Evolution lecture 4 & 5
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Macroevolution refers to large evolutionary changes that occur at or above the species level, in contrast to microevolution which involves smaller changes within a species. Adaptive radiation is the evolution of many descendant species from a single ancestor, with each species adapted to a different ecological niche. A classic example is Darwin's finches, which evolved from a common ancestor into 14 species with varying beak sizes and shapes adapted to different food sources. Orthogenesis proposed that evolutionary changes occur in a straight line, while allometry describes how body part size relates to overall body size and its evolution.
Evolution lecture 4 & 5
- 1. EvolutionLecture 4 Factors of large evolutionarychanges (Macroevolution) Macroevolution Macroevolutionarystudiesfocusonchange that occurs at or above the level of species,incontrastwith microevolution,whichreferstosmallerevolutionarychanges (typicallydescribedaschangesinallelefrequencies) withinaspeciesorpopulation. Adaptive radiations The term adaptive radiationwascoinedbyH.F.Osborn(1902). It isthe evolutionof anumberof descendantswitha great varietyof adaptationsto differentnichesfromasingle ancestor. The phenomenonof adaptive radiationisthe diversificationof adominantevolutionarygroupintoalarge numberof subsidiarytypesadaptedtomore restrictivemodesof life(differentadaptivezones)withinthe range of the larger group Accordingto George GaylordSimpson(1940, 1953), adaptive radiationisthe rapidproliferationof new taxa(species) froma single ancestral group. Certainauthorsof evolutionbiologysuchasSavage (1969),Stanley(1979) and Volpe (1985) have usedan entirelynew termmacroevolutionforthe Osborn’slaw of adaptive radiation. Examplesof Adaptive Radiation Adaptive radiationinDarwin’sfinches. A classical exampleof speciationinvolving the interplayof complex forces(suchas isolation,competition,adaptation,etc.) leadingtoadaptive radiationatthe specieslevel isprovidedbyDarwin’s finches. These birdsbelongtolargest familyof birds,the Fringillidae andlive in GalapagosIslandsinthe Pacificocean. The study of these small darkbirdsin theirnative habitatgave Darwinhisfirst insightintoevolutionaryprocesses. The present-dayassemblagesof Darwin’s finchesdescendedfromsmall sparrow- like birdsthatonce inhabitedthe mainlandof SouthAmerica. The ancestorsof Darwin’sfincheswere earlymigrantsto the GalapagosIslandsandprobablythe firstlandbirdsto reachthe islands.These earlycolonists have givenrise to14 distinctspecies,eachwelladaptedtoaspecificniche.Thirteenof these speciesoccur inthe Galapagos,one isfoundinthe small isolatedCocosIsland,northeastof Galapagos.Notall 13 speciesare foundon each island.These 14speciesbelongto4 genera: 1.Geospiza 2. Camarhynchus 3. Certhidea 4. Pinaroloxias. The most conspicuousdifference amongthe speciesare inthe sizesandshapesof the beak,whichare correlated withmarkeddifferencesinfeedinghabits. The ancestral finchwasa ground-dwelling,seed-eatingfinch.Afterthe burst of speciationinthe Galapagos,atotal of 14 speciesexist: ground-dwellingseed-eaters;some livingon cactusesand eatingseeds;some livingintreesandeatingseeds;andspeciesof tree-dwellinginsect-eaters. Orthogenesis Orthogenesisisthe assumptionthatevolutionarychangesorvariationsare producedinstraightlines.The observed tendencyof a part or organ to change progressivelyinsize iscalledorthogenesisorevolutioninastraightline. The theoryof orthogenesiswasproposedbyEimerin1897.
