[考研英語]考研英語閱讀模板(十二)

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1、[考研英語]考研英語閱讀模板（十二）閱讀原文來自：新東方在線?2011-10-2609:12:00?

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4、?更多▼??????SAMPLE12　　[生物學(xué)類]　　題目序號?題型歸類　　第1題?細(xì)節(jié)推導(dǎo)題型　　第2題?寫作手法題型　　第3題?審題定位與中心主旨題型　　第4題?詞匯題型　　第5題?審題定位題型　　　　Flatfish,suchastheflounder,areamongthefewvertebratesthatlackapproximatebilateralsymmetry(symmetryinwh

5、ichstructurestotheleftandrightofthebody’smidlinearemirrorimages).Moststrikingamongthemanyasymmetriesevidentinanadultflatfishiseyeplacement:beforematurityoneeyemigrates,sothatinanadultflatfishbotheyesareonthesamesideofthehead.Whileinmostspecieswithasymmetriesvirtuallyall

6、adultssharethesameasymmetry,membersofthestarryflounderspeciescanbeeitherleft-eyed(botheyesontheleftsideofhead)orright-eyed.InthewatersbetweentheUnitedStatesandJapan,thestarryflounderpopulationsvaryfromabout50percentleft-eyedofftheUnitedStatesWestCoast,throughabout70perc

7、entleft-eyedhalfwaybetweentheUnitedStatesandJapan,tonearly100percentleft-eyedofftheJapanesecoast.　　Biologistscallthiskindofgradualvariationoveracertaingeographicrangea“cline”andinterpretclinesasstrongindicationsthatthevariationisadaptive,aresponsetoenvironmentaldifferen

8、ces.Forthestarryflounderthisinterpretationimpliesthatageometricdifference(betweenfishthataremirrorimagesofoneanother)isadaptive,thatleft-eyednessintheJapanesestarryflounderhasbeenselectedfor,whichprovokesaperplexingquestion:whatistheselectiveadvantageinhavingbotheyesono

9、nesideratherthanontheother?　　Theeasewithwhichafishcanreversetheeffectofthesidednessofitseyeasymmetrysimplybyturningaroundhascausedbiologiststostudyinternalanatomy,especiallytheopticnerves,fortheanswer.Inallflatfishtheopticnervescross,sothattherightopticnerveisjoinedtoth

10、ebrain’sleftsideandviceversa.Thiscrossingintroducesanasymmetry,asoneopticnervemustcrossaboveorbelowtheother.G.H.Parkerreasonedthatif,forexample,aflatfish’slefteyemigratedwhentherightopticnervewasontop,therewouldbeatwistingofnerves,whichmightbemechanicallydisadvantageous

11、.Forstarryflounders,then,theleft-eyedvarietywouldbeselectedagainst,sinceinastarryfloundertheleftopticnerveisup