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UK BIO 304 - Lecture 18

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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 37Fig. 19-1Chapter 19: Population geneticsPopulation geneticsPopulation: interbreeding members of a speciesThree major principles of Darwinian evolutionary theory:• variation for traits exists within populations• selection applies to a subset of those traits (selection can act only upon variations)• traits are genetically transmittedPolymorphism: multiple forms of a gene are commonly found in a population(all studied populations are “wildly” polymorphic)• chromosomal polymorphisms• immunological polymorphisms• protein polymorphisms• nucleic acid sequence/structure polymorphismsp = (2 x MM) + (1 x MN) = frequency of M alleleq = (2 x NN) + (1 x MN) = frequency of N alleleTherefore, frequency of MN reflects the genetic variation in a populationFig. 19-2“Factoids” regarding protein polymorphisms in most species:• structural polymorphisms are displayed by about one-third of all proteins• typically, about 10% of the individuals in a large population are heterozygous for polymorphisms of an average geneTherefore, enormous protein-level variation exists in most populationsFig. 19-2Electrophoretic allelic variants of esterase-5 in DrosophilaFig. 19-3Electrophoretic variants of hemoglobin A in humansEnormous naturally-occuring variation (polymorphism)in protein sequenceEnormous naturally-occuring variation (polymorphism)in chromosomal rearrangementsFig. 19-5Enormous naturally-occuring variation (polymorphism)in nucleotide sequence Restriction sites within Drosophila xdh gene(58 wild chromosomes sampled) 4-base sites in 4.5 kb DNA* Site present in minority of chromosomes0/1 Site in ½ of chromosomesEnormous naturally-occuring variation (polymorphism)in tandem repeat arrays (VNTRs)Hardy-Weinberg EquilibriumRandom mating within a large population assures a stable equilibrium of genetic diversity in subsequent generationsprovided that certain assumptions apply:Mating is random(no biased mating, infinite population size)Allele frequencies do not change (no selection, no migration, etc.)Hardy-Weinberg EquilibriumFor a two-allele system, all genotypes exist as a simpleproduct of the frequency of each allele:homozygotes = p2 or q2 heterozygotes = 2pq p2 + 2pq + q2 = 1 Box 19-2Box 19-2Fig. 19-6Rare alleles are almost always found in heterozygotes, almost never homozygousAllele frequencies determine frequenciesof homozygotes and heterozygotesAnother measure of heterozygosity ishaplotype diversityHaplotype: combination of non-allelic alleles on a single chromosomeAllele and genotype frequencies can vary between populations,while exhibiting H-W equilibria within each populationMN allele and genotype frequenciesreflect Hardy-Weinberg assumptionsFig. 19-7Non-random mating: inbreeding (mating among relatives)Positive inbreeding:Mating among relativesis more common than randomIncreases frequenciesof homozygotes in a populationFig. 19-8Extreme inbreeding: self-fertilization results in loss of heterozygosityNo change in p or q; change only in heterozygosity and diversityNegative inbreeding (enforced outbreeding)-barriers to inbreeding are common attributes of successful populationsPositive assortative mating- individuals chose “like” mates (not necessarily relatives)Negative assortative mating- individuals choose dissimilar matesSources of variation• Mutation – very slow•Mutation is the ultimate source of variationBut spontaneous mutations occur at extremely low frequenciesBox 19-3Mutation frequency is influenced by allele frequencyMutation alone is a very slow evolutionary force and cannot directly account for diversity observed in populations.Sources of variation• Mutation – very slow• Recombination – rapidly mixes genes to provide new genetic combinations in a population • Migration – gene flow among different populations changes gene frequenciesSelection: directed change in genotypes in a populationFitness: survival and reproduction success; function of genotype and environmentFitness can be obvious (mortality, sterililty)• HbS/HbS: severe anemia, low survival• HbS/HbA: apparent resistance to malariaor more subtle/partial/conditionalFig. 19-9 Fitness (viability) of various homozygotes as a function of temperatureDrosophila pseudoobscuraFig. 19-11Enhanced fitness of a genotype will enrich those genesin subsequent generations of that populationFrequencies of positively selected genes increase over timeFrequencies of negatively selected genes decrease over timeChange in A frequency (p) is greatest where p = qFor a two-allele system, mean fitness (W) in a population is the proportional contribution of fitness by each genotype (A/A, A/a, a/a) W = p2WA/A + 2pqWA/a + q2Wa/a WA/A and WA/a > Wa/a p should increaseq should decreaseWa/a > WA/A and WA/a q should increasep should decreaseFig. 19-12p for malic dehydrogenase electrophoretic mobility variant MDHF where WS/S=1, WS/F=0.75, WF/F=0.4 Fitness can account for allele frequency changes over timeSelection: directed change in genotypes in a populationFitness: survival and reproduction success; function of genotype and environment Frequency independent selection: fitness is independent of genotype frequencyFrequency dependent selection: fitness changes as genotype frequency changesRandom genetic drift: random changes in gene frequency that can lead to extinction/fixation of genes • Requires no selection• Essentially “sampling error” inherent in each generation in achieving Hardy- Weinberg equilibrium• Most exaggerated in small populations (especially “founder effects”)• Allows isolated populations to diverge without differential selection (each experiences its own drift history)Fig. 19-13Model: history of emergence of ten mutations and their drift in a population over timeDrift to extinction for nine; drift to p=1 in oneDrift explains differences in unselected allelefrequencies in isolated populationsGenetic change is directed by diverse evolutionary forceswhich tend to increase (blue) or decrease (red) variationRecommended problems in Chapter 19: 3, 5, 12,


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