2. 外语
  3. METAMORPHOSIS (1) Organisms that metamorphose undergo radical changes over the course of their life cycle. A frog egg hatche

METAMORPHOSIS (1) Organisms that metamorphose undergo radical changes over the course of their life cycle. A frog egg hatche

admin2022-08-28  25
问题                                                 METAMORPHOSIS
    (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.
    (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. [A] 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. [B] Clearly, species that metamorphose must undertake complex genetic and physiological processes in the transformation. [C] These changes require complex regulatory mechanisms that involve turning on and off many genes at appropriate times. [D] 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) 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’s 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.
    (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.
    (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.
     (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 populations 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.
According to paragraph 3 and 4, what do frogs and butterflies have in common?
选项 A、Adults of both reproduce only when there is enough food in a habitat to sustain their offspring.
B、Adults of both eat only enough to supply the energy needed for dispersal and reproduction.
C、Immature forms of both disperse to new habitats when the habitats they are in can no longer sustain them.
D、Immature forms of both depend on aquatic environments.
答案B
解析 本题要求根据第3、4段找出青蛙和蝴蝶的共同点，属于事实信息题。原文的第4段第3句指出，蝴蝶的成虫并不会生长，它们进食也只是为了维持传播和繁殖所需的能量储备、而第4段第1句中的the saine strategy“相同的策略”回指第3段最后一句“成蛙通过进食获得的能量专门用于传播和繁殖”。由此可知，B项“成虫进食只是为了储存传播和繁殖所需的能量”为正确答案。A项“只有当柄息地有足够的食物来养活后代时，这两利，动物的成体才会繁殖”并无根据C项 “当两种动物的幼体所处的栖息地无法维持它们生存时，它们会迁移到新的柄息地”与原文不符，第3段第5句提到，青蛙幼体即便在栖息地不适合它们生存时，也无法迁移到新的池塘，而关于蝴蝶幼虫是否有这种情况，文中更是没有相关信息。D项“两种动物的幼体均依靠水生环境生存”、从第3段第3句可知，青蛙的幼体是依靠水生环境生存的，但是文中并未提及蝴蝶的幼虫也是依靠水生环境生存的，而且第4段第2句指出，蝴蝶幼虫以寄主植物为食，故不选。
转载请注明原文地址:https://kaotiyun.com/show/Gs5O777K
0

托福（TOEFL）

相关试题推荐
随机试题
最新回复(0)