- 2. Evidencesoforthogenesis Analogousorparallel variationsare of frequentobservation. Theyare the modificationof similarcharacterswhich appearin differentbranchesof the same large groupor inunrelatedgroup. For example the total reductionof side toesamongartiodactylinseveral unrelatedgenerasuchasgiraffe,camel and prong-buck (Antilocarpa) Anotherexampleexistedbetweenthe pseudo-horsesof SouthAmericaandtrue horsesof northern hemisphere, bothlinesshowingathree-toedandfinallyaone-toedmodification. There are several structureswhichevolvedbeyondthe pointof usefulness, suchastusksof Jeffersonmammothand antlersof Irishdeer. Allometry Allometryisan importantmethodfordescribingmorphologicalevolution. Itisthe relationbetweenthe size of an organismandthe size of anyof itsparts. Withina taxon,mostanimalsshare a commonbodyplanthat comprisesacertainnumberof body partsarranged in a particularway.For example,all insectsshare ahead,thorax,abdomen,one pairof antennae,twopairsof wings and three pairsof legs.Yetout of this simple bodyplanhasarisenthe enormousdiversityof insectformsthatwe see aroundus. Most of thisdiversityisaresultof changesinthe relative size of the differentbodyparts. Evolutionlecture 5 Modernconcept of natural selection Selection In a populationsome individualsare more successful thanothersinpassingalongtheirgenesintothe next generation.Thismayoccurfor manyreasons. Some organismsdie before theyreachreproductive age. Some organismsfail tofinda mate. Amongthose individualsthatdobreed,there are differencesinthe numberof offspringsproduced. The resultof these differencesmeansthatforone reasonorthe otherthere isa differential rate of reproduction amongmembersof a population,we call this selection. Selectionpatterns Both innatural selectionandartificial selection, the principle isthe same:some organismsbreedmore prolifically than others,thus,increasingthe frequencyof some genesanddecreasingthe frequencyof others. The followingthree patternsof natural selectionhave beenrecognizedbypopulationgeneticists: Directional, StabilizingandDisruptive (diversifying). 1. Directional Selection The directional selectiondescribesthe change thatoccurswhena populationshows aparticulartrendthroughtime. Such selectionprocess favors individualsthatare bestadaptedtonew situationsortonew ecological opportunities Under directional selection,the advantageousalleleincreasesasa consequence of differencesinsurvivaland reproductionamongdifferentphenotypes.The increasesare independentof the dominance of the allele,andevenif the allele is recessive,itwill eventuallybecomefixed. In fact,directional selectiontransformsthe gene pool of aspeciesorpopulationtowardthe highestlevel of adaptednessthatcanbe reachedinthe newenvironment. E.g Industrial melanisminmoth.
- 3. 2. StabilizingSelection Stabilizingselectionis atype of natural selection inwhich geneticdiversitydecreases. Itdescribesthe change when extreme individualsare eliminatedfromthe population.Sothe intermediate valuesforagiventraitare favoredover the extreme values.The resultof thisprocessisa reducedvariabilityinthe population. A classicexample of thisishumanbirthweight.Babiesof low weightloseheatmore quicklyandgetill from infectiousdiseasesmore easily,whereasbabiesof large bodyweightare more difficulttodeliverthroughthe pelvis. Infantsof a more mediumweightsurvivemuchmore often.Forthe largeror smallerbabies,the babymortalityrate ismuch higher. 3. Disruptive Selection Thisform of selectionoccurswhenthe extremevalueshave the highestfitness andthe intermediatevaluesare relativelydisadvantageousintermsof reproductive effectiveness. For example,shellpatternsof limpets(marine mollusks). Limpetstypicallydwellinone of twodistincthabitats,attachingeithertowhite goose-neckbarnacles,ortotan- colored rocks.As mightbe expected,the light-coloredlimpetsseekthe protectionof the white barnacles,whereas tan limpetslivebychoice almostexclusivelyonthe darkrocks.Limpetsof intermediateshell patternsare conspicuousand are intenselyselectedagainstbypredatoryshore birds. Levelsof selection The geneticmakeupof a populationisalteredthroughaninteractionwiththe ecologyof the organism.We referto thisinteractionasthe processof natural andsexual selection.The fundamentalpremise of Darwinianselectionis that natural selectionactsonthe individual,ormore properly,differencesinphenotype amongindividualswithina population In recentyearsa numberof authorshave argued thatselectionmightactat a numberof differentlevelsandthese levelsof selectionare looselystructuredaccordingto hierarchies of biologicalorganization: Genes->individuals-> kin -> groups -> species Laboratory and fieldexamplesregardingaction of natural selection 1. MelanisminMoths or Industrial Melanism The pepperedmoth(Bistonbetularia) isamothwhichflies duringthe nightand restson treesduringthe