The old belief that climatic stability accounts for the high level of species diversity in the Amazon
River basin of South America emerged, strangely enough, from observations of the deep sea. Sanders discovered high diversity among the mud-dwelling animals of the deep ocean. He argued that such diversity could be attributed to the absence of significant fluctuations in climate and physical conditions, without which the extinction of species should be rare. In the course of time new species would continue to evolve, and so the rate of speciation would be greater than the rate of extinction, resulting in the accumulation of great diversity. Sanders argued that the Amazon
tropical rain forest is analogous to the deep sea: because the rain forest has a stable climate, extinction should be rare. Evidence that some species of rain-forest trees have persisted for some 30 million years in the Amazon
basin, added to the absence of winter and glaciation, supports this view.
Recently however, several observations have cast doubt on the validity of the stability hypothesis and suggest that the climate of the Amazon
basin has fluctuated significantly in the past. Haffer noted that different species of birds inhabit different corners of the basin in spite of the fact that essentially unbroken green forest spreads from the western edge to the eastern edge of the region. This pattern presented a puzzle to biologists studying the distributions of plants and animals: why would different species inhabit different parts of the forest if the habitat in which they lived had a stable climate?
Haffer proposed a compelling explanation for the distribution of species. Observing that species found on high ground are different from those on low ground and knowing that in the Amazon
lowlands are drier than uplands, he proposed that during the ice ages the Amazon
lowlands became a near-desert arid plain; meanwhile, the more elevated regions became islands of moisture and hence served as refuges for the fauna and flora of the rain forest. Populations that were once continuous diverged and became permanently separated. Haffer’s hypothesis appears to explain the distribution of species as well as the unusual species diversity. The ice-age refuges would have protected existing species from extinction. But the periodic geographic isolation of related populations (there have been an estimated 13 ice ages to date) would have facilitated the development of new species as existing species on the lowlands adapted to changing climates.
Although no conclusive proof has yet been found to support Haffer’s hypothesis, it has led other researchers to gauge the effects of climatic changes, such as storms and flooding, on species diversity in the Amazon
basin. Their research suggests that climatic disturbances help account for the splendid diversity of the Amazon
15. As discussed in the first paragraph of the passage, Sanders’ analogy between the deep sea and the Amazon
basin involves which one of the following assumptions?
(A) Both the Amazon
basin and the deep sea support an unusually high rate of speciation.
(B) Both the rain-forest trees in the Amazon
basin and the mud-dwelling animals in the deep sea have survived for 30 million years.
(C) Both the deep sea and the Amazon
basin have not experienced dramatic changes in climate or physical conditions.
(D) A dependable supply of water to the Amazon
basin and the deep sea has moderated the rate of extinction in both habitats.
(E) The rate of speciation in the Amazon
basin is equivalent to the rate of speciation in the deep sea.
16. The author of the passage would most likely agree with which one of the following statements about Haffer’s hypothesis?
(A) It provides an intriguing and complete explanation for the high rate of species diversity in the Amazon
(B) It is partially correct in that a number of climatic disturbances account for species diversity in the Amazon
(C) It has not yet been verified, but it has had an influential effect on current research on species diversity in the Amazon
(D) It is better than Sanders’ theory in accounting for the low rate of species extinction in the Amazon
(E) It provides a compelling explanation for the distribution of species in the Amazon
basin but does not account for the high species diversity.
17. According to the passage, lowlands in the Amazon
basin currently differ from uplands in which one of the following respects?
(A) Lowlands are desertlike, whereas uplands are lush.
(B) Lowlands are less vulnerable to glaciation during the ice ages than are uplands.
(C) Uplands support a greater diversity of species than do lowlands.
(D) Uplands are wetter than are lowlands.
(E) Uplands are more densely populated than are lowlands.
18. Which one of the following best describes the organization of the passage?
(A) A hypothesis is discussed, evidence that undercuts that hypothesis is presented and a new hypothesis that may account for the evidence is described.
(B) A recently observed phenomenon is described, an explanation for that phenomenon is discussed, and the explanation is evaluated in light of previous research findings.
(C) Several hypotheses that may account for a puzzling phenomenon are described and discounted, and a more promising hypothesis is presented.
(D) A hypothesis and the assumptions on which it is based are described, and evidence is provided to suggest that the hypothesis is only partially correct.
(E) Two alternative explanations for a phenomenon are presented and compared, and experiments designed to test each theory are described.
19. The author of the passage mentions the number of ice ages in the third paragraph most probably in order to
(A) provide proof that cooler and drier temperatures are primarily responsible for the distribution of species in the Amazon
(B) explain how populations of species were protected from extinction in the Amazon
(C) explain how most existing species were able to survive periodic climatic disturbances in the Amazon
(D) suggest that certain kinds of climatic disturbances cause more species diversity than do other kinds of climatic disturbances
(E) suggest that geographic isolation may have occurred often enough to cause high species diversity in the Amazon
20. The passage suggests that which one of the following is true of Sanders’ hypothesis?
(A) He underestimated the effects of winter and glaciation in the Amazon
basin on the tropical rain forest.
(B) He failed to recognize the similarity in physical conditions of the Amazon
lowlands and the Amazon
(C) He failed to take into a count the relatively high rate of extinction during the ice ages in the Amazon
(D) He overestimated the length of time that species have survived in the Amazon
(E) He failed to account for the distribution of species in the Amazon
21. Which one of the following is evidence that would contribute to the “proof” mentioned in line 54?
(A) Accurately dated sediment cores from a freshwater lake in the Amazon
indicate that the lake’s water level rose significantly during the last ice age.
(B) Data based on radiocarbon dating of fossils suggest that the Amazon
uplands were too cold to support rain forests during the last ice age.
(C) Computer models of climate during global ice ages predict only insignificant reductions of monsoon rains in tropical areas such as the Amazon
(D) Fossils preserved in the Amazon
uplands during the last ice age are found together with minerals that are the products of an arid landscape.
(E) Fossilized pollen from the Amazon
lowlands indicates that during the last ice age the Amazon
lowlands supported vegetation that needs little water rather than the rain forests they support today.