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  3. Today, we are going to be continuing our discussion of evolution, specifically how the forces of natural selection and environme

Today, we are going to be continuing our discussion of evolution, specifically how the forces of natural selection and environme

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问题    Today, we are going to be continuing our discussion of evolution, specifically how the forces of natural selection and environmental pressures interact. Last time we said that for any species the most important urge is to reproduce and ensure survival of the species. Now any organism has a set amount of energy it can’t devote to the task of reproduction to produce eggs, find a mate, rear its offspring, etc. But different species have different strategies for how they use that energy. Let’s look at two examples to clarify this. For this purpose we’ll compare two aquatic animals: the blue whale and the American shad.
   The blue whale reproduces quite slowly. Females don’t reach sexual maturity until they are about ten years old. They take an entire year to give birth, and the calf is dependent on its mother for at least the next 6 months. That means at maximum, blue whales can reproduce once every two years, and it’s often more like every three years. In the context of this low birth rate, every calf is valuable, and a considerable amount of energy is devoted to caring for the calf and ensuring its survival.
   The American shad reach sexual maturity in as little as three years. They breed every single year. And in one breeding season a female American shad may lay as many as 300, 000 eggs. Of course, only a tiny fraction of these will survive to become mature shad themselves. After the female shad lays her eggs, she moves back into the ocean leaving her young to fend for themselves.
   So blue whales and American shad employ entirely different reproductive strategies that kind of lie on opposite ends of the spectrum. Blue whales invest all of their energy into a small number of offspring, while American shad put their energy into reproducing in huge numbers but with a very low chance of survival into maturity.
   Many other species follow one of these two reproductive strategies. Those that fit into the pattern of the blue whale are known as K-strategies, and those that reproduce in large numbers with low chances of survival for individual offspring are known as R-strategies. So what determines whether a species evolves to be a K-strategist or R-strategist? Well, it largely has to do with the environmental pressures that the species faces and how they fit into their overall habit.
   Blue whales are the biggest organisms in the ocean, and as such they have no natural predators. What’s more, their large bodies mean that they can build up large reserves of fat to sustain them when food is scarce. As a result, a mature blue whale has a very good chance of survival from year to year. In fact, if a blue whale manages to survive its childhood and reach maturity, it will most likely continue to live until it either dies of disease or old age. What all this means is that the blue whale population tends to be pretty stable. It generally doesn’t experience massive or sudden declines, or at least it didn’t until humans started disrupting its habitat.
   American shad lead a much riskier existence. Even when they reach maturity they are preyed upon by a number of larger fish. Moreover, their population is much more susceptible to food shortage and mass starvation. But this is balanced out by the shad’s ability to reproduce quickly, but it can always bounce back.
How does the passage explain why various species have evolved to be R-strategists or K-strategists?
选项 A、By discussing the environmental pressures faced by various species.
B、By explaining the life cycles of R-strategists and K-strategists.
C、By discussing the evolutionary origins of several species.
D、By comparing the chances of survival between R-strategists and K-strategists.
答案D
解析 本题细节定位于最后两段。由此可知，蓝鲸在海洋中没有天敌，而且它们巨大的身体还可以储备大量的脂肪，支撑它们度过食物匮乏时期。因此成年蓝鲸的生存率很高。相比而言，美国鲥鱼的生存风险却大得多，当它们长成后还会被一些较大的鱼捕食。此外，它们更容易受到食物短缺的影响。据此可以推知，文章通过比较两类物种的生存率来解释不同的物种进化成为R策略模式或K策略模式的原因，因此选择D。
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