day,where itis camouflagedtohide frombirds.The twomostcommon formsare called typicall,apale speckledmothwhichiswell- disguisedon light-coloredlichensgrowingontrees,andcarbonaria,ablack (ormelanic) mothwhichiseasytosee on the same background.Both are differentformsof the same species,likehumanswithblonde orbrownhair.Before the Industrial Revolution,whenthere wasmuchlesspollution,manytreeswere coveredinlichenand the typical formwas well-camouflagedwhenrestingonthem.Because the carbonariaformstoodoutagainstthis pale background,birdsfound carbonariamothsmuchmore easilythan typical,so carbonariawere more likelytobe eaten,and so lesslikelytosurvive topassontheirgenes.Thismeantthatthe carbonaria formwas rare. As coal-burningfactorieswere built,airpollutionincreasedsignificantly,whichkilledoff the lichensandblackened the treeswithsoot.On thisdark background,the pale typical mothswere nolongerwell-camouflagedandwere easilycaughtbybirds,butthe carbonariamothswere harderfor birdsto see,somore carbonariasurvivedtobreed and passon theirgenes,andthisdarktype of pepperedmothbecame more common.By1895, 98% of mothsin heavily-pollutedManchesterwere carbonaria.ArmedwithDarwin’snew theoryof natural selection,J.W.Tutt,an Englishentomologist, hypothesized in1896 that the change in colorwas due to selectionpressure basedon how oftenbirdswere able tospotthe moths.
- 4. 2. AustralianRabbits In 1859, a small colonyof 24 wildrabbits(Oryctolaguscuniculus)wasbroughtfromEurope toan estate inVictoriain the southeasterncornerof Australia.Fromsuchmodestbeginnings,the rabbitsmultipliedenormouslyandby1928 had spreadoverthe greaterpart of the Australiancontinent. Accordingto an estimate,the numberof adultrabbitwasover500 millioninanareaof about1 millionsquare miles. The rabbitscausedextensivedamage tosheep-grazingpasturesandtowheatcrop fields For controllingthe populationexplosionof these rabbits,the Australiangovernmentspenthuge sumsof moneyfor manyyears.Trapping,rabbit-proof fencing,poisoningof waterholes,andfumigationall provedtobe largely inadequate.Then,beginningin1950, outstandingsuccessinreducingthe rabbitpopulationwasachievedbya biological control method,i.e.,inoculatingrabbitswithavirusthat causesthe fatal disease myxomatosis The deadlymyxomaviruswasimplantedintothe tissuesof rabbitsinthe southernareaof Australia.Inaremarkably short periodof time,the virushadmade itsway,aidedbyinsectcarriers(mosquitoes),intomostof the rabbit- infestedareasof the continent.By1953, more than95 per centof the rabbitpopulationinAustraliahadbeen eradicated But, aftertheirdrasticdecline inthe early1950s, the rabbitpopulationsbegantobuildupagain. Evolutionary changeshave occurredin boththe pathogen (i.e.,myxomavirus) and the host (rabbit).Mutationsconferring resistance tothe myxomavirushave selectivelyaccumulatedinthe rabbitpopulations.Atthe same time,the viruses themselveshave undergone geneticchanges;lessvirulentstrainsof the virushave evolved(FrankJ.Fenner,1959). Laboratory example Gjedrem(1979) showedthatselectionof Atlanticsalmon(Salmosalar) ledtoanincrease inbodyweightby30% per generation.A comparative studyonthe performance of selectAtlanticsalmonwithwildfishwasconductedby AKVAFORSKGeneticsCentre inNorway.The traits,forwhichthe selectionwasdone includedgrowthrate,feed consumption,proteinretention,energyretention,andfeedconversionefficiency.Selectedfishhadatwice better growthrate, a 40% higherfeedintake,andanincreasedproteinandenergyretention.Thisledtoanoverall 20% betterFedConversionEfficiencyascomparedtothe wildstock. Atlanticsalmonhave alsobeenselectedfor resistance tobacterial andviral diseases.Selectionwas done tocheckresistance toInfectiousPancreaticNecrosis Virus(IPNV).The resultsshowed66.6%mortalityforlow-resistantspecieswhereasthe high-resistantspecies showed29.3% mortalitycomparedtowildspecies. Lecture 6 Modernconcept of natural selectioncontinued Natural selection Natural selectionisthe differential survivaland/orreproductionof organismsasa functionof theirphysical attributes.Because of theirphenotypes,whichare due tothe amalgam of traitsthat make up an individual,some individualsdobetterthanothers. Selectionpatterns Selectionisdefinedassome sortof functional relationshipbetweenfitnessandphenotype andwe caneasily describe fitnessin termsof three kindsof curves: 1. Directional selectioninwhichthe traitislinearlyrelatedtofitness, 2. Stabilizingselectioninwhichthere isanoptimal value forthe traitof interest,and 3. Disruptive selectioninwhichindividualswiththe smallestandlargestvaluesof the traithave the highestfitness and individualswithintermediate valuesare ata fitnessdisadvantage Directional Selection Favors variantsof one extreme. StabilizingSelection Acts uponextremesand favors the intermediate. DiversifyingSelection Favors variantsof opposite extremes.
