PrinciplesofEvolutionHowDidEvolutionaryThoughtEvolve?Earlybiologicalthoughtdidnotincludetheconceptofevolution.Explorationofnewlandsrevealedastaggeringdiversityoflife.FossildiscoveriesshowedthatlifehadchangedoverTime.Somescientistsdevisednon-evolutionaryexplanationsforfossils.Afewscientistsspeculatedthatlifehadevolvedwithtime.GeologyprovidedevidencethatEarthisexceedinglyold.Somepre-Darwinbiologistsproposedmechanismsforevolution.DarwinandWallaceproposedamechanismofevolution.DarwinstudiedagroupofcloselyrelatedspeciesoffinchesontheGalapagosIslands.Eachspeciesspecializesineatingadifferenttypeoffoodandhasabeakofcharacteristicsizeandshape,becausenaturalselectionhasfavoredtheindividualsbestsuitedtoexploiteachfoodsourceefficiently.Asidefromthedifferencesintheirbeaks,thefinchesarequitesimilar.

PrinciplesofEvolution

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Largegroundfinch,beaksuitedtolargeseedsSmallgroundfinch,beaksuitedtosmallseedsWarblerfinch,beaksuitedtoinsectsd. Vegetariantreefinch,beaksuitedtoleavesHowDoesNaturalSelectionWork?ModernGeneticsconfirmedDarwin’sassumptionofinheritance.NaturalSelectionmodifiespopulationsovertime.Fossilsprovideevidenceofevolutionarychangeovertime.ComparativeAnatomygivesevidenceofDescentwithModification.HomologousstructuresprovideevidenceofCommonAncestry.Functionlessstructuresareinheritedfromancestors.Manyorganismshavevestigialstructuresthatservenoapparentfunction.The(a)salamander,(b)whale,and(c)snakeallinheritedhindlimbbonesfromacommonancestor;thebonesremainfunctionalinthesalamanderbutarevestigialinthewhaleandsnake.HowDoWeKnowThatEvolutionHasOccurred?Someanatomicalsimilaritiesresultfromevolutioninsimilarenvironments

EmbryologicalsimilaritysuggestscommonancestryModernbiochemicalandgeneticanalysesrevealrelatednessamongdiverseorganisms.Convergentevolutioncanproduceoutwardlysimilarstructuresthatdifferanatomically.Thewingsof(a)insectsand(b)birdsandthesleek,streamlinedshapesof(c)sealsand(d)penguinsareexamplesofsuchanalogousstructures.Theearlyembryonicstagesofa(a)lemur,(b)pig,and(c)humanshowstrikinglysimilaranatomicalfeatures.WhatIstheEvidenceThatPopulationsEvolvebyNaturalSelection?ControlledBreedingModifiesOrganismsEvolutionbyNaturalSelectionOccursTodayWhenfewerpredatorsarepresent,brightercolorationcanevolve.NaturalSelectioncanleadtopesticideresistance.ExperimentscandemonstrateNaturalSelection.SelectionactsonrandomvariationtofavorthetraitsthatworkbestinparticularenvironmentsDogdiversityillustratesartificialselectionAcomparisonof(a)theancestraldog(thegraywolf,Canislupus)and(b)variousbreedsofmoderndogs.Artificialselectionbyhumanshascausedagreatdivergenceinsizeandshapeofdogsinonlyafewthousandyears.HowArePopulations,Genes,andEvolutionRelated?Genesandtheenvironmentinteracttodeterminetraits.Thegenepoolisthesumofthegenesinapopulation.Evolutionisthechangeovertimeofallelefrequencieswithinapopulation.Theequilibriumpopulationisahypotheticalpopulationthatdoesnotevolve.WhatCausesEvolution?MutationsarethesourceofGeneticVariabilityMutationsarerare,butimportantMutationsarenotGoal-DirectedMutationsoccurspontaneouslyGeneflowbetweenpopulationschangesAlleleFrequencies.Allelefrequenciesmaydriftinsmallpopulations.PopulationsizemattersApopulationbottleneckIsanexampleofGeneticDriftPopulationbottlenecksreducevariationTheEffectofPopulationSizeonGeneticDriftEachcoloredlinerepresentsonecomputersimulationofthechangeovertimeinthefrequencyofalleleAina(a)largeor(b)smallpopulationinwhichtwoalleles,Aanda,wereinitiallypresentinequalproportions,andinwhichrandomlychosenindividualsreproduced.PopulationBottlenecksReduceVariation(a)Apopulationbottleneckmaydrasticallyreducegeneticandphenotypicvariationbecausethefeworganismsthatsurvivemaycarrysimilarsetsofalleles.Both(b)thenorthernelephantsealand(c)thecheetahpassedthroughpopulationbottlenecksintherecentpast,resultinginanalmosttotallossofgeneticdiversity.WhatCausesEvolution?IsolatedfoundingpopulationsmayproducebottlenecksMatingwithinapopulationIsalmostneverrandomAllgenotypesarenotequallybeneficialAntibioticresistanceevolvesbynaturalselectionPenicillinresistanceillustrateskeypointsaboutevolutionAHumanExampleoftheFounderEffectAnAmishwomanwithherchild,whosuffersfromasetofgeneticdefectsknownasEllis–vanCreveldsyndrome(shortarmsandlegs,extrafingers,and,insomecases,heartdefects).ThefoundereffectaccountsfortheprevalenceofEllis–vanCreveldsyndromeamongtheAmishresidentsofLancasterCounty,Pennsylvania.ACompromiseBetweenOpposingPressures(a)Amalegiraffewithalongneckisatadefiniteadvantageincombattoestablishdominance.(b)Butagiraffe'slongneckforcesittoassumeanextremelyawkwardandvulnerablepositionwhendrinking.Thus,drinkingandmale–malecontestsplaceopposingevolutionarypressuresonnecklength.HowDoesNaturalSelectionWork?NaturalselectionstemsfromunequalreproductionNaturalselectionactsonphenotypesSomephenotypesreproducemoresuccessfullythanothers.Anenvironmenthasnon-livingandlivingcomponentsCompetitionactsasanagentofselectionBothpredatorandpreyactasagentsofselectionSexualselectionfavorstraitsthathelpanorganismmateSelection