AP Biology Chapter 23 The Evolution of Populations

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AP Biology Chapter 23 The Evolution of Populations. Campbell and Reece 10 th Edition. Individuals do not evolve, populations do over time. Individuals do not evolve , populations do. Medium Ground Finch from island of Daphne Major in Galpagos Islands - PowerPoint PPT Presentation

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<p>AP Biology Chapter 23 The Evolution of Populations</p> <p>Campbell and Reece 10th EditionAP BiologyChapter 23The Evolution of PopulationsIndividuals do not evolve, populations do over timeIndividuals do not evolve, populations doMedium Ground Finch from island of Daphne Major in Galpagos Islands</p> <p>Long period of drought altered their food supply to mostly larger nuts &amp; over the years those individuals with larger beaks were more successful</p> <p>Overall size of bird b/4 &amp; after drought years</p> <p>Average beak size &amp; size of individual birds larger after the drought so.The medium ground finch population had evolved by natural selection</p> <p>Genetic VariationasCause of EvolutionDarwin reasoned that natural selection acted on genetic variation of populationsHe knew nothing about genesFew yrs later: Mendels paper on inheritance in pea plants: stage set for understanding variationGenetic VariationGenotype inheritable, phenotypes are notExample: moth, Nemoria arizonaria, appears very different eating oak flowers vs. oak leaves</p> <p>Phenotype is result of : genotype + environmental influence</p> <p>In general, only the genetically determined part of a phenotype can affect evolution Discrete characters = either/or ( Mendels pea) = single geneMost heritable variations involve quantitative characters: vary along a continuum 2 genes</p> <p>way to quantify gene variabilityaverage % of loci that are heterozygous</p> <p>can calculate average: turns out if the average heterozygosity is 14% there is enough genetic variation for natural selection to act evolutionary change</p> <p>Average HeterozygosityGel Electrophoresis used to calculate heterozygosity</p> <p>Gel Electrophoresisdoes not show silent mutations (DNA changes but still codes for same a.a.)</p> <p>Geographic Variationdifferences in genetic composition of separate populations</p> <p>Gene Variation Between Populations</p> <p>Cline: a graded change in a character along a geographic axis</p> <p>MutationGene DuplicationSexual ReproductionOther process that results in new alleles or new genesSourcesofGenetic VariationOrganisms with short life spans new genetic variants arise fairly rapidlySources of genetic variation</p> <p>Mutations cant predict where in genome or what type mutationfor multicellular organisms only mutations in gametes cell line passed to new generations (most are in somatic cell line)most point mutations silent or only slightly harmful, rarely are they beneficialFormation of New Alleleschromosomal changes that delete, disrupt, or rearrange usually lethal or harmfulif genes left intact they may be neutral changesTranslocation:Part of 1 chromosome breaks off &amp; attaches to another chromosomeAltering Gene # or PositionTranslocations</p> <p>if large segments duplicated usually harmfulduplications of small pieces may be beneficial mutations accumulate over time eventually that duplication takes on new role</p> <p> end result: expanded genome</p> <p>Duplications of Chromosomesaverage mutation rate in plants &amp; animals is considered low ~ 1 mutation in every 100,000 genes / generation</p> <p>Rapid Reproduction</p> <p>Prokaryotic Mutation Ratesshorter generation spans allows for generation of genetic variation in a populationvirus populations, especially retroviruses process is fastest</p> <p>HIVsingle stranded RNA:less complicated to duplicatefewer RNA repair mechanisms in host cells</p> <p>HIVmost effective treatment for a quickly mutating retrovirus has been combination protocols</p> <p>Sexual Reproductionmost of genetic variation due to crossing over and independent assortment of chromosomes in meiosis and fertilization</p> <p>Hardy-Weinberg Equation can tell you if a Population is Evolvingpresence of genetic variation does not guarantee that population is evolving1 of 3 factors that cause evolution must be at work in a populationPopulation: group of same species in same area that interbreed, with fertile offspring</p> <p>Populationsexamples of isolated populations:IslandsLakeseven populations not strictly isolated members tend to breed with own population so are genetically closer to them than other groups</p> <p>Gene Poolsconsists of all copies of every allele at every locus in all members of a population</p> <p>Fixed Genesif there is only 1 allele for a locus that allele is said to be fixed in the gene pool; entire population is homozygous for that gene</p> <p>if there are 2 or more alleles for a locus then individuals may be homozygous or heterozygousEach allele has a frequency in the population</p> <p>to test whether natural selection is acting on a particular locus:Determine what the frequency would be if it were not evolvingThen compare that calculation with what you measure in the populationNo difference: not evolvingdifference: evolvingThe Hardy-Weinberg PrincipleHardy-Weinberg Principle 1908HardyWeinberg</p> <p>states that the frequencies of alleles &amp; genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation &amp; recombination of alleles are at workIf that is true the population is said to be in HARDY-WEINBERG EQUILIBRIUMHardy-Weinberg Principle Hardy-Weinberg Equilibrium</p> <p>Hardy-Weinberg Equilibrium</p> <p>assumes random mating</p> <p>http://nhscience.lonestar.edu/biol/hwe/q1d.html http://www.phschool.com/science/biology_place/labbench/lab8/intro.html Problem 2:If 9% of an African population is born with a severe form of sickle cell anemia (ss) what % of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?Hardy-Weinberg Problems 2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers) </p> <p>Answer to problem 2</p> <p>No MutationsRandom MatingNo Natural SelectionExtremely Large PopulationsNo Gene Flow</p> <p>Conditions for Hardy-Weinberg EquilibriumDeparture from any of the 5 conditions usually results in evolutionary changes</p> <p>A population may be evolving at some gene loci and in Hardy-Weinberg Equilibrium at other loci</p> <p>Can be