托福阅读真题：Metamorphosis
托福阅读真题：Metamorphosis
Metamorphosis
Paragraph 1: Organisms that metamorphose undergo radical changes over the course of their life cycle. A frog egg hatches a tadpole that metamorphoses into an adult frog within a few days or weeks. A fruit fly egg hatches a larva that feeds for a few hours or days and then enters the pupal stage during which it develops a protective covering. The changes that occur during the metamorphosis of a single species may be so great that the species occupies two separate and very different niches or places in an environment at different times. In fact, the larvae of two species may be more similar to each other than to the corresponding adult forms of their own species.
1. The word “radical” in the passage is closest in meaning to
O extreme
O distinctive
O periodic
O structural
2. According to paragraph 1, which of the following is true of the organisms that become fruit flies?
O They feed during the pupal stage.
O They become winged insects several days after the larvae hatch from eggs.
O They remain in the larval stage longer than they remain in the pupal stage.
O They need to be protected during the larval stage in order to reach the pupal stage.
Paragraph 2: Organisms that utilize different resources at different stages of their life cycle face an unusual evolutionary problem: exploiting different niches may be difficult with a single body plan. The solution is a juvenile (immature) form specialized for one niche, followed by metamorphosis to an entirely new body plan, adapted to a different niche in the adult. Clearly, species that metamorphose must undertake complex genetic and physiological processes in the transformation. These changes require complex regulatory mechanisms that involve turning on and off many genes at appropriate times. In addition, the reorganization of the body plan in a metamorphic species entails considerable energy costs. What sorts of ecological advantages could outweigh these complications?
3. It can be inferred from paragraph 2 that an advantage of metamorphosis is to
O develop regulatory mechanisms for turning genes on and off
O occupy different niches at different parts of the life cycle
O undertake complex genetic and physiological processes
O reduce their energy costs
4. Why does the author ask the question “What sorts of ecological advantages could outweigh these complications?”
O To suggest that there is no single advantage but many advantages
O To challenge the idea that metamorphic transformations are always beneficial
O To prepare readers for a discussion that may explain why metamorphosis occurs
O To identify a particular aspect of metamorphosis that is poorly understood
Paragraph 3: One prevailing hypothesis is that metamorphic species specialize so as to exploit habitats with high but transient (short term) productivity — and hence high potential for growth. Part of this strategy is that specializations for feeding, dispersal, and reproduction are separated across stages. A frog tadpole occupies an aquatic environment (such as a pond) with extremely high potential for growth. The existence of the pond or its high production may be transient, however. Whereas an aquatic larva is not capable of dispersal to new ponds if its habitat becomes unsuitable, the adult frog is. In this case rapid growth in the larva is separated from dispersal and reproduction in the adult. Although the adult feeds, its growth rate is far less than that of the tadpole. The energy adults obtain from feeding is dedicated to dispersal and reproduction.
5. The word “exploit” in the passage is closest in meaning to
O identify
O adapt to
O use to advantage
O become established in
6. According to paragraph 3, by changing from a tadpole into an adult frog, an adult frog is able to
O maintain a high growth rate
O provide a safer habitat for its offspring
O obtain more food
O move to a more suitable location
Paragraph 4: Many insects benefit from the same strategy. Although a butterfly larva feeds voraciously, often on a very specific set of host plant species, the adult does not grow. If it feeds, it does so only to maintain energy reserves required for dispersal and reproduction. The monarch butterfly exemplifies this strategy. Its larvae feed specifically on milkweeds. Monarch pupae also develop on this host plant. The emerging adults migrate long distances — from all over eastern North America to nine small sites in the Sierra Madre mountains of Mexico. There, females become sexually mature and migrate north, mating along the way and feeding only to maintain energy reserves. In this example, the feeding specialist stage is again separated from the dispersal and reproduction stages.
