Bài giảng Biology - Chapter 20: Genes Within Populations

Genes Within PopulationsChapter 201Darwin: Evolution is descent with modificationEvolution: changes through timeSpecies accumulate differenceDescendants differ from their ancestorsNew species arise from existing onesGenetic Variation and Evolution2Natural selection: proposed by Darwin as the mechanism of evolutionindividuals have specific inherited characteristics they produce more surviving offspringthe population includes more individuals with these specific characteristicsthe population evolves and is better adapted to its present environmentNatural selection: mechanism of evolutionary change3Darwin’s theory for how long necks evolved in giraffes4Natural selection: mechanism of evolutionary changeInheritance of acquired characteristics: Proposed by Jean-Baptiste LamarckIndividuals passed on physical and behavioral changes to their offspringVariation by experiencenot geneticDarwin’s natural selection: variation a result of preexisting genetic differences 5Lamarck’s theory of how giraffes’ long necks evolved6Measuring levels of genetic variationblood groupsenzymes Enzyme polymorphism A locus with more variation than can be explained by mutation is termed polymorphic.Natural populations tend to have more polymorphic loci than can be accounted for by mutation.DNA sequence polymorphismGene Variation in Nature7Godfrey H. Hardy: English mathematicianWilhelm Weinberg: German physicianConcluded that:The original proportions of the genotypes in a population will remain constant from generation to generation as long as five assumptions are metHardy-Weinberg Principle8Five assumptions : No mutation takes placeNo genes are transferred to or from other sourcesRandom mating is occurringThe population size is very largeNo selection occursHardy-Weinberg Principle9 Calculate genotype frequencies with a binomial expansion (p+q)2 = p2 + 2pq + q2p = individuals homozygous for first allele2pq = individuals heterozygous for both allelesq = individuals homozygous for second allelebecause there are only two alleles: p plus q must always equal 1Hardy-Weinberg Principle10 Hardy-Weinberg Principle11Using Hardy-Weinberg equation to predict frequencies in subsequent generationsHardy-Weinberg Principle12A population not in Hardy-Weinberg equilibrium indicates that one or more of the five evolutionary agents are operating in a populationFive agents of evolutionary change13Agents of Evolutionary ChangeMutation: A change in a cell’s DNAMutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.Ultimate source of genetic variationGene flow: A movement of alleles from one population to anotherPowerful agent of changeTends to homogenize allele frequencies1415Agents of Evolutionary ChangeNonrandom Mating: mating with specific genotypesShifts genotype frequenciesAssortative Mating: does not change frequency of individual alleles; increases the proportion of homozygous individualsDisassortative Mating: phenotypically different individuals mate; produce excess of heterozygotes16Genetic DriftGenetic drift: Random fluctuation in allele frequencies over time by chanceimportant in small populationsfounder effect - few individuals found new population (small allelic pool)bottleneck effect - drastic reduction in population, and gene pool size1718Genetic Drift: A bottleneck effect19Bottleneck effect: case study20SelectionArtificial selection: a breeder selects for desired characteristics21SelectionNatural selection: environmental conditions determine which individuals in a population produce the most offspring3 conditions for natural selection to occurVariation must exist among individuals in a populationVariation among individuals must result in differences in the number of offspring surviving Variation must be genetically inherited22Selection23Pocket mice from the Tularosa BasinSelection24Selection to match climatic conditionsEnzyme allele frequencies vary with latitudeLactate dehydrogenase in Fundulus heteroclitus (mummichog fish) varies with latitudeEnzymes formed function differently at different temperaturesNorth latitudes: Lactate dehydrogenase is a better catalyst at low temperatures25Selection for pesticide resistance26Fitness and Its MeasurementFitness: A phenotype with greater fitness usually increases in frequencyMost fit is given a value of 1Fitness is a combination of:Survival: how long does an organism liveMating success: how often it matesNumber of offspring per mating that survive27Body size and egg-laying in water stridersFitness and its Measurement28Interactions Among Evolutionary ForcesMutation and genetic drift may counter selectionThe magnitude of drift is inversely related to population size29Gene flow may promote or constrain evolutionary changeSpread a beneficial mutationImpede adaptation by continual flow of inferior alleles from other populationsExtent to which gene flow can hinder the effects of natural selection depends on the relative strengths of gene flow High in birds & wind-pollinated plantsLow in sedentary speciesInteractions Among Evolutionary Forces30Degree of copper toleranceInteractions Among Evolutionary Forces31Maintenance of VariationFrequency-dependent selection: depends on how frequently or infrequently a phenotype occurs in a populationNegative frequency-dependent selection: rare phenotypes are favored by selectionPositive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the populationStrength of selection changes through time32Negative frequency - dependent selectionMaintenance of Variation33Positive frequency-dependent selectionMaintenance of Variation34Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another timeGalápagos Islands ground finchesWet conditions favor big bills (abundant seeds)Dry conditions favor small billsMaintenance of Variation35Fitness of a phenotype does not depend on its frequencyEnvironmental changes lead to oscillation in selectionMaintenance of Variation36Heterozygotes may exhibit greater fitness than homozygotesHeterozygote advantage: keep deleterious alleles in a populationExample: Sickle cell anemia Homozygous recessive phenotype: exhibit severe anemia Maintenance of Variation37Homozygous dominant phenotype: no anemia; susceptible to malariaHeterozygous phenotype: no anemia; less susceptible to malariaMaintenance of Variation38Maintenance of VariationFrequency of sickle cell allele39Disruptive selection acts to eliminate intermediate typesMaintenance of Variation40Disruptive selection for large and small beaks in black-bellied seedcracker finch of west AfricaMaintenance of Variation41Directional selection: acts to eliminate one extreme from an array of phenotypesMaintenance of Variation42Directional selection for negative phototropism in DrosophilaMaintenance of Variation43Stabilizing selection: acts to eliminate both extremesMaintenance of Variation44Stabilizing selection for birth weight in humansMaintenance of Variation45Experimental Studies of Natural SelectionIn some cases, evolutionary change can occur rapidlyEvolutionary studies can be devised to test evolutionary hypothesesGuppy studies (Poecilia reticulata) in the lab and fieldPopulations above the waterfalls: low predationPopulations below the waterfalls: high predation46High predation environment - Males exhibit drab coloration and tend to be relatively small and reproduce at a younger age.Low predation environment - Males display bright coloration, a larger number of spots, and tend to be more successful at defending territories.Experimental Studies47The evolution of protective coloration in guppiesExperimental Studies48The laboratory experiment10 large pools2000 guppies4 pools with pike cichlids (predator)4 pools with killifish (nonpredator)2 pools as control (no other fish added)10 generationsExperimental Studies49The field experimentRemoved guppies from below the waterfalls (high predation)Placed guppies in pools above the falls10 generations later, transplanted populations evolved the traits characteristic of low-predation guppiesExperimental Studies50Evolutionary change in spot numberExperimental Studies51The Limits of SelectionGenes have multiple effectsPleiotropy: sets limits on how much a phenotype can be alteredEvolution requires genetic variationThoroughbred horse speedCompound eyes of insects: same genes affect both eyesControl of ommatidia number in left and right eye 52Selection for increased speed in racehorses is no longer effectiveExperimental Studies53Phenotypic variation in insect ommatidiaExperimental Studies54