文档介绍：2012年考研英语阅读理解模拟试题及答案(一) Flatfish,suchastheflounder,areamongthefewvertebratesthatlackapproximatebilateralsymmetry(symmetryinwhichstructurestotheleftandrightofthebody’smidlinearemirrorimages).Moststrikingamongthemanyasymmetriesevidentinanadultflatfishiseyeplacement:beforematurityoneeyemigrates,,membersofthestarryflounderspeciescanbeeitherleft-eyed(botheyesontheleftsideofhead)orright-,thestarryflounderpopulationsvaryfromabout50percentleft-eyedofftheUnitedStatesWestCoast,throughabout70percentleft-eyedhalfwaybetweentheUnitedStatesandJapan,tonearly100percentleft-eyedofftheJapanesecoast. Biologistscallthiskindofgradualvariationoveracertaingeographicrangea“cline”andinterpretclinesasstrongindicationsthatthevariationisadaptive,(betweenfishthataremirrorimagesofoneanother)isadaptive,thatleft-eyednessintheJapanesestarryflounderhasbeenselectedfor,whichprovokesaperplexingquestion:whatistheselectiveadvantageinhavingbotheyesononesideratherthanontheother? Theeasewithwhichafishcanreversetheeffectofthesidednessofitseyeasymmetrysimplybyturningaroundhascausedbiologiststostudyinternalanatomy,erves,,erve