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


  • 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?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.

  • Population GeneticsThe study of genetic variation in populations.Represents the reconciliation of Mendelism and Darwinism.

  • Modern SynthesisUses population genetics as the means to track and study evolution.Looks at the genetic basis of variation and natural selection.

  • PopulationA localized group of individuals of the same species.

  • SpeciesA group of similar organisms.A group of populations that could interbreed.

  • 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.

  • MicroevolutionChanges in the relative frequencies of alleles in the gene pool.

  • Hardy-Weinberg TheoremDeveloped in 1908. Mathematical model of gene pool changes over time.

  • Basic Equationp + q = 1p = % dominant alleleq = % recessive allele

  • Expanded Equationp + q = 1(p + q)2 = (1)2p2 + 2pq + q2 = 1

  • Genotypesp2 = Homozygous Dominants 2pq = Heterozygous q2 = Homozygous Recessives

  • Example CalculationLets look at a population where: A = red flowers a = white flowers

  • Starting PopulationN = 500Red = 480 (320 AA+ 160 Aa)White = 20Total Genes = 2 x 500 = 1000

  • Dominant AlleleA = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80%

  • Recessive Allelea = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20%

  • A and a in HW equationCross: Aa X AaResult = AA + 2Aa + aaRemember: A = p, a = q

  • Substitute the values for A and ap2 + 2pq + q2 = 1(.8)2 + 2(.8)(.2) + (.2)2 = 1.64 + .32 + .04 = 1

  • Dominant AlleleA = p2 + pq = .64 + .16 = .80 = 80%

  • Recessive Allelea = pq + q2 = .16 + .04 = .20 = 20%

  • ResultGene pool is in a state of equilibrium and has not changed because of sexual reproduction.No Evolution has occurred.

  • 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.

  • Example What is the frequency of the PKU allele?PKU is expressed only if the individual is homozygous recessive (aa).

  • PKU FrequencyPKU is found at the rate of 1/10,000 births.PKU = aa = q2 q2 = .0001 q = .01

  • Dominant Allelep + q = 1 p = 1- q p = 1- .01 p = .99

  • Expanded Equationp2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2 = 1.9801 + .0198 + .0001 = 1

  • Final ResultsNormals (AA) = 98.01%Carriers (Aa) = 1.98%PKU (aa) = .01%

  • 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).

  • Hardy-Weinberg Assumptions1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection

  • 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?

  • MicroevolutionCaused by violations of the 5 H-W assumptions.

  • Causes of Microevolution1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural Selection

  • Genetic DriftChanges in the gene pool of a small population by chance.Types:1. Bottleneck Effect2. Founder's Effect

  • By Chance

  • Bottleneck EffectLoss of most of the population by disasters.Surviving population may have a different gene pool than the original population.

  • ResultSome alleles lost.Other alleles are over-represented.Genetic variation usually lost.

  • ImportanceReduction of population size may reduce gene pool for evolution to work with. Ex: Cheetahs

  • Founder's EffectGenetic drift in a new colony that separates from a parent population.Ex: Old-Order Amish

  • ResultGenetic variation reduced.Some alleles increase in frequency while others are lost (as compared to the parent population).

  • ImportanceVery common in islands and other groups that don't interbreed.

  • Gene FlowMovement of genes in/out of a population.Ex:ImmigrationEmigration

  • ResultChanges in gene frequencies.

  • MutationsInherited changes in a gene.

  • ResultMay change gene frequencies (small population).Source of new alleles for selection.Often lost by genetic drift.

  • Nonrandom MatingFailure to choose mates at random from the population.

  • CausesInbreeding within the same neighborhood.Assortative mating (like with like).

  • ResultIncreases the number of homozygous loci.Does not in itself alter the overall gene frequencies in the population.

  • Natural SelectionDifferential success in survival and reproduction.Result - Shifts in gene frequencies.

  • CommentAs the Environment changes, so does Natural Selection and Gene Frequencies.

  • ResultIf the environment is "patchy", the population may have many different local populations.

  • Genetic Basis of Variation1. Discrete Characters Mendelian traits with clear phenotypes.2. Quantitative Characters Multigene traits with overlapping phenotypes.

  • PolymorphismThe existence of several contrasting forms of the species in a population.Usually inherited as Discrete Characteristics.

  • Examples Garter SnakesGaillardia

  • Human ExampleABO Blood GroupsMorphs = A, B, AB, O

  • Other examples

  • Quantitative CharactersAllow continuous variation in the population.Result Geographical VariationClines: a change along a geographical axis

  • Yarrow Plants and Altitude

  • Sources of Genetic VariationMutations.Recombination though sexual reproduction.Crossing-overRandom fertilization

  • Preserving Genetic Variation1. Diploidy - preserves recessives as heterozygotes.2. Balanced Polymorphisms - preservation of diversity by natural selection.

  • ExampleHeterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.

  • ResultCan't bred "true and the diversity of the population is maintained.Ex Sickle Cell Anemia

  • 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

  • Fitness - DarwinianThe relative contribution an individual makes to the gene pool of the next generation.

  • Relative FitnessContribution of one genotype to the next generation compared to other genotypes.

  • Rate of SelectionDiffers between dominant and recessive alleles.Selection pressure by the environment.

  • Modes of Natural Selection1. Stabilizing2. Directional3. Diversifying4. Sexual

  • StabilizingSelection toward the average and against the extremes.Ex: birth weight in humans

  • Directional SelectionSelection toward one extreme.Ex: running speeds in race animals.Ex. Galapagos Finch beak size and food source.

  • DiversifyingSelection toward both extremes and against the norm.Ex: bill size in birds

  • 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.

  • Sexual Mate selectionMay not be adaptive to the environment, but increases reproduction success of the individual.

  • ResultSexual dimorphism.Secondary sexual features for attracting mates.

  • CommentsFemales may drive sexual selection and dimorphism since they often "choose" the mate.

  • QuestionDoes evolution result in perfect organisms?

  • Answer - No1. Historical Constraints2. Compromises3. Non-adaptive Evolution (chance)4. Available variations

  • SummaryKnow the difference between a species and a population.Know that the unit of evolution is the population and not the individual.

  • SummaryKnow the H-W equations and how to use them in calculations.Know the H-W assumptions and what happens if each is violated.

  • SummaryIdentify various means to introduce genetic variation into populations.Know the various types of natural selection.