CHAPTER 23 The Evolution of Populations Section A: Population Genetics The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance A population’s gene pool is defined by its allele frequencies 3. The HardyWeinberg theorem describes a nonevolving population Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction • One obstacle to understanding evolution is the common misconception that organisms evolve, in a Darwinian sense, in their lifetimes • Natural selection does act on individuals by impacting their chances of survival and their reproductive success • However, the evolutionary impact of natural selection is onlyapparentintrackinghowapopulationoforganisms changesovertime ã Itisthepopulation,notitsindividual,thatevolves Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Evolutiononthescaleofpopulations,calledmicroevolution,is definedasachangeintheallelefrequenciesinapopulation ã Forexample,thebentgrass(Argrostistenuis)inthisphotoisgrowing onthetailingsof anabandonedmine, richintoxic heavymetals Fig.23.1 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Whilemanyseedslandontheminetailingseachyear, theonlyplantsthatgerminate,grow,andreproduce arethosethathadalreadyinheritedgenesenabling themtotoleratemetallicsoils ã Individualplantsdonotevolvetobecomemoremetalư tolerantduringtheirlifetimes Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã TheOriginofSpeciesconvincedmostbiologists thatspeciesaretheproductsofevolution,but acceptanceofnaturalselectionasthemain mechanismofevolutionwasmoredifficult • What was missing in Darwin’s explanation was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for the precise transmission of these variations from parents to offspring • Although Gregor Mendel and Charles Darwin were contemporaries, Mendel’s discoveries were unappreciated at the time, even though his principles of heredity would havegivencredibilitytonaturalselection Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 1.Themodernevolutionarysynthesis integratedDarwinianselectionand Mendelianinheritance ã WhenMendelsresearchwasrediscoveredinthe earlytwentiethcentury,manygeneticistsbelieved thatthelawsofinheritanceconflictedwith Darwinstheoryofnaturalselection • Darwin emphasized quantitative characters, those that vary along a continuum • These characters are influenced by multiple loci • Mendel and later geneticists investigated discrete “eitheror” traits Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • An important turning point for evolutionary theory was the birth of population genetics, which emphasizes the extensive genetic variation within populations and recognizes the importance of quantitative characters • Advances in population genetics in the 1930s allowed the perspectives of Mendelism and Darwinism to be reconciled • Thisprovidedageneticbasisforvariationand naturalselection Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Acomprehensivetheoryofevolution,themodernsynthesis,tookforminthe early1940s ã Itintegrateddiscoveriesandideasfrompaleontology,taxonomy,biogeography,and populationgenetics ã ThearchitectsofthemodernsynthesisincludedgeneticistsTheodosius DobzhanskyandSewallWright,biogeographerandtaxonomistErnstMayr, paleontologistGeorgeGaylordSimpson,andbotanistG.LedyardStebbins Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Themodernsynthesisemphasizes: (1)theimportanceofpopulationsastheunitsof evolution, (2)thecentralroleofnaturalselectionasthemost importantmechanismofevolution,and (3)theideaofgradualismtoexplainhowlarge changescanevolveasanaccumulationofsmall changesoverlongperiodsoftime ã Whilemanyevolutionarybiologistsarenow challengingsomeoftheassumptionsofthemodern synthesis,ithasshapedmostofourideasabouthow populationsevolve Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 2.Apopulationsgenepoolisdefinedby its allele frequencies • A population is a localized group of individuals that belong to the same species • One definition of a species (among others) is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in a nature • Populations of a species may be isolated from each other,suchthattheyexchangegeneticmaterial rarely,ortheymayintergradewithlowdensitiesin anintermediateregion Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Diversifyingselectioncanresultinbalanced polymorphism ã Forexample,twodistinctbilltypesarepresentinblackư belliedseedcrackersinwhichlargerưbilledbirdsaremore efficientwhenfeedingonhardseedsandsmallerưbilled birdsaremoreefficientwhenfeedingonsoftseeds. Fig.23.14 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Stabilizingselectionfavorsintermediatevariants andactsagainstextremephenotypes ã Stabilizingselectionreducesvariationandmaintains thepredominantphenotypes • Human birth weight is subject to stabilizing selection • Babies much larger or smaller than 34 kg have higher infant mortality. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3. Natural selection maintains sexual reproduction • Sex is an evolutionary enigma • It is far inferior to asexual reproduction as measured by reproductive output • If a population consisted of half sexual females and half asexual females, the asexual condition would increase • All offspring of asexual females would be reproductive daughters • Onlyhalfoftheoffspringofsexualfemaleswouldbedaughters; theotherhalfwouldbenecessarymales Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Theoretically,sexhasatwoưfolddisadvantage. ã Afemaleproducingtwooffspringpergenerationwould generateapopulationofeightfemalesafterfour generationsifreproducingasexually,butonlyonefemale ifreproducingsexually. Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings Fig.23.15 ã Sexmustconfersomeselectiveadvantageto compensateforthecostsofdiminishedreproductive output. ã Otherwise,amigrationofasexualindividualsormutation permittingasexualreproductionwouldoutcompete sexualindividualsandtheallelesfavoringsex • In fact, most eukaryotes maintain sex, even in those species that can also reproduce asexually Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings • The “textbook” explanation for the maintenance of sex is that the process of meiosis and fertilization generate genetic variation on which natural selection can act • However, this hypothesis that sex is maintained in spite of its disadvantages because it produces future adaptation in a variable world is difficult to defend • Natural selection acts on the present, favoring individuals here and now that best fit the current, local environment • A stronger hypothesis would present advantages to sex that place a value on genetic variation on a generationtogeneration time scale Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Resistancetodiseasemaybeonecurrentvalueof variabilityovercomingthedisadvantagesofsex ã Parasitesrecognizeandinfecttheirhostsbyattachingto receptormoleculesonthehostscells ã Amongthehosts,individualsarelikelytohavedifferent allelesforthesereceptormoleculesand,therefore,varyin their vulnerability to parasites • At the same time, parasites evolve very rapidly in their ability to use specific host receptors • Sex provides a mechanism for changing the distribution of alleles and varying them among offspring • This coevolution in a hostparasite relationship has been called the “Red Queen effect.” Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 4.Sexualselectionmayleadtopronounced secondarydifferencesbetweenthesexes ã Malesandfemalesofaspeciesdiffernotonlyin theirreproductiveorgans,butoftenalsoin secondarysexualcharacteristicsthatarenot directlyassociatedwithreproduction ã Thesedifferences,termedsexualdimorphism,may includesizedifferences,colorationdifferences,enlarged orexaggeratedfeatures,orotheradornments ã Malesareusuallythelargerandshowiersex,atleast amongvertebrates Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Sexualdimorphismisaproductofsexualselection ã Intrasexualselectionisdirectcompetitionamong individuals of one sex (usually males) for mates of the opposite sex • Competition may take the form of direct physical battles between individuals • The stronger individuals gain status • More commonly ritualized displays discourage lesser competitors and determine dominance Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Intersexualselectionormatechoiceoccurswhen membersofonesex(usuallyfemales)arechoosyin selectingamongindividualsoftheothersex ã Maleswiththemostmasculinefeaturesarethemost attractivetofemales ã Interestingly,thesefeaturesmaynotbeadaptiveinother waysandexposetheseindividualstoextrarisks Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã However,eveniftheseextravagantfeatureshave somecosts,individualsthatpossessthemwillhave enhancedreproductivesuccessiftheyhelpan individualgainamate ã Everytimeafemalechoosesamatebasedonappearance orbehavior,sheperpetuatestheallelesthatcausedherto makethatchoice. ã Shealsoallowsamalewiththatparticularphenotypeto perpetuatehisalleles Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Theunderlyingbasesoffemalechoiceareprobably notaesthetic ã Currentresearchisinvestigatingthehypothesisthat females use these sexual advertisements to measure the general health of a male • Individuals with infections or other problems are likely to have a relatively dull, disheveled plumage • These individuals are unlikely to win many females • For the female that chooses a healthy mate, even if his inclination is just a prewired response to visual signals, thebenefitisagreaterprobabilityofhavinghealthy offspring Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 5.Naturalselectioncannotfashionperfect organisms ã Thereareatleastfourreasonswhynaturalselection cannotproduceperfection 1.Evolutionislimitedbyhistoricalconstraints ã Evolutiondoesnotscrapancestralanatomyandbuild fromscratch ã Evolutioncoưoptsexistingstructuresandadaptsthemto newsituations Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 2.Adaptationsareoftencompromises ã Organismsareoftenfacedwithconflictingsituationsthatpreventan organismfromperfectinganyonefeatureforaparticularsituation • For example, because the flippers of a seal must not only allow it to walk on land, but also swim efficiently, their design is a compromise between these environments • Similarly, human limbs are flexible and allow versatile movements, but at the cost of injuries, such as sprains, torn ligaments, and dislocations • Better structural reinforcement would compromise agility Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings 3.Notallevolutionisadaptive ã Chanceaffectsthegeneticstructureofpopulationstoa greaterextentthanwasoncebelieved ã Forexample,foundersofnewpopulationsmaynot necessarilybethebestindividuals,butratherthose individualsthatarecarriedintotheopenhabitatby chance 4.Selectioncanonlyeditexistingvariations ã Selectionfavorsonlythefittestvariationsfromthose phenotypesthatareavailable ã Newallelesdonotariseondemand Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ... by the HardyWeinberg theory Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER? ?23? ? The Evolution of Populations Section B: Causes of Microevolution Microevolution is generationtogeneration change in a population’s ... minimize the impact of sampling errors. Fig .23. 5 Copyrightâ2002PearsonEducation,Inc.,publishingasBenjaminCummings ã Bottleneckingisanimportantconceptin conservationbiologyofendangeredspecies ã Populationsthathavesufferedbottleneckincidentshave... natural selection Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings CHAPTER? ?23? ? The Evolution of Populations Section C: Genetic Variation, the Substrate for Natural Selection