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RegisterLoginbyJohnTimmerNov172013,3:00pmESTLIFESCIENCES49FEATURESTORY(3PAGES)XboxOnereview:Morethanagameconsole,lessthanalivingroomrevolutionLimitedmultitaskingandvoicecommandsaren'tworthanextra$100ontheirown.
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comandabilitytousecitrateinsteadofglucose,leadingtoasignificantgrowthadvantage.
Otherspickedupgeneticchangesthatleftthemmoresusceptibletomutations,acceleratingtheirabilitytoadapt.
Butthesemutationstookplacetensofthousandsofgenerationsago.
Lenskidecidedtoseeifevolutionwasstilldrivingincreasesinfitness,orifthebacteriahadalreadyachievedalltheimprovementstheywerelikelytosee.
Toseeifthetestswereworthwhile,theresearcherstookthecurrentgroupofbacteria(about50,000generationsintotheexperiment)andcomparedthemtosomesamplessavedfromgeneration40,000.
Whencompetingforgrowthagainsteachotherinthesamemedium,generation50Kgrewfasterinnineofthe12cultures,anditwasroughlyequalintherest.
Togetafullfitnesscurve,Lenski'sgroupthensetall12culturesintocompetitionswiththeirownpasts,usingasmanyas40competitionsforeachculture.
Theendresultsforallcaseswereprettyclear:thebacteriamadelargegainsinfitnessduringthefirstfewthousandgenerations,afterwhichthepaceslowed.
Buttheincreaseinfitnessneverreallystopped.
Thefitnesscurvesweretestedtoseeifitlookedlikeahyperbola,whichapproachesbutneverexceedsafitnesslimit,orapowerlaw,inwhichcasefitnesswillcontinuetoincreaseovertimeevenasitspaceslows.
Theresultswereprettyclear:"thepowerlawoutperformsthehyperbolicmodelwithaposterioroddsratioof~30million.
"Theteamthenattemptedtobuildamathematicalmodelthatwoulddescribethefitnessincreasesbasedonthemutationrate.
Oneofthenotablefeaturesoftheirmodelisthatitleftouttheoccurrenceofharmfulmutationsentirely,anditstillproducedaccuratefitstothefitnesscurves.
Inotherwords,evolutionseemstobesoefficientatgettingridofharmfulmutationsthattheycansafelybeignored.
Anotherwaythisbecameapparentisthehypermutatorstrains.
Halfoftheoriginaldozenstrainspickedupmutationsingenesthatresultedinfurthermutationsaccumulatingata100foldhigherpace.
Ifnegativemutationsweremuchofaproblem,thenthehypermutatorsshouldshowfitnesscurvesthatwerebelowthoseoftheremainingstrains.
Instead,theyachievedhigherlevelsoffitnessthantheirpeersby10,000generationsandcontinuedtostayaheadforover40,000generations.
Thisisn'ttosaythattherewon'tcomeachoicewhereahighermutationratewillbecomeaproblem.
Butit'sclearthat,whileadaptingtoaradicallydifferentenvironment,it'sasignificantbenefit.
Theauthorsnotethatthebacteriahaveonethinggoingagainstthem:theydon'thavesex.
It'sentirelypossiblethatsomehelpfulmutationsoccurredinthesepopulationsthatweresimplyoutcompetedbythingsthatmadeanevenlargerimprovementinfitness.
Hadthepopulationbeenexchanginggenes,itwouldhavebeenpossibletogetbothmutationsintothesameorganism.
Finally,theauthorsnotethatthere'snoindicationofthingsslowingdown.
Infact,theypointoutthatmanyadaptivemutationsalsocausesomeminornegativesideeffects.
Furtherevolutioncanundothenegatives,increasingthefitnessofthepopulationevenfurther.
Science,2013.
DOI:10.
1126/science.
1243357(AboutDOIs).
JohnTimmer/JohnbecameArsTechnica'sscienceeditorin2007afterspending15yearsdoingbiologyresearchatplaceslikeBerkeleyandCornell.
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