Chapter 23: The Evolution of Populations. Chapter 23 Assignment 1. Define the terms population, species, gene pool, relative fitness, and neutral variation.

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Chapter 23: The Evolution of Populations

Chapter 23: The Evolution of PopulationsSChapter 23 AssignmentDefine the terms population, species, gene pool, relative fitness, and neutral variationList the five conditions of Hardy-Weinberg equilibrium.Hardy Weinberg Equilibrium Problems (to be distributed later)Overview: The Smallest Unit of EvolutionOne misconception is that organisms evolve, in the Darwinian sense, during their lifetimesNatural selection acts on individuals, but only populations evolveGenetic variations in populations contribute to evolutionMicroevolution is a change in allele frequencies in a population over generations3Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possibleSTwo processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals5Genetic Variation Variation in individual genotype leads to variation in individual phenotypeNot all phenotypic variation is heritableNatural selection can only act on variation with a genetic component6

Fig. 23-2(a)(b)Nonheritable variation: these caterpillars display phenotypes based on their diets, not their genes.7Figure 23.2 Nonheritable variationVariation Within a PopulationBoth discrete and quantitative characters contribute to variation within a populationDiscrete characters can be classified on an either-or basisQuantitative characters vary along a continuum within a population8Average Heterozygosity Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levelsAverage heterozygosity measures the average percent of loci that are heterozygous in a population9Variation Between PopulationsGeographic variation: differences between gene pools of separate populations or population subgroups10

Fig. 23-313.1719XX10.169.128.1112.43.145.1867.159.1012.1911.1213.1715.183.84.165.146.7XX11Figure 23.3 Geographic variation in isolated mouse populations on Madeira

Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axisExamples: frequency of an allele present in the population`12How do new alleles arise?Mutations:Gene (point mutations)ChromosomalSexual reproductionMutationMutations are changes in the nucleotide sequence of DNAMutations cause new genes and alleles to ariseOnly mutations in cells that produce gametes can be passed to offspring14Point MutationsA point mutation is a change in one base in a genePoint mutations can vary in severity:Noncoding regions point mutation has no effect on gene expressionCan be more severe sickle-cell disease

Rarely do mutations increase the organisms fitness most mutations have a negative effect on the organism

15Mutations That Alter Gene Number or SequenceChromosomal mutations that delete, disrupt, or rearrange many loci are typically harmfulDuplication of large chromosome segments is usually harmfulDuplication of small pieces of DNA is sometimes less harmful and increases the genome sizeDuplicated genes can take on new functions by further mutation16Mutation RatesMutation rates are low in animals and plantsWhy?The average is about one mutation in every 100,000 genes per generationMutations rates are often lower in prokaryotes and higher in virusesWhy are mutations higher in viruses?17Sexual Reproduction3 mechanisms:Crossing overIndependent assortmentFertilization Shuffle existing alleles into new combinationsIn organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible18Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolvingSGene Pools and Allele FrequenciesA population is a localized group of individuals capable of interbreeding and producing fertile offspringA gene pool consists of all the alleles for all loci in a population20

Fig. 23-5Porcupine herdPorcupineherd rangeBeaufort SeaNORTHWESTTERRITORIESMAPAREAALASKACANADAFortymileherd rangeFortymile herdALASKAYUKON21Figure 23.5 One species, two populationsHardy-Weinberg PrincipleDetermine if a population is evolving or notCalculate allele frequencies of hypothetical non-evolving populations and compare to actual frequencies of sample populationAllele frequencies should remain constant from one generation to the next if there is no evolutionThe frequency of an allele in a population can be calculatedFor diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles23By convention, if there are 2 alleles at a locus, p and q are used to represent their frequenciesThe frequency of all alleles in a population will add up to 1For example, p + q = 1

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Fig. 23-6Frequencies of allelesAlleles in the populationGametes producedEach egg:Each sperm:80%chance80%chance20%chance20%chanceq = frequency ofp = frequency ofCR allele = 0.8CW allele = 0.225Figure 23.6 Selecting alleles at random from a gene pool

Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene poolIf p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, thenp2 + 2pq + q2 = 1where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype26Hardy-Weinberg Equilibrium Equationp2 + 2pq + q2 = 1Where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Fig. 23-7-1SpermCR (80%)CW (20%)80% CR ( p = 0.8) CW (20%)20% CW (q = 0.2) 16% ( pq) CRCW 4% (q2) CW CWCR (80%) 64% ( p2) CRCR 16% (qp) CRCWEggs28Figure 23.7 The Hardy-Weinberg principle

