gre考试阅读真题解析(B)_GRE阅读

gre考试阅读真题解析(B)

　　Several things, however, suggest that the conventional view needs revising. For example, the genus Lingula has an unbroken fossil record extending over more than half a billion years to the present. Thus, if longevity is any measure, brachiopods are the most successful organisms extant. Further, recent studies suggest that diversity among species is a less important measure of evolutionary success than is the ability to withstand environmental change, such as when a layer of clay replaces sand on the ocean bottom. The relatively greater uniformity among the existing brachiopod species may offer greater protection from environmental change and hence may reflect highly successful adaptive behavior.

　　The adaptive advantages of uniformity for brachiopods can be seen by considering specialization, a process that occurs as a result of prolonged colonization of a uniform substrate. Those that can survive on many surfaces are called generalists, while those that can survive on a limited range of substrates are called specialists. One specialist species, for example, has valves weighted at the base, a characteristic that assures that the organism is properly positioned for feeding in mud and similar substrates; other species secrete glue allowing them to survive on the face of underwater cliffs. The fossil record demonstrates that most brachiopod lineages have followed a trend toward increased specialization. However, during periods of environmental instability, when a particular substrate to which a specialist species has adapted is no longer available, the species quickly dies out. Generalists, on the other hand, are not dependent on a particular substrate, and are thus less vulnerable to environmental change. One study of the fossil record revealed a mass extinction of brachiopods following a change in sedimentation from chalk to clay. Of the 35 brachiopod species found in the chalk, only 6 survived in the clay, all of them generalists.