Anole lizard species that occur together (sympatrically) on certain Caribbean islands occupy different habitats: some live only in the 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. [highlight]What is striking about these lizards is not that coexisting species differ in morphology and habitat use (such differences are common among closely related sympatric species), but that the same three types of habitat specialists occur on each of four islands: Puerto Rico, Cuba, Hispaniola, and Jamaic[/highlight]a. 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 traveled over water to colonize other islands. Or this ancestral species might have evolved at a time when the islands were 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 occurred 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 the 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 to 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.
1. The primary purpose of the passage is to
A. describe some unusual features of anole lizard species
B. account for a particular type of behavior found among anole lizard species
C. contrast two types of evidence that have been used to support a particular hypothesis concerning anole lizard species
D. explain how researchers resolved a particular scientific question concerning anole lizard species
E. examine different explanations for a particular trait common to certain anole lizard species
2. Which of the following best describes the purpose of the highlighted sentence?
A. It raises a question about why coexisting anole lizard species occupy the different types of habitats mentioned in the first sentence.
B. It introduces a fact about anole lizard species that the passage will go on to explore.
C. It identifies a particular aspect of anole lizard behavior that distinguishes anoles from other lizard species.
D. It explains why one aspect of anole lizard species' habitat use has been difficult to account for.
E. It points out a surprising relationship between morphology and habitat use that is explained in the concluding paragraph.
3. It can be inferred form the passage that which of the following is true of the Cuban tree-dwelling anole lizard and the Jamaican tree-dwelling anole lizard?
A. They share a morphology characterized by stocky bodies and long legs.
B. They have bodies that are relatively slender compared to their stubby legs.
C. They differ significantly form one another in size.
D. They differ significantly from one another in behavior and habitat use.
E. They are genetically closely related to one another.
4. The passage suggests that if a grass-dwelling anole lizard species evolved on one island and then traveled over water to colonize a second island, the grass-dwelling anoles on the two islands would eventually
A. develop very different DNA sequences
B. develop into different species that are more distantly related to each other than to tree- and twig-dwelling anoles on their own islands
C. come to differ significantly from one another in habitat use
D. develop into different, but closely related, species
E. evolve significant morphological differences
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