Bài giảng Biology - Chapter 23: The Evolution of Populations

Chapter 23The Evolution of PopulationsOverview: The Smallest Unit of EvolutionOne common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimesNatural selection acts on individuals, but populations evolveGenetic variations in populationsContribute to evolutionFigure 23.1Concept 23.1: Population genetics provides a foundation for studying evolutionMicroevolutionIs change in the genetic makeup of a population from generation to generationFigure 23.2The Modern SynthesisPopulation geneticsIs the study of how populations change genetically over timeReconciled Darwin’s and Mendel’s ideasThe modern synthesisIntegrates Mendelian genetics with the Darwinian theory of evolution by natural selectionFocuses on populations as units of evolutionGene Pools and Allele FrequenciesA populationIs a localized group of individuals that are capable of interbreeding and producing fertile offspringMAPAREAALASKACANADABeaufort SeaPorcupineherd range Fairbanks WhitehorseFortymileherd rangeNORTHWESTTERRITORIESALASKAYUKONFigure 23.3The gene poolIs the total aggregate of genes in a population at any one timeConsists of all gene loci in all individuals of the populationThe Hardy-Weinberg TheoremThe Hardy-Weinberg theoremDescribes a population that is not evolvingStates that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at workMendelian inheritancePreserves genetic variation in a populationFigure 23.4Generation1CRCRgenotypeCWCWgenotypePlants mateAll CRCW(all pink flowers)50% CRgametes50% CWgametesCome together at randomGeneration2Generation3Generation425% CRCR50% CRCW25% CWCW50% CRgametes50% CWgametesCome together at random25% CRCR50% CRCW25% CWCWAlleles segregate, and subsequentgenerations also have three typesof flowers in the same proportionsPreservation of Allele FrequenciesIn a given population where gametes contribute to the next generation randomly, allele frequencies will not changeHardy-Weinberg EquilibriumHardy-Weinberg equilibriumDescribes a population in which random mating occursDescribes a population where allele frequencies do not changeA population in Hardy-Weinberg equilibriumFigure 23.5Gametes for each generation are drawn at random fromthe gene pool of the previous generation:80% CR (p = 0.8)20% CW (q = 0.2)SpermCR(80%)CW(20%)p264%CRCR16%CRCW16%CRCW4%CWCWqpCR(80%)EggsCW(20%)pqIf the gametes come together at random, the genotypefrequencies of this generation are in Hardy-Weinberg equilibrium:q264% CRCR, 32% CRCW, and 4% CWCWGametes of the next generation:64% CR fromCRCR homozygotes16% CR fromCRCW homozygotes+=80% CR = 0.8 = p16% CW fromCRCW heterozygotes+=20% CW = 0.2 = qWith random mating, these gametes will result in the samemix of plants in the next generation:64% CRCR, 32% CRCW and 4% CWCW plantsp24% CW fromCWCW homozygotesIf p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, thenp2 + 2pq + q2 = 1And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotypeConditions for Hardy-Weinberg EquilibriumThe Hardy-Weinberg theoremDescribes a hypothetical populationIn real populationsAllele and genotype frequencies do change over timeThe five conditions for non-evolving populations are rarely met in natureExtremely large population sizeNo gene flowNo mutationsRandom matingNo natural selectionPopulation Genetics and Human HealthWe can use the Hardy-Weinberg equationTo estimate the percentage of the human population carrying the allele for an inherited diseaseConcept 23.2: Mutation and sexual recombination produce the variation that makes evolution possibleTwo processes, mutation and sexual recombinationProduce the variation in gene pools that contributes to differences among individualsMutationMutationsAre changes in the nucleotide sequence of DNACause new genes and alleles to ariseFigure 23.6Point MutationsA point mutationIs a change in one base in a geneCan have a significant impact on phenotypeIs usually harmless, but may have an adaptive impact Mutations That Alter Gene Number or SequenceChromosomal mutations that affect many lociAre almost certain to be harmfulMay be neutral and even beneficialGene duplicationDuplicates chromosome segmentsMutation RatesMutation ratesTend to be low in animals and plantsAverage about one mutation in every 100,000 genes per generationAre more rapid in microorganismsSexual Recombination In sexually reproducing populations, sexual recombinationIs far more important than mutation in producing the genetic differences that make adaptation possibleConcept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic compositionThree major factors alter allele frequencies and bring about most evolutionary changeNatural selectionGenetic driftGene flowNatural SelectionDifferential success in reproductionResults in certain alleles being passed to the next generation in greater proportionsGenetic DriftStatistically, the smaller a sampleThe greater the chance of deviation from a predicted resultGenetic driftDescribes how allele frequencies can fluctuate unpredictably from one generation to the nextTends to reduce genetic variationFigure 23.7CRCRCRCWCRCRCWCWCRCRCRCWCRCWCRCWCRCRCRCROnly 5 of10 plantsleaveoffspringCWCWCRCRCRCWCRCRCWCWCRCWCWCWCRCRCRCWCRCWOnly 2 of10 plantsleaveoffspringCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRGeneration 2p = 0.5q = 0.5Generation 3p = 1.0q = 0.0Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3The Bottleneck EffectIn the bottleneck effectA sudden change in the environment may drastically reduce the size of a populationThe gene pool may no longer be reflective of the original population’s gene poolOriginalpopulationBottleneckingeventSurvivingpopulationFigure 23.8 A(a)Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.Understanding the bottleneck effectCan increase understanding of how human activity affects other speciesFigure 23.8 B(b)Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction.The Founder EffectThe founder effectOccurs when a few individuals become isolated from a larger populationCan affect allele frequencies in a populationGene FlowGene flowCauses a population to gain or lose allelesResults from the movement of fertile individuals or gametesTends to reduce differences between populations over timeConcept 23.4: Natural selection is the primary mechanism of adaptive evolutionNatural selectionAccumulates and