- 5. Each mode of selectionaltersthe meanorvariance of the phenotypictraitina populationorspecies.Inthe long term,directional selectioncanhave the mostdramaticimpact onthe evolutionof aspecies.Directionalselection can leadto the formation of a newtype froman existingtype.Thiscontrastswiththe actionof stabilizingselection whichmaintainsthe existingtype withoutchange inmeanoverlongperiodsof time.Stabilizingselectioneliminates the extremesinadistributionof phenotypes, andassuchit leadstoa refinementof the existingtype.Byeliminating individualsfromthe centerof the distribution,disruptiveselectionfavorsthe individualsinthe tailsormore extreme valuesof the phenotype.Disruptive selectioncanleadtothe formationof twonew typesfroma single existingtype. Action of natural selectionleadingtoconvergence, radiation,regression and extinction Radiation Natural selectioncanultimatelyleadtothe formationof new species.Sometimesmanyspecies evolve fromasingle ancestral species.Fourteenspecies of Darwinfinchesevolvedfromone ancestral species.Suchanevolutionary pattern,inwhichmanyrelatedspeciesevolvedfromasingle ancestral species,iscalledadaptiveradiation.Adaptive radiationmostcommonlyoccurswhena speciesof organismssuccessfullyinvadesanisolatedregionwhere few competingspeciesexist.If newhabitatsare available,new specieswillevolve. Extinction Natural selectionisaprocessthroughwhichthe traitsthat are helpful inthe survivalof anorganismare passedonto the nextgenerationwhile those thatcanhindersurvival are eliminated.Natural selectionthenonlyallowsthe very well adaptedspeciestoreproduce.Thisisdone toensure survival inthe ever-changingenvironmental conditions. Due to the evolutionsthathave beentakingplace,the habitatof organismchange andtocope up withthese changes,the organismsdevelopsome geneticcharacteristicsthatwouldenhance theirsurvival.However,evenwith natural selection,studieshave shownthatsome organisms,bothplantsandanimalshave become extinct.Dinosaurs became extinctsixtyfive millionyearsago.Anotherexampleisthe thylacine tigerof Australia. The minorityof the organismsinthe environmentare the onesthatbecome extincttogetherwiththose thatare unable tosurvive the changesinthe environment.The minorityare unable tocompete withthe majorityforsurvival means.Moreover,whennewspeciesare beingformedormodifiedtheyexertpressure onthe existingonesand finallyeliminate them.The introductionof newbreedsof animalsleadstothe extinctionof the inferiorones. The main reasonwhyextinctionstill occursisthatnatural selectionisgoodatmaintainingandevenimproving speciesadaptationswhileonthe otherhandit eliminatesthosespeciesthatare the minorityandthose thatare not well adapted.Itisthenclearthat natural selectiondoesnotonlyplayarole of preventingextinction,italsosupports extinction of the speciesthatare not well adaptedforsurvival.Thisbeingthe case then,extinctionisboundto prevail evenif natural selectionisstill there. ConvergentEvolution Speciesfromdifferentevolutionarybranches maycome to resemble one anotherif theylive in verysimilar environments. For example,flighthas evolvedinbothbatsand insects,andtheybothhave wings,whichare adaptationstoflight. However,the wingsof batsandinsectshave evolvedfromverydifferentoriginal structures Regression Directional selection stabilizingselection disruptive selection
