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An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the amount of life on earth. Such an event is identified by a sharp change in the diversity and abundance of macroscopic life. It occurs when the rate of extinction increases with respect to the rate of speciation. Because the majority of diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.
Over 98% of documented species are now extinct, but extinction occurs at a very uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organism fossils.
Since life began on Earth, the five major mass extinctions have significantly exceeded the background extinction rate for animal and plant species. The most recent, the Cretaceous–Paleocene extinction event, which occurred approximately 66 million years ago, was a large-scale mass extinction of animal and plant species in a geologically short period of time. In the past 540 million years, during each of these five major events, over 50% of animal and plant species died. Mass extinctions seem to be a Phanerozoic-era phenomenon, with extinction rates low before large complex organisms arose.
(1) It can be inferred from the passage that:
(A) in an extinction event, there is a dramatic decrease in microbial life on earth.
(B) over 90% of easily observed, biologically complex species become extinct during a biotic crisis.
(C) the background extinction rate of animal and plant species is well below 50%.
(D) marine fossils are easier to find and thus more useful to those studying mass extinction events.
(E) new species do not emerge during a mass extinction event.
(2) It can be inferred from the passage that the Phanerozoic era was:
(A) a time period that existed before 540 million years ago.
(B) a time period during which few new species emerged.
(C) a time period before the Cretaceous–Paleocene era.
(D) a time period during which large, complex organisms existed.
(E) a time period during which fewer than 50% of animal and plant species died.
(3) The author would most probably agree with which of the following statements?
(A) The diversity of microbial life has changed dramatically during mass extinction events.
(B) The Cretaceous–Paleocene extinction event was the most significant in the past 540 million years.
(C) There were many mass extinction events prior to 540 million years ago.
(D) Extinction rates have varied widely over the past 540 million years.
(E) Mass extinctions are less likely now than in the past 540 million years.
(4) The primary purpose of the passage is to:
(A) warn readers about the possible dangers of mass extinction events.
(B) discuss the causes of a particular scientific phenomenon.
(C) present new evidence to support the theory of mass extinction events.
(D) describe an important scientific and historical occurrence
(E) suggest that mass extinctions are likely to continue in the future.
参考答案： CDDD
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发表于 2021-5-30 21:37:35
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