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GeneralEcology(BIO160) Worksheet#6 Dept.ofBiologicalSciences SacramentoStateUniversity Worksheet6:Intraspecificvariation 1. Whatisabsolutefitness?Relativefitness?Howaretheymeasured?Absolutefitnessisthetotal numberofsurvivingoffspringanindividualproducesinitslifetime.Itisquantifiedaccordinglyby simplycountingsurvivingoffspring.Relativefitnessisderivedfromtheratiooftheabsolutefitness ofagenotypedividedbytheabsolutefitnessofthefittestgenotype. 2. Darwinmadeasimpleandcogentargumentfornaturalselectionasamechanismofevolution. DescribethethreeempiricalobservationsthatDarwinmadeuponwhichhebasedhisargumentand thelogicaldeductionthatfollowsfromtheseobservations.Hisfirstobservationwasthatindividuals ofaspeciesvaryintheirtraitsandthatthisvariationwasheritable.Second,heobservedthat organismsproducemoreoffspringthansurvived.Third,herecognizedtheroleofcompetitionin natureandthedegreetowhichcompetitionforlimitedresourcesactedtolimitindividualsurvival andreproduction.ThelogicalconsequenceDarwinputforthbasedontheseobservationswasthat themostcompetitiveindividualswouldpersistandreproduce,whereasthosethatarenotwould die.Overtime,individualswiththesetraitswillbecomemorecommoninthepopulation. 3. Doindividualsorpopulationsevolvethroughtheprocessofnaturalselection?Explainyouranswer. Populationsevolve,butindividualsdonot.Naturalselectionisaprocessinwhichindividualswho arebetteradaptedtotheirenvironmentsurviveandproducemoreoffspringthanthosewhoare lesswelladapted.Becausenaturalselectionoccursatthelevelofthepopulation,thepopulationis consideredtheunitofevolution. 4. Whatisthedifferencebetweenageneandanallele?AgeneisaheritablesequenceofDNAthat codesforaparticularcharacteristicofandorganism(e.g.,flowercolor).Analleleisoneoftwoor morealternateformsofagene(e.g.,white,purple,etc.). 5. Individualsinpopulationsvaryintheirgenotypesandintheirphenotypes.Whatistherelationship betweengenotypeandphenotype?Isiteverpossibleforindividualswiththesamegenotype (clones)tohavedifferentphenotypes?Explainyouranswer.Thegenotypeisthetotalsumofgenes inanindividual,whereasthephenotypeisthephysicalexpressionorappearanceofthegenotype. Therefore,thephenotypeisinfluencedbyboththegeneticmake-upoftheindividualandits environment.Yes,itispossibleforindividualswiththesamegenotypetodifferinphenotype.This isduetotherangeofinfluencesthattheenvironmentcanhaveontheexpressionofthegenotype; thepossibilityofagenotypetogiverisetodifferentphenotypesundercontrastingenvironmental conditionsiscalled“phenotypicplasticity.” 6. Inaclassicstudy,Clausen,Keck,andHiesey(CKH)studiedthesourceofphenotypicvariationin Yarrow(Achilleamillefolium),aplantspeciesthatoccursacrossawiderangeofelevationsin California.ItvariesconsiderablyinheightfromthelowelevationcoastofCalifornia(whereitis taller)tothehighalpinezoneoftheSierraNevada(whereitismarkedlyshorter).Answerthe followingquestionsbasedontheirwork. a. WhatexperimentalapproachdidCKHusetodistinguishbetweenageneticandan environmentalbasisforphenotypicheightvariationinYarrow?Theyusedareciprocal transplantstudy. GeneralEcology(BIO160) Worksheet#6 Dept.ofBiologicalSciences SacramentoStateUniversity b. WhatdidCKHconcludeaboutthesourceofphenotypicvariationinYarrow?Hint:youranswer shouldaddresswhetherthevariationwasgenetic,environmental,orsomecombinationofthe twoandwhatevidencesupportsthatconclusion.Theheightvariationinyarrowacrossthis elevationgradientisduetoacombinationofgeneticandenvironmentalinfluences.Yarrows fromthehighelevationsmaintainedtheirsmallstature,suggestingthatgenotypedetermined theirgrowth.Theotheryarrowsshowedphenotypicplasticityintheirgrowthwhenplantedat locationsotherthanwheretheyoriginated,suggestingthattheenvironmentwasprimarily responsiblefortheirgrowthform. 