Fig. 23-7-2Gametes of this generation:64% CRCR, 32% CRCW, and 4% CWCW64% CR + 16% CR= 80% CR = 0.8 = p4% CW + 16% CW= 20% CW = 0.2 = q29Figure 23.7 The Hardy-Weinberg principle

Fig. 23-7-3Gametes of this generation:64% CRCR, 32% CRCW, and 4% CWCW64% CR + 16% CR= 80% CR = 0.8 = p4% CW + 16% CW= 20% CW = 0.2 = q64% CRCR, 32% CRCW, and 4% CWCW plantsGenotypes in the next generation:30Figure 23.7 The Hardy-Weinberg principleConditions for Hardy-Weinberg EquilibriumThe Hardy-Weinberg theorem describes a hypothetical populationIn real populations, allele and genotype frequencies do change over time

31The five conditions for nonevolving populations are rarely met in nature:

No mutations Random mating No natural selection Extremely large population sizeNo gene flow32Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

33Hardy-Weinberg Practice Problems and Homework SPractice Problem 1You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:The frequency of the "aa" genotype.The frequency of the "a" allele.The frequency of the "A" allele.The frequencies of the genotypes "AA" and "Aa.The frequencies of the two possible phenotypes if "A" is completely dominant over "a."Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a populationSThree major factors alter allele frequencies and bring about most evolutionary change:Natural selectionGenetic driftGene flow37Natural SelectionDifferential success in reproduction results in certain alleles being passed to the next generation in greater proportions38Genetic DriftThe smaller a sample, the greater the chance of deviation from a predicted resultGenetic drift describes how allele frequencies fluctuate unpredictably from one generation to the nextGenetic drift tends to reduce genetic variation through losses of alleles2 examples: Founder Effect, Bottleneck Effect39

Fig. 23-8-1Generation 1p (frequency of CR) = 0.7q (frequency of CW ) = 0.3CW CW CR CRCR CWCR CRCR CRCR CRCR CRCR CWCR CWCR CW40Figure 23.8 Genetic drift

Fig. 23-8-2Generation 1p (frequency of CR) = 0.7q (frequency of CW ) = 0.3Generation 2p = 0.5q = 0.5CW CW CR CRCR CWCR CRCR CRCR CRCR CRCR CWCR CWCR CWCR CWCR CWCR CWCR CWCW CW CW CW CW CW CR CRCR CRCR CR41Figure 23.8 Genetic drift

Fig. 23-8-3Generation 1CW CW CR CRCR CWCR CRCR CRCR CRCR CRCR CWCR CWCR CWp (frequency of CR) = 0.7q (frequency of CW ) = 0.3Generation 2CR CWCR CWCR CWCR CWCW CW CW CW CW CW CR CRCR CRCR CRp = 0.5q = 0.5Generation 3p = 1.0q = 0.0CR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CR42Figure 23.8 Genetic driftThe Founder EffectThe founder effect occurs when a few individuals become isolated from a larger populationAllele frequencies in the small founder population can be different from those in the larger parent population43The Bottleneck EffectThe 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 populations gene poolIf the population remains small, it may be further affected by genetic drift44

Fig. 23-9OriginalpopulationBottleneckingeventSurvivingpopulation45Figure 23.9 The bottleneck effectEffects of Genetic Drift: A SummaryGenetic drift is significant in small populationsGenetic drift causes allele frequencies to change at randomGenetic drift can lead to a loss of genetic variation within populationsGenetic drift can cause harmful alleles to become fixed46Gene FlowGene flow consists of the movement of alleles among populationsAlleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)Gene flow tends to reduce differences between populations over timeGene flow is more likely than mutation to alter allele frequencies directly47Gene flow can decrease the fitness of a populationIn bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soilsWindblown pollen moves these alleles between populationsThe movement of unfavorable alleles into a population results in a decrease in fit between organism and environment48

