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1、13.Themajortransitionsinevolutionpage13-113.ThemajortransitionsinevolutionthUntilthemidofthe19century,naturalhistorywasdominatedbyaviewthatpostulatesaprogressivedevelopmentthatstartedwithnon-livingmaterialandprogressedfromplantsoversimpletomoreelaborateanimals.Lamarckpostulatedthatallliving
2、beingswouldhaveaninherenttendencytoclimbaladderofprogression.CharlesDarwindisagreedonthisnotion.Ratherthanhavinganintrinsictendencyforprogression,theprocessesofmutationandselectionleadtoorganismsthatarewelladaptedtotheirenvironment.Todayevolutionarybiologistsareawareofthefactthatbacteriaare
3、notlesssuccessfulintheirenvironmentthane.g.humans,noraretheylesswelladapted.However,withoutdoubtsomeorganismsaremorecomplexthanothers.Again,thereisneitheranytheoreticalreasontoexpect,noranytrueempiricalevidence,thatevolutionarylineagesincreaseincomplexity.Yetsometaxaaremorecomplexthanothers
4、,e.g.eukaryotesareobviouslymorecomplexthanprokaryotes(Figure13.1).JohnMaynardSmithandEörsSzathmarydevelopedthefruitfulideathatincreasesincomplexitymayhaveachievedasaresultofaseriesofmajorevolutionarytransitions,whichinvolvedchangesinthewayinformationisstoredand41transmitted.13.1.Whatisamajo
5、rtransition?ComplexityasinformationAssaidabove,anincreaseincomplexityisnotauniversalorinevitableevolutionarytrendinalltaxa.Sometaxamayevenshowdecreasingcomplexityifsuchreductionleadstoadaptationtotheirenvironment,atrendthatisoftenobservedinparasites.However,theprevailingimpressionisthatline
6、ageshaveincreasedincomplexity.Thisraisesthequestionhowtomeasurecomplexity.Forsure,thistaskhasnotbeenconvincinglysolvedasyet,butonewaytodealwiththeproblemistofocusonbiologicalinformation.SinceinformationisstoredasDNAinmostorganisms,theamountofDNAmayserveasanestimatorofthedegreeofcomplexity(T
7、able13.1).Ingeneral,thereisindeedatrendforincreasedgenomesizeoverevolutionaryhistory.Thisestimatebecomesmoreconvincingwhentheproportionofthegenomethatencodesforproteinsistakenintoaccount,whichcanbequitelowinsometaxa,suchasnewts.Ingeneral,thelargeragenome
