Môn Sinh học - Chapter 23: The evolution of populations

Ch. 22/23 Warm-upList 5 different pieces of evidence for evolution.(Review) What are the 3 ways that sexual reproduction produces genetic diversity?What is 1 thing you are grateful for today?Ch. 23 Warm-upIn a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb).The allele frequencies of this population are: B allele: ___ b allele: ___The genotype frequencies are: BB: ___ Bb: ___ bb: ___Use the above info to determine the genotype frequencies of the next generation: B (p): ___ b (q): ___ BB (p2): ___ Bb (2pq): ___ bb (q2): ___Chapter 23The Evolution of PopulationsWhat you must know:How mutation and sexual reproduction each produce genetic variation.The conditions for Hardy-Weinberg equilibrium.How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving.Smallest unit of evolutionMicroevolution: change in the allele frequencies of a population over generationsDarwin did not know how organisms passed traits to offspring1866 - Mendel published his paper on geneticsMendelian genetics supports Darwin’s theory  Evolution is based on genetic variationSources of Genetic VariationPoint mutations: changes in one base (eg. sickle cell)Chromosomal mutations: delete, duplicate, disrupt, rearrange  usually harmfulSexual recombination: contributes to most of genetic variation in a populationCrossing Over (Meiosis – Prophase I)Independent Assortment of Chromosomes (during meiosis)Random Fertilization (sperm + egg)Population genetics: study of how populations change genetically over timePopulation: group of individuals that live in the same area and interbreed, producing fertile offspringGene pool: all of the alleles for all genes in all the members of the populationDiploid species: 2 alleles for a gene (homozygous/heterozygous)Fixed allele: all members of a population only have 1 allele for a particular traitThe more fixed alleles a population has, the LOWER the species’ diversityHardy-Weinberg PrincipleHardy-Weinberg Principle: The allele and genotype frequencies of a population will remain constant from generation to generationUNLESS they are acted upon by forces other than Mendelian segregation and recombination of allelesEquilibrium = allele and genotype frequencies remain constantConditions for Hardy-Weinberg equilibriumNo mutations.Random mating.No natural selection.Extremely large population size.No gene flow.If at least one of these conditions is NOT met, then the population is EVOLVING!Hardy-Weinberg PrincipleAllele Frequencies:Gene with 2 alleles : p, q p = frequency of dominant allele (A) q = frequency of recessive allele (a)Note:1 – p = q1 – q = pHardy-Weinberg EquationGenotypic Frequencies:3 genotypes (AA, Aa, aa)p2 = AA (homozygous dominant)2pq = Aa (heterozygous)q2 = aa (homozygous recessive)Allele frequenciesGenotypic frequenciesStrategies for solving H-W Problems:If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles.If you know phenotypes, then use “aa” to find q2, and then q. (p = 1-q)Use p2 + 2pq + q2 to find genotype frequencies.If p and q are not constant from generation to generation, then the POPULATION IS EVOLVING!Hardy-weinberg practice problem #1The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths. AA = 1469 Aa = 138 aa = 5Allele Frequencies: A = a =Genotypic Frequencies: AA = Aa = aa =Hardy-weinberg practice problem #2:PTC TastersTaster = AA or Aa Nontaster = aaTasters = ____ Nontasters = ___q2 = q =p + q = 1 p = 1 – q =p2 + 2pq + q2 = 1Causes of evolutionConditions for Hardy-Weinberg equilibriumNo mutations.Random mating.No natural selection.Extremely large population size.No gene flow.If at least one of these conditions is NOT met, then the population is EVOLVING!Minor Causes of Evolution:#1 - MutationsRare, very small changes in allele frequencies#2 - Nonrandom matingAffect genotypes, but not allele frequenciesMajor Causes of Evolution:Natural selection, genetic drift, gene flow (#3-5)Major Causes of Evolution#3 – Natural SelectionIndividuals with variations better suited to environment pass more alleles to next generationMajor Causes of Evolution#4 – Genetic DriftSmall populations have greater chance of fluctuations in allele frequencies from one generation to anotherExamples:Founder EffectBottleneck EffectGenetic DriftFounder EffectA few individuals isolated from larger populationCertain alleles under/over representedPolydactyly in Amish populationBottleneck EffectSudden change in environment drastically reduces population sizeNorthern elephant seals hunted nearly to extinction in CaliforniaMajor Causes of Evolution#5 – Gene FlowMovement of fertile individuals between populationsGain/lose allelesReduce genetic differences between populationsHow does natural selection bring about adaptive evolution?Fitness : the contribution an individual makes to the gene pool of the next generationNatural selection can alter frequency distribution of heritable traits in 3 ways:Directional selectionDisruptive (diversifying) selectionStabilizing selectionDirectional Selection: eg. larger black bears survive extreme cold better than small onesDisruptive Selection: eg. small beaks for small seeds; large beaks for large seedsStabilizing Selection: eg. narrow range of human birth weightSexual selectionForm of natural selection – certain individuals more likely to obtain matesSexual dimorphism: difference between 2 sexesSize, color, ornamentation, behaviorSexual selectionIntrasexual – selection within same sex (eg. M compete with other M)Intersexual – mate choice (eg. F choose showy M)Preserving genetic variationDiploidy: hide recessive alleles that are less favorableHeterozygote advantage: greater fitness than homozygoteseg. Sickle cell diseaseRunning Time: 14:03 minHHMI Video:Natural Selection in HumansNatural selection cannot fashion perfect organisms.Selection can act only on existing variations.Evolution is limited by historical constraints.Adaptations are often compromises.Chance, natural selection, and the environment interact.Sample ProblemDefine the following examples as directional, disruptive, or stabilizing selection:Tiger cubs usually weigh 2-3 lbs. at birthButterflies in 2 different colors each represent a species distasteful to birdsBrightly colored birds mate more frequently than drab birds of same speciesFossil evidence of horse size increasing over time