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1、把握文章结构对于做好与全文主题相关的题型非常有效。例如， SymbioticRelationships Asymbioticrelationshipisaninteractionbetweentwoormorespeciesinwhichonespecieslivesinoronanotherspecies.Therearethreemaintypesofsymbioticrelationships:parasitism,commensalism,andmutualism.Thefirstandthethirdcanbekeyfactorsinthestructureofabi
2、ologicalcommunity;thatis,allthepopulationsoforganismslivingtogetherandpotentiallyinteractinginaparticulararea. Parasitismisakindofpredator-preyrelationshipinwhichoneorganism,theparasite,derivesitsfoodattheexpenseofitssymbioticassociate,thehost.Parasitesareusuallysmallerthantheirhosts.Anexampl
3、eofaparasiteisatapewormthatlivesinsidetheintestinesofalargeranimalandabsorbsnutrientsfromitshost.Naturalselectionfavorstheparasitesthatarebestabletofindandfeedonhosts.Atthesametime,defensiveabilitiesofhostsarealsoselectedfor.Asanexample,plantsmakechemicalstoxictofungalandbacterialparasites,alo
4、ngwithonestoxictopredatoryanimals(sometimestheyarethesamechemicals).Invertebrates,theimmunesystemprovidesamultipledefenseagainstinternalparasites. Attimes,itisactuallypossibletowatchtheeffectsofnaturalselectioninhost-parasiterelationships.Forexample,Australiaduringthe1940swasoverrunbyhundreds
5、ofmillionsofEuropeanrabbits.TherabbitsdestroyedhugeexpansesofAustraliaandthreatenedthesheepandcattleindustries.In1950,myxomavirus,aparasitethataffectsrabbits,wasdeliberatelyintroducedintoAustraliatocontroltherabbitpopulation.Spreadrapidlybymosquitoes,thevirusdevastatedtherabbitpopulation.Thevi
6、ruswaslessdeadlytotheoffspringofsurvivingrabbits,however,anditcausedlessandlessharmovertheyears.Apparently,genotypes(thegeneticmake-upofanorganism)intherabbitpopulationwereselectedthatwerebetterabletoresisttheparasite.Meanwhile,thedeadlieststrainsofthevirusperishedwiththeirhostsasnaturalselect
7、ionfavoredstrainsthatcouldinfecthostsbutnotkillthem.Thus,naturalselectionstabilizedthishost-parasiterelationship. Incontrasttoparasitism,incommensalism,onepartnerbenefitswithoutsignificantlyaffectingtheother.Fewcasesofabsolutecommensal