used to estimate the frequency of a gene causing inherited disease in a populationMust assume:No new mutationsRandom matingIgnore any effects of differential survival &amp; reproductive successNo genetic driftApplying the Hardy-Weinberg PrincipleHow Allele Frequencies are Altered in a Population</p> <p>No Mutations: not usually significant unless mutation produces new alleles that have a strong influence in a (+) or (-) wayRandom Mating: not usually significantNo Natural Selection cause mostExtremely Large Populations evolutionary No Gene Flow change</p> <p>Conditions Necessary for H-W Equilibriumis based on differential success in survival &amp; reproductionif NS consistently favoring some alleles over others, NS can cause adaptive evolution (dfn: evolution that results in a better match between organisms &amp; their environment)NATURAL SELECTIONprocess in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the nextthe smaller the population the more pronounced the effectGENETIC DRIFTGENETIC DRIFT</p> <p>Founder Effectgenetic drift that occurs when a few individuals become isolated from a larger population &amp; form a new population whose gene pool composition is not reflective of original population2 Examples of Genetic DriftFounder Effect</p> <p>Founder EffectTristan da Cunha15 British colonist in 1814</p> <p>1 colonist carried recessive allele for retinitis pigmentosaTristan da Cunha</p> <p>By late 1960s, there were 240 descendants of the original founders4 had retinitis pigmentosaThis frequency is 10x higher than frequency of retinitis pigmentosa in EnglandTristan da Cunha2. Bottleneck Effect: occurs when the size of a population is reduced, as by a natural disaster or human actions. The resulting population is genetically different than original population. 2nd Example of Genetic Drift Bottleneck Effect</p> <p>Genetic Drift :is significant in small populationscan cause allele frequencies to change at randomcan lead to a loss of genetic variation w/in populationscan cause harmful alleles to become fixed</p> <p>Summarizing Effects of Genetic Driftthe transfer of alleles from one population to another as result of movement of fertile individuals or their gametesGENE FLOWGene Flow</p> <p>transferred alleles may increase a populations ability to adapt to local conditions</p> <p>Culex pipiensspread of insecticide-resistant alleles used to treat mosquitoes to prevent spread of West NileGene Flow</p> <p>outcome of NS is not randomNS increases frequencies of alleles that provide reproductive advantage so, leads to adaptive evolutionNS acts directly on the phenotype &amp; indirectly on the genotypeNatural Selection is only mechanism that consistently causes adaptive evolution </p> <p>the contribution an individual makes to the gene pool of the next generation, relative to the contribution of other individuals in the populationRelative FitnessDIRECTIONAL SELECTION conditions favor individuals favoring one extreme of a phenotypeshifts curve in one direction or other</p> <p>Types of Selection</p> <p>2. DISRUPTIVE SELECTIONconditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypesTypes of Selection</p> <p>3. STABILIZING SELECTIONconditions favor the intermediate phenotype and act against both extremesreduces variationTypes of Selection</p> <p>Natural selection will increase the frequencies of alleles that enhances survival &amp; reproduction so over time adaptations arise</p> <p>genetic drift &amp; gene flow may cause changes that are either advantageous or disastrousADAPTIVE EVOLUTIONindividuals with certain inherited characteristics are more likely to obtain matescan result in sexual dimorphism: marked differences between the 2 sex characteristics Sexual SelectionSexual Dimorphism: House Finches</p> <p>Intrasexual Selectionselection w/in same sexAlpha maleIntersexual Selectionaka mate choicefemales choosy about their mate (often depends on male showiness)Mechanisms of Sexual SelectionIntrasexual Selection</p> <p>Intersexual Selection: Sandpiper male</p> <p>1 hypothesis: females have linked good genes with traitstudy: gray tree frog2nd hypothesisfemales have linked good health with these traitsstudy: birds How do female preferences devlop?neutral variation: differences in DNA that do not confer an advantage or disadvantageWhy dont all genes move toward neutrality?tendency for directional or stabilizing selection countered by mechanisms that preserve or restore variationThe Preservation ofGenetic Variationrecessive alleles hidden and carried forward in heterozygotesheterozygote protection maintains a huge pool of alleles that might not be favored under present conditions, but could bring benefits in environment changesDiploidyAdvantages determined by Alleles</p> <p>occurs when natural selection maintains 2 or more forms in a population</p> <p>2 types: heterozygote advantagefrequency-dependent selectionBalancing SelectionHeterozygotes have survival advantage</p> <p>If phenotype of a heterozygote is intermediate between the 2 homozygotes then this advantage is: stabilizing selectionIf phenotype of heterozygote same as dominant homozygote this advantage is directional selectionHeterozygote AdvantageExample: Sickle Cell</p> <p>Heterozygote Advantage</p> <p>SShomozygous dominantno protection against malariaSsheterozygousprotection against malariafew sickle cells but not harmfulsshomozygous recessivedie young of sickle cellSickle Cell Allelethe fitness of a phenotype depends on how common it is in the populationFrequency-Dependent SelectionScale-Eating Fish in Lake Tanganyika</p> <p>eat scales off flank of preysome left-mouthed some right-mouthedright-mouthed dominant to left-mouthedScale-eating Fishselection favors whichever mouth phenotype is least common (prey fish learn to avoid attacks from more common </p> <p>Why Natural Selection does not Result in a Perfect Organism1. Selection can only act on existing variationsNS favors only the fittest available phenotypes</p> <p>2. Evolution is limited by historical constraintsNS has to work with existing structures</p> <p>3. Adaptations are often compromiseseach organisms must do many things: some structures are a compromise (Walrus fins great for swim, not so good for walking on rocks)</p> <p>4. Chance, natural selection, &amp; the environment interactfounding population may not carry best alleles for new environment; environments can change </p>