nAturAl sElECtion
First CoinED ByDArwin, thE tErm nAturAl sElECtion rEFErs to thE proCEss At thE CorE oF EvolutionAry thEory. simply put,DArwin BEliEvED thAt EvEry living orgAnism hAD to strugglE to survivE AnD rEproDuCE.By rAnDom ChAnCE, somE inDiviDuAls in A spECiEs woulD hAvE ChArACtEristiCs thAt mADE thEm BEttEr ADAptED to this strugglE thAn thEir ContEmporAriEs. suCh inDiviDuAls woulD BE morE likEly to rEproDuCE AnD pAss thEir ChArACtEristiCs on to thEir oFFspring. ovEr mAny gEnErAtions, A sEriEs oF smAll DiFFErEnCEs in ChArACtEristiCs woulD ACCumulAtE, until EvEntuAlly thE CrEAturEs thAt wErE proDuCED woulD BE so DiFFErEnt From thEir originAl AnCEstors thAt thEy woulD ConstitutE A nEw spECiEs. ■
A、AlthoughDArwin DiD not know how vAriAtions BEtwEEn inDiviDuAl mEmBErs oF thE sAmE spECiEs wErE ABlE to ArisE in thE First plACE or how suCh trAits CoulD thEn BE pAssED on From spECiEs to spECiEs, wE now know thAt this proCEss oCCurs through gEnEs. ■
B、gEnEs DEtErminE whAt ChArACtEristiCs An orgAnism will possEss, From its sizE to its skin Color to its gEnDEr. ■
C、somEtimEs thEsE gEnEs BEComE DAmAgED or AltErED--A proCEss known As mutAtion. ■
D、most mutAtions ArE EithEr too smAll to hAvE Any EFFECt, or hAvE DEtrimEntAl EFFECts on thE orgAnism.A、FEw, howEvEr, CAn BE BEnEFiCiAl to hElping An orgAnism survivE. thE orgAnisms with thEsE BEnEFiCiAl mutAtions ArE thE onEs thAt .survivE AnD rEproDuCE, pAssing thEir pArtiCulAr gEnEtiC mutAtion on to FuturE gEnErAtions.
originAlly,DArwin thought thAt two pArtiCulAr typEs oF trAits wErE nAturAlly sElECtED For Evolution. thE First typE inCluDED Any AnD All trAits thAt hElpED An orgAnism survivE, EithEr By giving it EAsiEr ACCEss to FooD, BEttEr protECtion From prEDAtors, or supErior ADAptABility to shiFting EnvironmEntAl ConDitions. sEvErAl ExAmplEs oF thE nAturAl sElECtion oF suCh trAits hAvE BEEn oBsErvED sinCEDArwin First proposED his thEory. thE most oFtEn CitED ExAmplE is thE ColorAtion oF moths nEAr inDustriAl CEntErs. somE oF thEm hAvE whitE wings AnD thE othErs hAvE BlACk wings. unDEr normAl ConDitions, thE lightEr-ColorED moths prEDominAtE sinCE whitE moths ArE lEss visiBlE AgAinst thE light-ColorED BArk oF thE trEEs in thE ForEsts whErE thEy livE, whiCh mEAns thEy CAn morE EAsily AvoiD prEDAtors. nEAr inDustriAl CEntErs, howEvEr, DArkEr-ColorED moths prEDominAtE, As thE pollution From thE FACtory hAs DArkEnED thE BArk oF thE nEArBy trEEs. moths lAnDing on thEsE trEEs ArE muCh morE DiFFiCult to sEE iF thEy hAvE BlACk wings. this trAit hAs now BEComE most Common in moth populAtions living in ArEAs whErE pollutAnts hAvE ChAngED thE Color oF thE trEE BArk.
thE sEConD typE oF nAturAlly sElECtED trAits EnvisionED ByDArwin involvED thosE ChArACtEristiCs oF An orgAnism thAt mADE it EAsiEr For thAt orgAnism to FinD A mAtE. this Form oF nAturAl sElECtion, oFtEn CAllED sExuAl sElECtion, wAs thought to ExplAin thE ExistEnCE oF highly DEvElopED trAits in CErtAin AnimAls thAt sEEmED to plAy no pArtiCulAr rolE in thEir survivAl, AnD thAt in somE CAsEs sEEmED to mAkE it EvEn hArDEr For thEm to survivE.A、primE ExAmplE oF suCh A trAit is thE pEACoCk’s ElABorAtE, ColorFul tAil FEAthErs.A、long, unwiElDy tAil thAt CAn ExpAnD into A DisplAy oF Bright Colors thAt DrAws thE AttEntion oF Any prEDAtor in thE viCinity sEEms lEss likEly thAn thAt oF othEr kinDs oF BirDs to hElp thE BirD survivE. howEvEr, rEsEArChErs DisCovErED thAt pEAhEns invAriABly ChosE to mAtE with thosE pEACoCks whosE tAil FEAthErs proDuCED thE most ArtistiC DisplAys. thus, Although thE pEACoCk’s tAil FEAthErs hAvE no ADAptivE FunCtion in tErms oF Coping with survivAl prEssurE, thE gEnE thAt Controls FEAthEr Color is thE onE thAt is most likEly to BE pAssED on BECAusE oF its rolE in thE mAting proCEss.
mAny sCiEntists now BEliEvE thAt A thirD typE oF nAturAlly sElECtED ChArACtEristiCs Exists, nAmEly thosE thAt mAy BE hArmFul to An inDiviDuAl orgAni