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9/27/16 Genetic Variation Genetic Variation: The genetic substrate for natural selection Sex: Sources of Genotypic Variation Dr. Carol E. Lee, University of Wisconsin • If there is no genetic variation, neither genetic drift nor natural selection would be able to change allele frequencies, because there would be nothing to change • Natural Selection requires genetic variation upon which it could act • So, I’m going to talk about genetic variation today, to prepare you for the lecture on Natural Selection in a few lectures Sources of Genetic Variation OUTLINE: (1) ReductionofVariation:GeneticDriftà Inbreeding (2) SourcesofAllelic Variation:Mutations (3) SourcesofGenotypic Variation:Sex(Meiosis) (4)Heritablevariationwithoutchangesinthegenetic code:EpigeneticInheritance • LastlecturefocusedonMutation • Sex: Shufflingofcombinationsofalleles Nonovelalleles,onlynovelgenotypes SEX: SourceofGenotypicVariation SEX Meiosis With Crossing Over VideoReviews: http://highered.mcgrawhill.com/sites/9834092339/student_view 0/chapter11/meiosis_with_crossing_ove r.html http://www.youtube.com/watch?v=4B07 1d9Ywbc What is SEX? Changes composition along chromosomes and shuffles alleles into new genotypes 1 9/27/16 What happens during SEX? Meiosis Meiosis (produce gametes): Separates homologous chromosomes What happens during SEX? Meiosis Meiosisgeneratesgenotypic diversityintwoways: Recombination: exchange of genetic information between the pairs of chromosomes (1)Physicalexchangeofhomologous chromosomalregionsbyhomologous recombinationresultsinnew combinationsofDNAwithin chromosomes. Random Mating (shuffle the gametes): Newly formed haploid chromosomes (sperm & eggs) come together à Shuffling of allelic combinations into new genotypes (2) Separationofhomologous chromosomepairsà allowsrandom andindependentshufflingofhaploid chromosomes(gametes)during randommating Genetic Recombination • Definition:aprocessbywhichamolecule ofnucleicacid(usuallyDNA,butcanalso beRNA)isbrokenandthenjoinedtoa differentone. Consequence of Sex During Eukaryotic Recombination, the crossover process leads to the offspring having different combinations of alleles from those of their parents along the chromosomes. Consequence of Sex: GeneticRecombinationà reducesLinkage Disequilibrium:breaksupassociationsofallelesalong achromosome Consequence of Sex: GeneticRecombination:Mixesupcombinationsof allelesacrossloci(reducesLinkageDisequilibrium) And, And, RandomMatingà increasesgenotypicvariation: RandomMating:Mixesupcombinationofallelesat Mixesupcombinationsofallelesatagivenlocus agivenlocus(increasesgenotypicvariation) 2 9/27/16 AnimportantconsequenceofEukaryoticSex Meiosis & Crossing over Consequence of recombination: Reduce Linkage Disequilibrium Linkage Disequilibrium • Thenon-randomassociationofallelesattwoormore loci,notnecessarilyonthesamechromosome. • Theoccurrenceofsomecombinationsofallelesor geneticmarkersinapopulationmoreoftenorless oftenthanwouldbeexpectedfromarandom formationofhaplotypesfromallelesbasedontheir frequencies. • Linkagedisequilibriumcanbecausedbyevolutionary factorssuchasnaturalselectionandgeneticdrift. • Recombinationwillbreakdownlinkage disequilibrium Linkage Disequilibrium The nonrandom association of alleles is often caused by natural selection or genetic drift For example, in Northern latitudes, lack of sunlight might simultaneously allow light hair, light skin and blue eyes to evolve, even though these traits are encoded by different genes So that these traits are associated with each other in a nonrandom manner However, recombination with dark hair, dark skin, dark eyed individuals will break down the nonrandom associations, and breakdown linkage disequilibrium, so that you might end up with individuals with dark skin and light hair, or dark skin and blue eyes 3 9/27/16 Linkage Disequilibrium (LD) • Manysmallandisolatedpopulationsareinlinkage disequilibrium(example:Finnishdiseaseheritage) • ManyagriculturalspecieshavehighlevelsofLDdueto strongartificialselection(human-induced) Whydowecare? • Usefultoknowwhichallelesarecommonlyassociated withoneanother • PatternsofLDcantellusabouttheevolutionary historyofthepopulation(geneticdrift,natural selection) Random mating during SEX Consequence of Sex: The haploid chromosomes (sperm and egg) come together Can get combination of favorable mutations more quickly GeneticRecombination:Mixesupcombinationsof allelesacrossloci(reducesLinkageDisequilibrium) And, Possible genotype combinations Favorable Mutations in different alleles RandomMating:Mixesupcombinationofallelesat agivenlocus(increasesgenotypicvariation) This shuffling of alleles is thought to have many advantages, as a major engine of generating genotypic variation Random mating during SEX Random mating during SEX Can remove unfavorable mutations more quickly Through sex can bring the favorable mutations