Fig. 23-12NON-MINESOILMINESOILNON-MINESOILPrevailing wind directionIndex of copper toleranceDistance from mine edge (meters)7060504030201002002002040608010012014016049Figure 23.12 Gene flow and selectionGene flow can increase the fitness of a populationInsecticides have been used to target mosquitoes that carry West Nile virus and malariaAlleles have evolved in some populations that confer insecticide resistance to these mosquitoesThe flow of insecticide resistance alleles into a population can cause an increase in fitness50Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolutionSOnly natural selection consistently results in adaptive evolution52A Closer Look at Natural SelectionNatural selection brings about adaptive evolution by acting on an organisms phenotype 53Relative FitnessThe phrases struggle for existence and survival of the fittest are misleading as they imply direct competition among individualsReproductive success is generally more subtle and depends on many factors54Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individualsSelection favors certain genotypes by acting on the phenotypes of certain organisms55Directional, Disruptive, and Stabilizing SelectionThree modes of selection:Directional selection favors individuals at one end of the phenotypic rangeDisruptive selection favors individuals at both extremes of the phenotypic rangeStabilizing selection favors intermediate variants and acts against extreme phenotypes56

Fig. 23-13aOriginal population(a) Directional selectionPhenotypes (fur color)Frequency of individualsOriginal populationEvolved population57Figure 23.13 Modes of selection

Fig. 23-13bOriginal population(b) Disruptive selectionPhenotypes (fur color)Frequency of individualsEvolved population58Figure 23.13 Modes of selection

Fig. 23-13cOriginal population(c) Stabilizing selectionPhenotypes (fur color)Frequency of individualsEvolved population59Figure 23.13 Modes of selectionThe Key Role of Natural Selection in Adaptive EvolutionNatural selection increases the frequencies of alleles that enhance survival and reproductionAdaptive evolution occurs as the match between an organism and its environment increases60

Fig. 23-14(a) Color-changing ability in cuttlefish(b) Movable jaw bones in snakesMovable bones61Figure 23.14 Examples of adaptationsBecause the environment can change, adaptive evolution is a continuous processGenetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment62Sexual SelectionSexual selection is natural selection for mating successIt can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics63

Fig. 23-1564Figure 23.15 Sexual dimorphism and sexual selectionIntrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sexIntersexual 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 males chances of attracting a female, while decreasing his chances of survival65How 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 for66

Fig. 23-16SC male graytree frogFemale graytree frogLC male graytree frogEXPERIMENTSC sperm Eggs LC spermOffspring ofLC fatherOffspring ofSC fatherFitness of these half-sibling offspring comparedRESULTS1995Fitness Measure1996Larval growthLarval survivalTime to metamorphosisLC betterNSDLC better(shorter)LC better(shorter)NSDLC betterNSD = no significant difference; LC better = offspring of LC malessuperior to offspring of SC males.67Figure 23.16 Do females select mates based on traits indicative of good genes?

Fig. 23-16aSC male graytree frogFemale graytree frogLC male graytree frogSC sperm Eggs LC spermOffspring ofLC fatherOffspring ofSC fatherFitness of these half-sibling offspring comparedEXPERIMENT68Figure 23.16 Do females select mates based on traits indicative of good genes?

Fig. 23-16bRESULTS1995Fitness Measure1996Larval growthLarval survivalTime to metamorphosisLC betterNSDLC better(shorter)LC better(shorter)NSDLC betterNSD = no significant difference; LC better = offspring of LC malessuperior to offspring of SC males.69Figure 23.16 Do females select mates based on traits indicative of good genes?

The Preservation of Genetic VariationVarious mechanisms help to preserve genetic variation in a population70DiploidyDiploidy maintains genetic variation in the form of hidden recessive alleles71Balancing SelectionBalancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population72Heterozygote AdvantageHeterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotesNatural selection will tend to maintain two or more alleles at that locusThe sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance73

Fig. 23-1702.5%Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)Frequencies of thesickle-cell allele2.55.0%7.510.0%5.07.5%>12.5%10.012.5%74Figure 23.17 Mapping malaria and the sickle-cell allele

Frequency-Dependent SelectionIn frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the populationSelection can favor whichever phenotype is less common in a population75

Fig. 23-18Right-mouthed1981Left-mouthedFrequency ofleft-mouthed individualsSample year1.00.5082838485868788899076Figure 23.18 Frequency-dependent selection in scale-eating fish (Perissodus microlepis)

Neutral VariationNeutral variation is genetic variation that appears to confer no selective advantage or disadvantageFor example, Variation in noncoding regions of DNAVariation in proteins that have little effect on protein function or reproductive fitness77Why Natural Selection Cannot Fashion Perfect OrganismsSelection can act only on existing variationsEvolution is limited by historical constraintsAdaptations are often compromisesChance, natural selection, and the environment interact

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