A、Extinctions during the past 25 million years have tended to be more intense every 2.6 million years. B、The theory that the per

Moderator:
    Welcome to "Science World". Today we have Dr. Michael Hudson, Chief of the Department of Life Science in Boston University. It is estimated that over 99 percent of all species that ever existed have become extinct. What causes extinction? Dr. Hudson:
    Well, 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 approximately 95 percent of all species died. Mass extinctions can be caused by a relatively rapid change in the environment and can be worsened by the close interrelationship of many species.
    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 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 or inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially random events.
    23.What does Dr. Hudson say regarding the vast majority of species in Earth’s history at the beginning of the lecture?
    24.What does the evidence from fossils suggest?
    25.What can we learn about the ’Interesting and controversial" finding?