together Possible genotype combinations X Deleterious (bad) Mutations 4 9/27/16 Random mating during SEX Sex will occasionally bring the deterious mutations together X WhataretheBenefitsofSex? As homozygotes, the deleterious mutations can now be selected OUT of the population, as they are no longer masked in the heterozygous state, but are exposed as homozygotes BenefitsofSex • • Genetic Recombination à Breakdown of Linkage Disequilibrium Random Mating à – Increase genotypic diversity: Drosophila have 10,000 functional loci; if an individual is heterozygous at 10% loci, can produce 21000 or 10300 different gametes (different combinations of loci) – – Bringtogetherfavorablemutationsacrossloci – create individualsfreeofdeleteriousmutations Bringtogetherunfavorablemutationsacrossloci – deleteriouscombinationscanbeselectedoutofthe population Cost of Sex BenefitsofSex? • Randommatingbenefitsthespeciesbyincreasing therateofevolution – Increasestherateatwhichadvantageousmutations couldbebroughttogether – Increasestherateatwhichdeleteriousmutationscould bebroughttogetherandremovedbyselection • RemovalofDeleteriousmutationsmorequicklyà Allowssexuallyreproducingorganismstoavoid Muller'sratchet,inwhichthegenomesofan asexualpopulationaccumulatedeleterious mutations. (Weismann1886;Muller1932;CrowandKimura,1965) Sexisnotuniversal • SexoccursinEukaryotes,andnotinBacteriaorArchaea Loss of Fitness relative to clonal populations: Reduces population growth rate by 1/2, because males cannot reproduce – Bacteria:highmutationrateandhorizontalgenetransfer – Viruses:costofmistakes(mutations)arenotcriticalbecausethe hostisproducingalltheoffspring • Asexualityoccursofteninplants,andinmanyinvertebrates • Someeukaryotesareasexualuntiltheyexperiencealarge accumulationofdeleteriousmutations(mutationalmeltdown)or stress,thentheyhavesex(Daphnia,ciliates,dinoflagellates) • Someeukaryoteshavemorethan2sexes(someciliateshave32) 5 9/27/16 Within Eukaryotes • Sexualspeciestendtolastlonger • Asexualspeciesareoftengoodearly colonizersofnovelhabitatsbecauseof rapidgrowthrate(manyinvasiveplantsare asexual) • InEukaryotes,lessthan1%areasexual SEX: Benefits and Costs Benefits: § § § § § BreakdownLinkageDisequilibrium IncreaseinGenotypicVariation Purgedeleteriousmutationsmoreeasily Bringtogetherfavorablemutations Evolutionof“individuality” Costs: § § § § LowerReproductionRate(1/2) Havetofindmates(notallindividualsreproduce) Passononly½ofyourgenome Deathofuniqueindividualsintheparentalgeneration 1. Which of the following is most FALSE regarding the genetic substrate (variation) on which selection acts? (A) Sex creates new combinations of genotypes (B) Genetic drift could reduce the levels of allelic and genotypic variation (C) Inbreeding, caused by genetic drift, results in a high level of homozygosity in a population (D) Mutations are a source of allelic variation (E) Epigenetic modifications give rise to allelic diversity Cost of Individuality Many clonal organisms grow and divide and are in a sense “immortal” We are unique individuals and produce novel genetic architectures before dying (SEX = DEATH) Questions (1) What are the sources of genetic variation? (2) What are mutations and are they harmful or beneficial? (3) Why are there sex differences in mutation rate in the germ line? (4) What is sex and why did it evolve? (5) What are the costs and benefits of Sex? (6) What is the relationship between Genetic Variation and Natural Selection? 2. Which of the following alleles would tend to be removed MOST quickly from a population through natural selection? (Hint: play with the Allele A1 software and think about the results) (A) (B) (C) (D) Dominant highly deleterious allele Dominant slightly deleterious allele Recessive slightly deleterious allele Recessive highly deleterious allele 6 9/27/16 3. Which of the following is FALSE regarding inbreeding? 4. Which is NOT a consequence of Recombination (Sex)? (A) Inbreeding results from genetic drift (B) Populations with lower allelic diversity tend to have lower genotypic diversity (more homozygous) (C) Selection acts more slowly in inbred populations to remove deleterious recessive alleles (A) The increase in allelic diversity (B) Death (in an evolutionary sense) (C) The creation of many new genotypes across the genome (Evolution of individuality) (D) Reduction in population growth rate relative to clonal reproduction (1/2 of the population does not bear offspring) (D) One way to reduce inbreeding in a population is to bring in migrants from another population 5. Which of the following are INCORRECT regarding mutations? (A) Mutations can be harmful (B) Mutations can be beneficial (C) Mutations generate genetic variation (D) Most mutations have significant effects on fitness (E) Mutations accumulate to a much greater degree in the male germline (sperm) than in the female germline (eggs) with age • Answers: 1E 2A 3C 4A 5D 7