beneficial alleles are “sorted” and favored by natural selection ! Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes Parus major ! Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage ! The flow of insecticide resistance alleles into a population can cause an increase in fitness 77 © 2014 Pearson Education, Inc. 78 © 2014 Pearson Education, Inc. 79 © 2014 Pearson Education, Inc. 80 © 2014 Pearson Education, Inc. Natural Selection: A Closer Look Relative Fitness ! Natural selection brings about adaptive evolution by acting on an organism’s phenotype Directional, Disruptive, and Stabilizing Selection ! The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals ! Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals ! Reproductive success is generally more subtle and depends on many factors ! Selection favors certain genotypes by acting on the phenotypes of individuals ! There are three modes of selection ! Directional selection favors individuals at one extreme 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 81 © 2014 Pearson Education, Inc. 82 © 2014 Pearson Education, Inc. 83 © 2014 Pearson Education, Inc. Figure 23.14 Frequency of individuals Figure 23.13 Original Evolved population population The Key Role of Natural Selection in Adaptive Evolution Original population 84 © 2014 Pearson Education, Inc. Figure 23.14a Bones shown in green are movable. ! Striking adaptations have arisen by natural selection ! For example, certain octopuses can change color rapidly for camouflage Phenotypes (fur color) Ligament ! For example, the jaws of snakes allow them to swallow prey larger than their heads (a) Directional selection (b) Disruptive selection (c) Stabilizing selection 85 © 2014 Pearson Education, Inc. 86 © 2014 Pearson Education, Inc. 87 © 2014 Pearson Education, Inc. 88 © 2014 Pearson Education, Inc. Figure 23.15 Sexual Selection ! Natural selection increases the frequencies of alleles that enhance survival and reproduction ! Adaptive evolution occurs as the match between a species and its environment increases ! 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 ! Because the environment can change, adaptive evolution is a continuous process 89 © 2014 Pearson Education, Inc. ! Sexual selection is natural selection for mating success ! It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics 90 © 2014 Pearson Education, Inc. 91 © 2014 Pearson Education, Inc. Figure 23.16 ! Intrasexual selection is direct 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 Experiment Recording of LC male’s call Recording of SC male’s call Offspring of SC father Female gray tree frog LC male gray SC male tree frog gray tree SC sperm × Eggs × LC sperm frog Offspring of LC father Survival and growth of these half-sibling offspring compared Results Offspring Performance 1996 1995 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. 94 © 2014 Pearson Education, Inc. Recording of LC male’s call Female gray tree frog SC male LC male gray gray tree tree frog SC sperm × Eggs × LC sperm frog ! The “good genes” hypothesis suggests that if a trait is related to male genetic quality or health, both the male trait and female preference for that trait should increase in frequency ! Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival © 2014 Pearson Education, Inc. Figure 23.16a Experiment Recording of SC male’s call ! How do female preferences evolve? 93 92 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Offspring of SC father Offspring of LC father Survival and growth of these half-sibling offspring compared 95 © 2014 Pearson Education, Inc. 96 Figure 23.16b Balancing Selection Results Offspring Performance 1995 1996 Larval survival LC better NSD Larval growth NSD LC better Time to metamorphosis LC better (shorter) LC better (shorter) Heterozygote Advantage ! Diploidy maintains genetic variation in the form of recessive alleles hidden from selection in heterozygotes ! Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes ! A mutation in an allele that codes for part of the hemoglobin protein causes sickle-cell disease, but also confers malaria resistance ! Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population ! Natural selection will tend to maintain two or more alleles at that locus ! In regions where the malaria parasite is common, selection favors individuals heterozygous for the sickle-cell allele ! Heterozygote advantage can result from stabilizing or directional selection ! Balancing selection includes NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males. ! Heterozygote advantage ! Frequency-dependent selection 97 © 2014 Pearson Education, Inc. 98 © 2014 Pearson Education, Inc. Figure 23.17a Figure 23.17b Sickle-cell allele on chromosome Figure 23.17d MAKE CONNECTIONS: The Sickle-Cell Allele Evolution in Populations Events at the Molecular Level 100 © 2014 Pearson Education, Inc. Figure 23.17c MAKE CONNECTIONS: The Sickle-Cell Allele MAKE CONNECTIONS: The Sickle-Cell Allele 99 © 2014 Pearson Education, Inc. MAKE CONNECTIONS: The Sickle-Cell Allele Effects on Individual Organisms Template strand Consequences for Cells Fiber An adenine replaces a Sickle-cell hemoglobin Wild-type thymine. allele Low-oxygen conditions Normal hemoglobin (does not aggregate into fibers) Sickled red blood cell Normal red blood cell Key Frequencies of the sickle-cell allele 3.0–6.0% 6.0–9.0% Distribution of malaria 9.0–12.0% 12.0–15.0% caused by Plasmodium falciparum >15.0% (a parasitic unicellular eukaryote) 101 © 2014 Pearson Education, Inc. Infected mosquitos spread malaria when they bite people. 102 © 2014 Pearson Education, Inc. 103 © 2014 Pearson Education, Inc. Figure 23.18 Figure 23.UN02 Frequency-Dependent Selection Why Natural Selection Cannot Fashion Perfect Organisms “Left-mouthed” P. microlepis ! Selection favors whichever phenotype is less common in a population ! For example, frequency-dependent selection results in approximately equal numbers of “rightmouthed” and “left-mouthed” scale-eating fish 1. Selection can act only on existing variations 1.0 Frequency of “left-mouthed” individuals ! In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population 2. Evolution is limited by historical constraints “Right-mouthed” P. microlepis 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact 0.5 0 105 © 2014 Pearson Education, Inc. 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year 106 © 2014 Pearson Education, Inc. Figure 23.UN03a 104 © 2014 Pearson Education, Inc. 107 © 2014 Pearson Education, Inc. Figure 23.UN03b 108 © 2014 Pearson Education, Inc. Figure 23.UN04 Figure 23.UN05 Sampling sites (1–8 represent pairs of sites) Original population 1 2 3 5 4 7 6 8 9 10 11 Allele frequencies Evolved population 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 Directional selection Disruptive selection Stabilizing selection 1 Long Island 2 3 Sound N 109 © 2014 Pearson Education, Inc. 110 © 2014 Pearson Education, Inc. 111 © 2014 Pearson Education, Inc. 5 6 7 8 9 10 11 © 2014 Pearson Education, Inc. 4 Atlantic Ocean 112 Figure 23.UN06 113 © 2014 Pearson Education, Inc.