Chapter 23: The Evolution of Populations

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Chapter 23: The Evolution of Populations. Question?. Is the unit of evolution the individual or the population ? Answer while evolution effects individuals, it can only be tracked through time by looking at populations. So what do we study?. - PowerPoint PPT Presentation


<ul><li><p>Chapter 23:The Evolution of Populations</p></li><li><p>Question?Is the unit of evolution the individual or the population?Answer while evolution effects individuals, it can only be tracked through time by looking at populations.</p></li><li><p>So what do we study?We need to study populations, not individuals.We need a method to track the changes in populations over time. This is the area of Biology called population genetics.</p></li><li><p>Population GeneticsThe study of genetic variation in populations.Represents the reconciliation of Mendelism and Darwinism.</p></li><li><p>Modern SynthesisUses population genetics as the means to track and study evolution.Looks at the genetic basis of variation and natural selection.</p></li><li><p>PopulationA localized group of individuals of the same species.</p></li><li><p>SpeciesA group of similar organisms.A group of populations that could interbreed.</p></li><li><p>Gene PoolThe total aggregate of genes in a population.If evolution is occurring, then changes must occur in the gene pool of the population over time.</p></li><li><p>MicroevolutionChanges in the relative frequencies of alleles in the gene pool.</p></li><li><p>Hardy-Weinberg TheoremDeveloped in 1908. Mathematical model of gene pool changes over time.</p></li><li><p>Basic Equationp + q = 1p = % dominant alleleq = % recessive allele</p></li><li><p>Expanded Equationp + q = 1(p + q)2 = (1)2p2 + 2pq + q2 = 1</p></li><li><p>Genotypesp2 = Homozygous Dominants 2pq = Heterozygous q2 = Homozygous Recessives</p></li><li><p>Example CalculationLets look at a population where: A = red flowers a = white flowers</p></li><li><p>Starting PopulationN = 500Red = 480 (320 AA+ 160 Aa)White = 20Total Genes = 2 x 500 = 1000</p></li><li><p>Dominant AlleleA = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80%</p></li><li><p>Recessive Allelea = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20%</p></li><li><p>A and a in HW equationCross: Aa X AaResult = AA + 2Aa + aaRemember: A = p, a = q</p></li><li><p>Substitute the values for A and ap2 + 2pq + q2 = 1(.8)2 + 2(.8)(.2) + (.2)2 = 1.64 + .32 + .04 = 1</p></li><li><p>Dominant AlleleA = p2 + pq = .64 + .16 = .80 = 80% </p></li><li><p>Recessive Allelea = pq + q2 = .16 + .04 = .20 = 20%</p></li><li><p>ResultGene pool is in a state of equilibrium and has not changed because of sexual reproduction.No Evolution has occurred.</p></li><li><p>Importance of Hardy-WeinbergYardstick to measure rates of evolution.Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold.Way to calculate gene frequencies through time.</p></li><li><p>Example What is the frequency of the PKU allele?PKU is expressed only if the individual is homozygous recessive (aa).</p></li><li><p>PKU FrequencyPKU is found at the rate of 1/10,000 births.PKU = aa = q2 q2 = .0001 q = .01</p></li><li><p>Dominant Allelep + q = 1 p = 1- q p = 1- .01 p = .99</p></li><li><p>Expanded Equationp2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2 = 1.9801 + .0198 + .0001 = 1</p></li><li><p>Final ResultsNormals (AA) = 98.01%Carriers (Aa) = 1.98%PKU (aa) = .01%</p></li><li><p>AP Problems Using Hardy-WeinbergSolve for q2 (% of total).Solve for q (equation).Solve for p (1- q).H-W is always on the national AP Bio exam (but no calculators are allowed).</p></li><li><p>Hardy-Weinberg Assumptions1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection</p></li><li><p>If H-W assumptions hold true:The gene frequencies will not change over time.Evolution will not occur.But, how likely will natural populations hold to the H-W assumptions?</p></li><li><p>MicroevolutionCaused by violations of the 5 H-W assumptions.</p></li><li><p>Causes of Microevolution1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural Selection</p></li><li><p>Genetic DriftChanges in the gene pool of a small population by chance.Types:1. Bottleneck Effect2. Founder's Effect</p></li><li><p>By Chance</p></li><li><p>Bottleneck EffectLoss of most of the population by disasters.Surviving population may have a different gene pool than the original population.</p></li><li><p>ResultSome alleles lost.Other alleles are over-represented.Genetic variation usually lost.