[考研英语]考研英语阅读模板(十二)

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1、[考研英语]考研英语阅读模板（十二）阅读原文来自：新东方在线 2011-10-2609:12:00 

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4、 更多▼      SAMPLE12　　[生物学类]　　题目序号 题型归类　　第1题 细节推导题型　　第2题 写作手法题型　　第3题 审题定位与中心主旨题型　　第4题 词汇题型　　第5题 审题定位题型　　　　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