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Anole Lizards on the Caribbean Islands
Anole lizard species that occur together (sympatrically) on certain Caribbean islands occupy different habitats: some live only in grass, some only on tree trunks, and some only on twigs. These species also differ morphologically: grass dwellers are slender with long tails, tree dwellers are stocky with long legs, twig dwellers are slender but stubby-legged. What is striking about these lizards is not that coexisting species differ in morphology and habitat use (such differences are common among closed related sympatric species), but that the same three type of habitat specialists occur on each of the four islands: Puerto Rico, Cuba, Hispaniola, and Jamaica. Moreover, the Puerto Rican twig species closely resembles the twig species of Cuba, Hispaniola, and Jamaica in morphology, habitat use, and behavior. Likewise, the specialists for other habitats are similar across the islands. The presence of similar species on different islands could be variously explained. An ancestral species might have adapted to exploit a particular ecological niche on one island and then travelled over water to colonize on other islands. Or this ancestral species might have evolved at a time when the islands were once connected, which some of these islands may once have been. After the islands separated, the isolated lizard populations would have become distinct species while also retaining their ancestors’ niche adaptations. Both of these scenarios imply that specialization to each niche occur only once. Alternatively, each specialist could have arisen independently on each of the islands. If each type of specialist evolved just once, then similar specialists on different islands would be closely related. Conversely, if specialists evolved independently on each island, then a specialist on one island would be more closely related to other types of anoles on the same island – regardless of their ecological niches – than it would be a similar specialist on a different island. Biologists can infer how species are related evolutionarily by comparing DNA sequences for the same genes in different species. Species with similar DNA sequences for these genes are generally more closely related to each other than to species with less-similar DNA sequences. DNA evidence concerning the anoles led researchers to conclude that habitat specialists on one island are not closely related to the same habitat specialists elsewhere, indicating that specialists evolved independently on each island.
It can be inferred that the DNA evidence mentioned in the highlight text supports which of the following statements about the twig-dwelling anole lizards of any one of the four Caribbean islands mentioned in the passage ○They evolved from an ancestral species of twig dwellers at a time when their island was connected with the other three islands. ○They were once more similar morphologically to twig dwellers on the other three islands than they are at present. ⊙They are more closely related to tree- and grass-dwelling anoles on their island than they are to twig dwellers on the other three islands. ○Their habitat specialization as twig dwellers was influenced by the differences in the types of vegetation on the other three islands. ○Their DNA more closely resembles that of non-anole lizard species on their island than it does the DNA of anoles on the other three islands.
According to the passage, sympatric anole lizard species resemble many other closely related sympatric species in that anole lizard species ________. ⊙differ from one another in morphology and habitat use ○evolved on islands that were once connected ○appear to have adapted unusually quickly to new habitats ○have been able to colonize areas remote from their ancestral habitat ○occupy the same range of habitats in a number of different locations
It can be inferred from the passage that which of the following is true of the Cuban tree-dwelling anole lizard and the Jamaican tree-dwelling anole lizard? ⊙They share a morphology characterized by stock bodies and long legs. ○They have bodies that are relatively slender compared to their stubby legs. ○They differ significantly from one another in size. ○They differ significantly from one another in behavior and habitat use. ○They are genetically closely related to one another.
Which of the following best describes the purpose of the sentence in the highlighted text? ○It raises a question about why coexisting anole lizard species occupy the different type of habitats mentioned in the first sentence. ⊙It introduce a fact about anole lizard species that the passage will go on to explore. ○It identifies a particular aspect of anole lizard behavior that distinguishes anoles from other lizard species. ○It explains why one aspect of anole lizard species’ habitat use has been difficult to account for. ○It points out a surprising relationship between morphology and habitat use that is explained in the concluding paragraph.
这篇在做Practice test 1的两次中都遇到了，第一次还有一道主旨题，但是想不起来怎么问了。 貌似和RC机经里的“不同岛上的R动物”有异曲同工之妙，也许就是同一篇，希望提供机经的同志能来帮忙确认下，谢谢。 | 
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发表于 2009-8-30 05:25:00