7. Ensatinasalamandersoccuratmid-elevationsintheCoastRangeandSierraNevadaofCalifornia andarelivingexamplesofevolutionarydivergenceandecotypevariation. a. Explainhowthispatternofecotypicvariationcouldoccur.Ensatinasalamandersoccurovera largegeographicrangeinCAthatitselfhasawiderangeoflocalhabitatvariation.About5 millionyearsago,acommonancestralspeciesmigratedsouthwardfromOregontooccupymidelevationsintheCoastRangesandSierraNevada(formingaring-shapedgeographicrange). Sub-populationsofEnsatinaadaptedtolocalenvironmentalconditionsacrosstheirgeographic range.Neighboringsub-populationscaninterbreed.[However,thoseoccurringinthefarsouth oneithersideoftheringeithercannotinterbreedordosorarely.Thusareproductivebarrier existsthatlikelyoccurredbecausethosesouthernpopulationsoneithersideoftheringhave beenseparatedforalongtimeandappeartohavedivergedgeneticallytothepointwherethey havebecomereproductivelyisolated.] b. Whatevolutionaryforcescouldcausetheobservedcolorandpatterndifferencesontheskin amongsub-populationsofEnsatinasalamanders?Whymightthesecolorandpattern differenceshaveoccurred?Whattypeofselectionpressuremighthavebeenresponsible? Mutationandnaturalselectionaremostlikelytointroducenewcolorvariantsinthe populations.Suchcolorandpatterndifferenceslikelyaroseduetonaturalselectionthat favoredthosecolor/patterns.Alikelyselectionpressurethatmighthaveledtotheobserved color/patternvariationispredationbecausecolorandpatterncanactascamouflage(called crypticcoloration)andprovideprotectionfromvisualpredatorslikebirdsofprey.Thecryptic colorationspecifictoasup-populationwouldbespecifictothelocalhabitatconditions,suchas thecolorpatternofthesoilandvegetation. 8. Threegeneraltypesofselectionoccurinpopulations.Usingafrequencydistributiontorepresent thevariationofatraitwithinapopulation,illustratethethreetypesofselectionandexplainhow theyoccur.Figure2.11inyourtextillustratesfrequencydistributionsforeachtypeofselection. Directionalselectionoccurswhenthetraitsfavoredbyselectionareatoneendofthedistribution causingashiftinallelefrequenciestoanewoptimum.Stabilizingselectionoccurswhenthetraits favoredbyselectionarethoseneartheaverage,causingtheallelefrequencytoremainstable. Disruptiveselectionoccurswhenthetraitsfavoredbyselectionarealternativetraitsateitherendof thedistribution(extremes)andcausetheallelefrequencydistributiontobecomebimodal. 9. Figure2.12inyourtext(pg23)illustratestherangeofcolorpatternsobservedinMimulusflowers. Thesepatternsserveasnectarguidesforpollinators.Namethekindofselectionoccurringinthis plantspeciesandexplainwhyitoccurredinthisspecies.Thisisanexampleofdisruptiveselection becausetwocontrastingtraitsareselectedforsimultaneouslywhiletheintermediatetraitsare GeneralEcology(BIO160) Worksheet#6 Dept.ofBiologicalSciences SacramentoStateUniversity selectedagainst.Disruptiveselectionoccursbecausetwotypesofpollinatorsvisittheflowersand eachexertsdifferentselectionpressuresonthemorphologyofthenectarguides.Insectsprefer largeguidespointingtowardthecenteroftheflowerandhummingbirdssmallheartshapedguides. 10. PeterandRosemaryGrantarescientistswhohavestudiedtheevolutionaryecologyofDarwin’s finchesontheGalapagosIslandsformanyyears.Refertopage22andFigure2.10inyourtextto answerthefollowingquestions. a. Howdidamajordroughtaffectbeakdepthinthepopulationofmediumgroundfincheson DaphneIsland?Aseveredroughtoccurred(causedbyaLaNinaevent).Thedroughtcaused greatermortalityofplantsproducingsmallthanlargeseeds.Birdswithsmallerbeakssuffered greatermortalitythanlargebeakedbirds,therebyincreasingtheproportionoflargebeaked birdsinthepopulation. b. Whydidfincheswithlargebeakshavehigherfitnessthanthosewithsmallbeaks?Largebeaked birdswereabletocrackopenthelargehardseeds,whereassmallerbeakedbirdsinthe populationcouldnot.Asaconsequence,birdswithlargerbeakssurvivedbetterandhad greaterreproduction(i.e.,fitness)thanksmallbeakedbirds. c. Whattypeofselection(directional,stabilizing,ordisruptive)wasevidencedinthispopulationof groundfinches?Whatevidencecanyousitetosupportyouranswer?Directionalselection. Thisisevidencedbytheobservedshiftinthefrequencydistributionofbeaksizetowardoneend ofthedistributionforthattrait.