</p></li><li><p>ImportanceReduction of population size may reduce gene pool for evolution to work with. Ex: Cheetahs</p></li><li><p>Founder's EffectGenetic drift in a new colony that separates from a parent population.Ex: Old-Order Amish</p></li><li><p>ResultGenetic variation reduced.Some alleles increase in frequency while others are lost (as compared to the parent population).</p></li><li><p>ImportanceVery common in islands and other groups that don't interbreed.</p></li><li><p>Gene FlowMovement of genes in/out of a population.Ex:ImmigrationEmigration</p></li><li><p>ResultChanges in gene frequencies.</p></li><li><p>MutationsInherited changes in a gene.</p></li><li><p>ResultMay change gene frequencies (small population).Source of new alleles for selection.Often lost by genetic drift.</p></li><li><p>Nonrandom MatingFailure to choose mates at random from the population.</p></li><li><p>CausesInbreeding within the same neighborhood.Assortative mating (like with like).</p></li><li><p>ResultIncreases the number of homozygous loci.Does not in itself alter the overall gene frequencies in the population.</p></li><li><p>Natural SelectionDifferential success in survival and reproduction.Result - Shifts in gene frequencies.</p></li><li><p>CommentAs the Environment changes, so does Natural Selection and Gene Frequencies.</p></li><li><p>ResultIf the environment is "patchy", the population may have many different local populations.</p></li><li><p>Genetic Basis of Variation1. Discrete Characters Mendelian traits with clear phenotypes.2. Quantitative Characters Multigene traits with overlapping phenotypes.</p></li><li><p>PolymorphismThe existence of several contrasting forms of the species in a population.Usually inherited as Discrete Characteristics.</p></li><li><p>Examples Garter SnakesGaillardia</p></li><li><p>Human ExampleABO Blood GroupsMorphs = A, B, AB, O</p></li><li><p>Other examples</p></li><li><p>Quantitative CharactersAllow continuous variation in the population.Result Geographical VariationClines: a change along a geographical axis</p></li><li><p>Yarrow Plants and Altitude</p></li><li><p>Sources of Genetic VariationMutations.Recombination though sexual reproduction.Crossing-overRandom fertilization</p></li><li><p>Preserving Genetic Variation1. Diploidy - preserves recessives as heterozygotes.2. Balanced Polymorphisms - preservation of diversity by natural selection.</p></li><li><p>ExampleHeterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.</p></li><li><p>ResultCan't bred "true and the diversity of the population is maintained.Ex Sickle Cell Anemia</p></li><li><p>CommentPopulation geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.Ex Sickle Cell and Malaria, Tay-Sachs and Tuberculosis</p></li><li><p>Fitness - DarwinianThe relative contribution an individual makes to the gene pool of the next generation.</p></li><li><p>Relative FitnessContribution of one genotype to the next generation compared to other genotypes.</p></li><li><p>Rate of SelectionDiffers between dominant and recessive alleles.Selection pressure by the environment.</p></li><li><p>Modes of Natural Selection1. Stabilizing2. Directional3. Diversifying4. Sexual</p></li><li><p>StabilizingSelection toward the average and against the extremes.Ex: birth weight in humans</p></li><li><p>Directional SelectionSelection toward one extreme.Ex: running speeds in race animals.Ex. Galapagos Finch beak size and food source.</p></li><li><p>DiversifyingSelection toward both extremes and against the norm.Ex: bill size in birds</p></li><li><p>CommentDiversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations dont interbreed.</p></li><li><p>Sexual Mate selectionMay not be adaptive to the environment, but increases reproduction success of the individual.</p></li><li><p>ResultSexual dimorphism.Secondary sexual features for attracting mates.</p></li><li><p>CommentsFemales may drive sexual selection and dimorphism since they often "choose" the mate.</p></li><li><p>QuestionDoes evolution result in perfect organisms?</p></li><li><p>Answer - No1. Historical Constraints2. Compromises3. Non-adaptive Evolution (chance)4. Available variations</p></li><li><p>SummaryKnow the difference between a species and a population.Know that the unit of evolution is the population and not the individual.</p></li><li><p>SummaryKnow the H-W equations and how to use them in calculations.Know the H-W assumptions and what happens if each is violated.</p></li><li><p>SummaryIdentify various means to introduce genetic variation into populations.Know the various types of natural selection.</p></li></ul>


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