badshah

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.