7. According to paragraph 3 and 4, what do frogs and butterflies have in common?
O Adults of both reproduce only when there is enough food in a habitat to sustain their offspring.
O Adults of both eat only enough to supply the energy needed for dispersal and reproduction.
O Immature forms of both disperse to new habitats when the habitats they are in can no longer sustain them.
O Immature forms of both depend on aquatic environments.
8. The phrase “the same strategy” in the passage refers to
O dispersing to new habitats as adults
O spending most of the life cycle on dispersal and reproduction
O feeding on a specific set of host plant species
O separating specialization for feeding from dispersal and reproduction
Paragraph 5: In the previous examples, the reproductive function is delegated to the adult. Under certain ecological conditions, however, it is apparently advantageous for reproduction to occur in the larval stage. Thus, even the reproductive function typically fulfilled by the adult can apparently be modified under certain circumstances. Species that show this modification of a metamorphic life cycle are said to demonstrate neoteny, a life cycle in which the larvae of some populations or races become sexually mature and no longer metamorphose into adult. Some populations of the salamander Ambystoma maculatum show this trait. In fact, the larvae of this species were originally classified as a separate species before it was recognized that they are neotenic forms.
9. The word “apparently” in the passage is closest in meaning to
O usually
O especially
O seemingly
O certainly
10. According to paragraph 5, why were the larvae of some populations of the salamander species Ambystoma maculatum once thought to be members of a separate species?
O Because they were shaped differently than other larvae of Ambystoma maculatum.
O Because they were discovered far away from other members of Ambystoma maculatum.
O Because they were sexually mature and could reproduce.
O Because the neotenic form of Ambystoma maculatum looks very different from the neotenic forms of other salamanders.
Paragraph 6: The selective factors leading to neoteny are not well understood. We know, however, that neotenic forms are more frequently found in extreme environments, often high altitudes or latitudes. High-altitude populations of certain salamanders have higher frequencies of neoteny than do low-elevation population of these species. If the larval environment is rich compared to the harsh adult environment, selection may favor neoteny. One research study, has ruled out simple food effects: supplemental food did not increase the frequency with which organisms reached the adult stage. This suggests that neoteny may be a genetically determined feature of some amphibian life histories.
11. The word “harsh” in the passage is closest in meaning to
O severe
O typical
O restricted
O available
12. Paragraph 6 indicates that greater frequency of neoteny in salamanders may be associated with all of the following EXCEPT
O an environment that is richer for larvae than for adults
O an inadequate food supply for larvae
O a high-altitude location
O a genetic makeup favoring neoteny
Paragraph 2: Organisms that utilize different resources at different stages of their life cycle face an unusual evolutionary problem: exploiting different niches may be difficult with a single body plan. ■The solution is a juvenile (immature) form specialized for one niche, followed by metamorphosis to an entirely new body plan, adapted to a different niche in the adult. ■Clearly, species that metamorphose must undertake complex genetic and physiological processes in the transformation. ■These changes require complex regulatory mechanisms that involve turning on and off many genes at appropriate times. ■In addition, the reorganization of the body plan in a metamorphic species entails considerable energy costs. What sorts of ecological advantages could outweigh these complications?
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
Metamorphosis, however, comes with its own set of difficulties.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
In metamorphic species, an organism’s body plan changes at different points in its life cycle.
Answer Choices
O Organisms that metamorphose use resources less efficiently than do organisms with a single body plan.
O Because of the high energy costs associated with metamorphosis, only a few types of niches are rich enough to support metamorphic species.
O The larvae of some population found in extreme environments reproduce at the juvenile stage and never achieve the adult form.
O Metamorphosis enables an organism to make use of environments that are highly productive for a short period of time but not for the organism’s entire life span.
O In many metamorphic species, the juvenile stage is focused on eating and growing and the adult stage is focused on dispersal and reproduction.
O Some population remain at the larval stage in part because they lack sufficient nutrition to metamorphose into adults.
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