maintains favorable genotypes in a populationGenetic Variation Genetic variationOccurs in individuals in populations of all speciesIs not always heritableFigure 23.9 A, B(a) Map butterflies thatemerge in spring:orange and brown(b) Map butterflies thatemerge in late summer:black and whiteVariation Within a PopulationBoth discrete and quantitative charactersContribute to variation within a populationDiscrete charactersCan be classified on an either-or basisQuantitative charactersVary along a continuum within a populationPolymorphismPhenotypic polymorphismDescribes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeableGenetic polymorphismsAre the heritable components of characters that occur along a continuum in a populationMeasuring Genetic VariationPopulation geneticistsMeasure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular levelAverage heterozygosityMeasures the average percent of loci that are heterozygous in a populationVariation Between PopulationsMost species exhibit geographic variationDifferences between gene pools of separate populations or population subgroups12.43.145.1867.15XX1913.1710.169.128.1112.193.84.165.146.7XX15.1813.1711.129.10Figure 23.10Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axisFigure 23.11EXPERIMENT Researchers observed that the average sizeof yarrow plants (Achillea) growing on the slopes of the Sierra Nevada mountains gradually decreases with increasing elevation. To eliminate the effect of environmental differences at different elevations, researchers collected seeds from various altitudes and planted them in a common garden. They then measured the heights of theresulting plants.RESULTS The average plant sizes in the commongarden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments.CONCLUSION The lesser but still measurable clinal variationin yarrow plants grown at a common elevation demonstrates therole of genetic as well as environmental differences.Mean height (cm)Atitude (m)Heights of yarrow plants grown in common gardenSeed collection sitesSierra NevadaRangeGreat BasinPlateauA Closer Look at Natural SelectionFrom the range of variations available in a populationNatural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generationsEvolutionary FitnessThe phrases “struggle for existence” and “survival of the fittest”Are commonly used to describe natural selectionCan be misleadingReproductive successIs generally more subtle and depends on many factorsFitnessIs the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individualsRelative fitnessIs the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locusDirectional, Disruptive, and Stabilizing SelectionSelectionFavors certain genotypes by acting on the phenotypes of certain organismsThree modes of selection areDirectionalDisruptiveStabilizingDirectional selectionFavors individuals at one end of the phenotypic rangeDisruptive selectionFavors individuals at both extremes of the phenotypic rangeStabilizing selectionFavors intermediate variants and acts against extreme phenotypesThe three modes of selectionFig 23.12 A–C(a) Directional selection shifts the overallmakeup of the population by favoringvariants at one extreme of thedistribution. In this case, darker mice arefavored because they live among darkrocks and a darker fur color conceals themfrom predators.(b) Disruptive selection favors variantsat both ends of the distribution. Thesemice have colonized a patchy habitatmade up of light and dark rocks, with theresult that mice of an intermediate color areat a disadvantage.(c) Stabilizing selection removesextreme variants from the populationand preserves intermediate types. Ifthe environment consists of rocks ofan intermediate color, both light anddark mice will be selected against.Phenotypes (fur color)Original populationOriginalpopulationEvolvedpopulationFrequency of individualsThe Preservation of Genetic VariationVarious mechanisms help to preserve genetic variation in a populationDiploidyDiploidyMaintains genetic variation in the form of hidden recessive allelesBalancing SelectionBalancing selectionOccurs when natural selection maintains stable frequencies of two or more phenotypic forms in a populationLeads to a state called balanced polymorphismHeterozygote AdvantageSome individuals who are heterozygous at a particular locusHave greater fitness than homozygotesNatural selectionWill tend to maintain two or more alleles at that locusThe sickle-cell alleleCauses mutations in hemoglobin but also confers malaria resistanceExemplifies the heterozygote advantageFigure 23.13Frequencies of thesickle-cell allele0–2.5%2.5–5.0%5.0–7.5%7.5–10.0%10.0–12.5%>12.5%Distribution ofmalaria caused byPlasmodium falciparum(a protozoan)Frequency-Dependent Selection In frequency-dependent selectionThe fitness of any morph declines if it becomes too common in the populationAn example of frequency-dependent selectionPhenotypic diversityFigure 23.14Parental population sampleExperimental group samplePlain backgroundPatterned backgroundOn pecking a moth imagethe blue jay receives afood reward. If the bird does not detect a mothon either screen, it pecksthe green circle to continueto a new set of images (anew feeding opportunity). 0.060.050.040.030.02020406080100Generation numberFrequency-independent controlNeutral VariationNeutral variationIs genetic variation that appears to confer no selective advantageSexual SelectionSexual selectionIs natural selection for mating successCan result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristicsIntrasexual selectionIs a direct competition among individuals of one sex for mates of the opposite sexIntersexual selectionOccurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sexMay depend on the showiness of the male’s appearanceFigure 23.15The Evolutionary Enigma of Sexual ReproductionSexual reproductionProduces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicapFigure 23.16Asexual reproductionFemaleSexual reproductionFemale MaleGeneration 1Generation 2Generation 3Generation 4If sexual reproduction is a handicap, why has it persisted?It produces genetic variation that may aid in disease resistanceWhy Natural Selection Cannot Fashion Perfect OrganismsEvolution is limited by historical constraintsAdaptations are often compromisesChance and natural selection interactSelection can only edit existing variations