It is estimated that over 99 percent of all species that ever existed have become extinct. What causes extinction? When a species is no longer adapted to a changed environment, it may perish. The exact causes of a species’ death vary from situation to situation. Rapid ecological change may render an environment hostile to a species. For example, temperatures may change and a species may not be able to adapt. Food resources may be affected by environmental changes, which will then cause problems for a species requiring these resources. Other species may become better adapted to an environment, resulting in competition and, ultimately, in the death of a species.

The fossil record reveals that extinction has occurred throughout the history of Earth. Recent analyses have also revealed that on some occasions many species become extinct at the same time- a mass extinction. One of the best- known examples of mass extinction occurred 65 million years ago with the demise of dinosaurs and many other forms of life. Perhaps the largest mass extinction was the one that occurred 225 million years ago, when appropriately 95 percent of all species died. Mass extinction can be caused by a relatively rapid change in the environment and can be worsened by the close interrelationship of many species. If, for example, something were to happen to destroy much of the plankton in the oceans, then the oxygen content of Earth would drop, affection even organisms not living in the ocean. Such a change would probably lead to a mass extinction. One interesting, and controversial, finding is that extinctions during the past 250 million years have tended to be more intense every 26 million years. This periodic extinction might be due to intersection of the Earth’s orbit with a cloud of comets, but this theory is purely speculative. Some researchers have also speculated that extinction may often be random. That is, certain species may be eliminated and others may survive for no particular reason. A species’ survival may have nothing to do with its ability to adapt. If so, some of evolutionary history may reflect a sequence of essentially random events.

According to paragraph 2, evidence from fossil suggests that ...............

Câu hỏi liên quan

• The word “downside” in paragraph 3 is closest in meaning to_________.

• As used in paragraph 2, the word "seeking" could best be replaced by which of the following?

• In order to operate a computer, a student does not have to be _______

ZUNIA12

• What is the main point of the passage?

• The word “reluctant” in line 13 can best be replaced by ________.

• It can be inferred from the passage that social workers, employers, and politicians concern themselves with the study of learning because they need to ______.

• The word "discrete" in line 16 is closest in meaning to ......

• According to the text, all of the following sentences are true EXCEPT _____.

• What can the word adorning be best replaced by?

• It can be inferred from paragraph 1 that___________

• According to the passage, what does “slip away” mean?

• According to the passage the first fire used by people was probably obtained ..........

• It can be inferred from paragraph 3 that _______.

• What is the similar meaning to the word “octogenarian”?

• The changes possibly caused by the cell phones are mainly concerned with ..........

• We can infer from the passage that ..........

• The Daylight Saving Time Energy Act of 1973 was responsible for ..

• What does “they” refer to?

• The first to support the equality of the sexes was .................

• Which of the following helps to create the Earth’s magnetic fields?