badshah

1. Lecture 6
Modern concept of natural selection continued
Natural selection
Natural selectionisthe differential survivaland/orreproductionof organismsasa functionof theirphysical attributes.
Because of theirphenotypes,whichare due tothe amalgamof traits that make upan individual,some individualsdo
betterthanothers.
Selectionpatterns
Selectionisdefinedassome sortof functional relationshipbetweenfitnessandphenotype andwe caneasilydescribe
fitnessintermsof three kindsof curves:
1. Directional selectioninwhichthe traitislinearlyrelatedtofitness,
2. Stabilizingselectioninwhichthere isanoptimal value forthe traitof interest,and
3. Disruptive selectioninwhichindividualswiththe smallestandlargestvaluesof the traithave the highestfitnessand
individualswithintermediate valuesare ata fitnessdisadvantage
 Directional Selection Favors variantsof one extreme.
 StabilizingSelection Acts uponextremesand favors the intermediate.
 DiversifyingSelection Favors variantsof opposite extremes.
Each Mode of selectionaltersthe meanorvariance of the phenotypictraitina populationorspecies.In the longterm,
directional selectioncanhave the mostdramaticimpact onthe evolutionof aspecies.Directionalselectioncanleadto
the formationof a newtype froman existingtype.Thiscontrastswiththe actionof stabilizingselectionwhichmaintai ns
the existingtype withoutchange inmeanoverlongperiodsof time.Stabilizingselectioneliminatesthe extremesina
distributionof phenotypes,andassuchit leadstoa refinementof the existingtype.Byeliminatingindividualsfromthe
centerof the distribution,disruptive selectionfavorsthe 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.Sometimesmanyspeciesevolve fromasingle
ancestral species.Fourteenspeciesof Darwinfinchesevolvedfromone ancestral species. Suchanevolutionarypattern,
inwhichmany relatedspeciesevolvedfromasingle ancestral species,iscalledadaptive radiation.Adaptive radiation
mostcommonlyoccurs whenaspeciesof organismssuccessfullyinvadesanisolatedregionwhere few competing
speciesexist.If newhabitatsare available,new specieswillevolve.
Extinction
Natural selectionisaprocessthroughwhichthe traitsthat are helpful inthe survivalof anorganismare passedonto
the nextgenerationwhile those thatcanhindersurvival are eliminated.Natural selectionthenonlyallowsthe verywell
adaptedspeciestoreproduce.Thisisdone toensure survival inthe ever-changingenvironmental conditions.Due tothe
evolutionsthathave beentakingplace,the habitatof organism change andto cope up withthese changes,the
organismsdevelopsome geneticcharacteristicsthatwouldenhance theirsurvival.However,evenwithnatural
selection,studieshave shownthatsome organisms,bothplantsandanimalshave becomeextinct.Dinosaursbecame
extinctsixtyfive millionyearsago.Anotherexample isthe thylacine tigerof Australia.
The minorityof the organismsinthe environmentare the onesthatbecome extincttogetherwiththose thatare unable
to survive the changesinthe environment.The minorityare unable tocompete withthe majorityforsurvivalmeans.
Moreover,whennewspeciesare beingformedormodifiedtheyexertpressure onthe existingonesandfinally
eliminatethem.The introductionof newbreedsof animalsleads tothe extinctionof the inferiorones.
The main reasonwhyextinctionstill occursisthatnatural selectionisgoodatmaintainingandevenimprovingspecies
adaptationswhile onthe otherhanditeliminatesthose speciesthatare the minorityandthose that are not well
adapted.Itis thenclearthat natural selectiondoesnotonlyplayarole of preventingextinction,italsosupports
Directional selection stabilizingselection disruptive selection

2. extinctionof the speciesthatare not well adaptedforsurvival.Thisbeingthe case then,extinctionisboundtoprevail
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
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,replacement of gillsbylungswhenvertebratesemerged ontodry land.
Co-evolution
Evolutionary change,inwhichone speciesact as a selective force ona second species,inducingadaptationsthatinturn
act as selective force onthe firstspecies.
In evolutionarybiology, mimicryisasimilarityof one speciestoanotherwhichprotectsone orboth.
Examplesof coevolution:mimicry
The classical examplesof mimicryillustrate nicelydifferenttypesof co-evolutionaryinteractions.
Müllerianmimicry describesthe convergenceof unpalatablemodelstoasimilarphenotype,i.e.reciprocal evolution
betweenspeciesall of whichare distasteful.Itisthuscharacterizedasa +/+ mutualisticinteraction,i.e.all involved
speciesbenefit
Batesianmimicry describesthe convergence of apalatable speciestounpalatable models.A non-toxic,ediblespecies
mimicsthe warningcolorof a toxic,noxiousmodel.Thisisa0/+ interaction,becauseonlyone speciesbenefits.The
systemworkswell aslongasthe mimicdoesnotbecome toofrequent.Otherwise, the noxiousmodel mayhave a
disadvantage (i.e.the interactionmayturnintoantagonism, –/+),because predatorsdonotavoidthe warningcolorany
more.
Sexual selection
Natural selectiontendstoproduce individualsthatare well adaptedtotheirenvironment.However,sexual selection
doesnotadapt the individual tothe environmentbutdoesenhance traitsinvolvedinmate acquisition.Moreover,sexual
selectioncanproduce individualswithsuchelaborateornamentsthattheymustbe eitherenergetically costlyto
develop,costlytomaintain,orevenleadtoa directsurvival costforthe individual thatbearsthe ornament.Darwin's
theoryof sexual gave a plausibleexplanationforthe originof manysplendidif notbizarre ornaments.
Darwin’s second“major” book: 1871. On the Descentof Man, and SelectioninRelationtoSex.Darwinneededatheory
to explainthe 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 basicprinciples
are identical –selectionfavorswhatevergetsmore genesintothe nextgeneration.Insexual selection,fitnessis
measuredrelativetomembersof 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 greatlyimpairshis mobility…how couldsuchatraitevolve?
Long-tailedwidowbird:Maleswithunnaturallylongtailsattractfemalesawayfromthe nestsof “normal”malesor
maleswithshortenedtails
Hypothesesfor male ornaments
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 developandmaintain Choosingamate with
“good genes”requiresanhonest signal of geneticquality. Onlymalesingoodcondition(those withgoodgenes) willbe
able to fullydevelopandmaintainanornament.