- 6. Natural selectionmayfavorRegression.Regression isaprocess of partial or complete education of organsthat have losttheiradaptive significance. For example, replacementof the notochord bya cartilaginous skeleton andlaterbya bony skeleton inthe process of vertebrate evolution, replacementof gillsbylungswhen vertebratesemerged ontodry land. Co-evolution Evolutionary change,inwhichone speciesact as a selective force ona second species,inducingadaptationsthatin turn act as selective force onthe firstspecies. In evolutionarybiology, mimicryisasimilarityof one speciestoanotherwhichprotectsone orboth. Examplesof coevolution:mimicry The classical examplesof mimicryillustrate nicelydifferenttypesof co-evolutionary interactions. Müllerianmimicry describesthe convergenceof unpalatablemodelstoasimilarphenotype,i.e.reciprocal evolution betweenspeciesall of whichare distasteful.Itisthus characterized asa +/+ mutualisticinteraction,i.e.all involved speciesbenefit Batesianmimicry describesthe convergence of apalatable speciestounpalatable models.A non-toxic,edible speciesmimicsthe warning colorof atoxic,noxiousmodel.Thisisa 0/+ interaction,because onlyone species benefits. The systemworkswellaslongas the mimicdoesnotbecome toofrequent.Otherwise,the noxiousmodel may have a disadvantage (i.e.the interactionmayturnintoantagonism, –/+),because predatorsdonotavoid the warningcolorany more. Sexual selection Natural selectiontendstoproduce individualsthatare well adaptedtotheirenvironment.However,sexual selection doesnotadapt the individual tothe environmentbutdoesenhance traitsinvolvedinmate acquisition.Moreover, sexual selectioncanproduce individualswithsuchelaborate ornamentsthattheymustbe eitherenergeticallycostly to develop,costlytomaintain,orevenleadtoa directsurvival costforthe individual thatbearsthe ornament. Darwin'stheoryof sexual gave a plausibleexplanationforthe originof manysplendidif notbizarre ornaments. Darwin’s second“major” book: 1871. On the Descentof Man, and SelectioninRelationtoSex Darwinneededatheorytoexplainthe manyextravaganttraitsthatseemtoreduce survival e.g.the peacock’stail. What is sexual selection? Sexual selectionisdistinguishedfromnatural selectionbythe followingcriterion:Sexual selectionarisesthrough variance inmatingsuccess Is sexual selectiondifferentfromnatural selection? Darwinsaw themas distinct - onlysexual selectioncouldproduce traitsthatcompromise survival.The basic principlesare identical –selectionfavorswhatevergetsmore genesintothe nextgeneration.Insexual selection, fitnessismeasuredrelativetomembersof the same sex Two kinds of sexual selection Intrasexual selection– matingsuccessdeterminedby within-sex interactions. e.g.,male-malecombat Intersexual selection–matingsuccessdeterminedby between-sexinteractions.e.g.,female choiceof males Extravagant male ornaments:The peacock’stail greatlyimpairshismobility…how couldsuchatraitevolve? Long-tailedwidowbird:Maleswithunnaturallylongtailsattractfemalesawayfromthe nestsof “normal”malesor maleswithshortenedtails Hypothesesfor male ornaments
- 7. Fisher’s“runaway” hypothesis:Mate choice originallyevolvedtofacilitate adaptive choice fortraitsconferringa survival advantage.Once female preference evolved,anygenesthat conferredsurvival advantage butcompromised attractivenesswouldnotbe passedonbecause survivingmaleswouldfail tomate Zahavi’s “handicap” hypotheses: Extravagantmale traitsare costlyto developand maintainChoosingamate with “good genes”requiresanhonest signal of geneticquality.Onlymalesingoodcondition(those withgoodgenes) will be able to fullydevelopandmaintainanornament.