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  3. Part Ⅱ Reading Comprehension (Skimming and Scanning)Directions: In this part, y

Part Ⅱ Reading Comprehension (Skimming and Scanning)Directions: In this part, y

游客2024-05-07  16
问题 Part Ⅱ Reading Comprehension (Skimming and Scanning)
Directions: In this part, you will have 15 minutes to go over the passage quickly and answer the questions on Answer Sheet 1.
For questions 1-4, mark
Y (for YES)                if the statement agrees with the information given in the passage;
N (for NO)                 if the statement contradicts the information given in the passage;
NG (for NOT GIVRN)         if the information is not given in the passage.
For questions 5-10, complete the sentences with the information given in the passage.
                                        Unity and Diversity
    Many physicists are engaged in the search for a "theory of everything". Biologists, smugly, think they have found one already. Organisms that survive long enough to reproduce and are attractive enough to find a mate pass their genes on to the next generation. Those that do not are evolutionary cul-de-sacs. But the details—how you go on from the basic principles of evolution to explain large-scale patterns in biology—are more divisive. Scientific camps form. Their leaders step onto soap boxes. And only rarely do people concede that their own theories and those of their opponents are not always mutually exclusive.
    Since the early 1970s, the two grandest patterns of life—how species are arranged in space and how they are arranged in time—have divided their opposing camps quite neatly. Those who squabble over space disagree about why there are more species in the tropics than anywhere else. To them, the tropics are either where species are more often born (cradles of diversity) or where they tend not to die (museums of diversity). By contrast, biologists concerned with patterns in time tenaciously debate whether new species come into being in a smooth and gradual manner, or whether the history of life is actually a series of bursts of change that are interspersed with periods when nothing much happens.
    Two papers just published in Science have cast light on these questions, and their findings, if not necessarily resulting in compromise, do show the value of taking leaves out of other people’s books. The "space biologists" have looked into time, namely the fossil record over the past 11m years. Meanwhile the "time biologists" have looked at the here and now and found evidence in living species for periods of rapid evolution in their genes.
Biological Spacetime
    The space biologists have the advantage that they agree about the pattern they are trying to explain. Almost all groups of life that have been studied—be they fungi, plants, vertebrates or invertebrates, and no matter whether they occur in forests, streams or seas—seem to have more species the closer they are to the equator.
    To decide whether the tropics are a cradle or a museum, though, involves picking this pattern apart with statistics. And statistics work best when you have more than one sample. That is the reason for reaching into the past.
    David Jablonski, of the University of Chicago, and his colleagues created their samples by dividing the past 11m years into three periods. For simplicity’s sake, they also chopped the Earth’s surface into two: tropical regions and everywhere else, which they called the "extratropics".
    To avoid sampling bias, they restricted their analysis to one group of animals—the bivalve molluscs—that fossilise well. This allowed them to follow 431 "lineages" of marine bivalve through the course of geological time. The vast majority of these lineages appear in the tropics and then spread into the extratropics, in other words, the tropics do, indeed, act as cradles of biodiversity.
    In fact, the pattern Dr Jablonski reports is probably more marked than his data suggest. That is because palaeontologists themselves are generally a temperate species and are most commonly found in the northern hemisphere. That means rocks in this region have been oversampled compared with those in the tropics. Also, tropical rocks tend to experience deep weathering, rarely poking above the ground as outcrops. That makes sampling them harder, even if you bother to look in the first place.
    The tropics do, indeed, act as cradles of biodiversity. Both of these facts mean it is likely that some lineages which seem to make their first appearance in the extratropical fossil record actually started out near the equator. The cradle hypothesis, then, looks strong. But that does not necessarily mean the museum hypothesis is wrong—for, at the same time as the tropics were generating diversity they seem to have been preserving it as well. Although the bivalve lineages Dr Jablonski studied spread out from the equator in waves, they did not become extinct in the wake of these waves. Instead of being forced out of tropical regions as they travelled poleward, they accumulated there.
    Common ground appears to be forming in time biology as well. The dispute is between the gradualists and those who think that the sudden shifts in fossil types seen in the geological record are real, rather than a consequence of the irregular way that rocks are laid down. It got rather personal a few years ago, with references to evolution by creeps and evolution by jerks. But, in a similar way to Dr Jablonski’s study, which suggests that models of cradles and museums present a false dichotomy, a paper from a member of the gradualist camp of time biology endorses some features of jerky evolution.
Some Jerks, Some Creeps
    Mark Pagel and his colleagues at the University of Reading, in England, reasoned that the theory of punctuated equilibria (the formal name given to evolution in bursts) predicts a relation between the rate at which new species are formed and the rate of genetic change in an organism’s recent past. A lineage that has spun off a lot of species will show more genetic change than one that has not. The alternative view, that evolutionary changes tick along gradually, suggests mutations would accumulate incrementally as time goes by, independently of how many new species a lineage spins off.
    Dr Pagel studied 122 family trees this way. He found that in about a third of them, there was more change in the DNA in those trees where more species had been generated. He also found that punctuated evolution explained about 22% of such genetic changes, with the remainder unfolding smoothly through time. This means, among other things, that when biologists have calculated how long ago two lineages diverged by assuming change is regular, they may have their dates wrong.
    Both Dr Jablonski and Dr Pagel, then, have found a degree of resolution between opposing ideas by persistently hacking away at the data. Both have also posed more questions to their respective fields. Why does punctuation contribute more to some lineages than to others? (Plants and fungi are more affected than animals.) How do lineages spread into the colder climates of higher latitudes?
    Some of the old questions, though, are still rattling around like skeletons in a cupboard. Dr Jablonski’s study does not, for instance, help to answer whether the tropics are a wellspring of species because a hotter climate brings a higher mutation rate, or because more intense interactions between species select for more rapid evolutionary change. In the spirit of compromise, there seems no reason why it could not be a bit of both. [br] The bivalve lineages were forced out of tropical regions.
选项 A、Y
B、N
C、NG
答案 B
解析 见文章第10段，该物种分布虽然向南北扩展，在赤道地区仍有相当数量。
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