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.

Why is “plankton” mentioned in the second paragraph?

Câu hỏi liên quan

• The word "others" in paragraph 3 refers to ................

• Which of the following is closest in meaning to “frigid” in paragraph 1?

• Faugel’s research showed that beta blockers given to his sample ...............

ZUNIA12

• The phrase ‘as well as’ in paragraph 4 is closest in meaning to ___.

• According to the writer, students today are different from those she knew in that they are ____________.

• The word “displaced” in line 6 is closest in meaning to _________ .

• All of the following are true about the 1970 survey of Antarctica EXCEPT that it________.

• What does the passage mainly discuss?

• The most suitable title for the passage could be ..............

• Which of the following would NOT be a problem for the construction plan?

• The word “they” in paragraph 4 refers to ...........

• Most people are afraid of being operated on ............

• Why does the author mention a dog’s bark?

• What does the word "run" in the last paragraph mean?

• According to the passage,which of the following is a universal rule of etiquette?

• Which of the following best expresses the main idea of the second paragraph of the passage?

• The word “benign” in paragraph 3 is closest in meaning to ________.

• The word “These” in paragraph 3 refers to__________.

• What is the main topic of the passage?

• According to the passage, writing ................