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  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

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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.
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 sentences 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.
In metamorphic species, an organism’s body plan changes at different points in its life cycle.
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Answer Choices
(A) Organisms that metamorphose use resources less efficiently than do organisms with a single body plan.
(B) Because of the high energy costs associated with metamorphosis, only a few types of niches are rich enough to support metamorphic species.
(C) The larvae of some populations found in extreme environments reproduce at the juvenile stage and never achieve the adult form.
(D) 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.
(E) In many metamorphic species, the juvenile stage is focused on eating and growing and the adult stage is focused on dispersal and reproduction.
(F) Some populations remain at the larval stage in part because they lack sufficient nutrition to metamorphose into adults.
选项
答案C,D,E
解析 本题为文章总结题。C项“人们发现，极端环境中的一些物种在幼体阶段繁殖，永远不会发育成成体的形态”是对第5段第4句所指出的“在这个(幼态持续)状态中，一些种群或物种的幼体性发育成熟，且不再发育为成体”，以及第6段第2句所说的“幼态持续更常见于极端的环境当中”的概括总结。D项 “变态发育能够使生物利用环境，在短时间内，而不是整个生命周期，大量繁殖”对应第3段第1句，以及之后所举的青蛙和蝴蝶的例子。这些都说明了变态物种可利用环境在短期内大量繁殖，而当发育为成虫后，则不再快速生长，而是进入传播和繁殖的阶段。E项“许多变态物种在幼虫期以进食和生长为主，在成虫期以传播和繁殖为主”是对第3段和第4段青蛙和蝴蝶的例子所说明的观点的整合。A项“变态生物的资源利用率比单一身体形态生物的低”在原文中并未被提及。B项“因为变态发育耗能多，所以变态物种只有几种生态位”中的前半部分可对应第2段倒数第2句，但文中并无该项后半部分的信息。F项“一些种群仍处于幼虫期，部分原因是它们缺乏足够的营养，无法变态发育为成虫”，但文中第6段倒数第2句说明，食物因素对幼虫进入成虫期没有影响，故F项错误。
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