(单词翻译:单击)
Sometimes it's easy to understand how an evolutionary change took place, but it's more difficult to understand why.
有时候去理解生物进化演变的过程很容易，想知道原因却很难。
For example, consider the beautiful birds of paradise which live in the forests of New Guinea. The males of many bird-of-paradise species have evolved colorful plumage, used for dramatic courtship displays. How this plumage evolved is no mystery: The healthiest males usually have the most spectacular plumage, so females choose whichever male has the gaudiest feathers. With this criterion, it's not surprising that male plumage would grow more and more elaborate over the generations.
就拿居住在新几内亚森林中的极乐鸟作例子吧。极乐鸟种群中的雄性常常会利用它们绚丽多彩的羽毛上演一幕又一幕的求爱话剧。这羽毛是怎样进化来的并不神秘：最健壮的雄性往往拥有最绚丽的羽毛，所以雌性往往会选择有艳丽羽毛的雄性。在这种标准的要求下，雄性鸟儿拼命想长出比同类更抢眼，更精致的羽毛，这一点不足为奇。
Understanding why the plumage evolved is a little trickier. After all, there are some bird-of-paradise species that are really quite drab. These birds devote most of their energy to finding food and raising their young, instead of wasting it on courtship. What makes these near neighbors so different?
了解羽毛色泽为什么会进化就有点棘手了。毕竟，有几种极乐鸟的颜色十分单调。这些鸟把它们主要的精力都放在寻找食物和抚养后代上了，而不是用于求爱。是什么导致这些近邻如此不同？
The answer is diet. Those species with the most elaborate plumage also have the most diverse and reliable diet: a variety of insects, and fruits rich in complex nutrients. With this healthy, reliable food supply, females can raise the hatchlings without help from the males. This frees the males for their extravagant courtship behavior, and the complex diet provides them with chemicals to make their brightly colored feathers.
答案就是日常饮食。那些有着最美丽羽毛的鸟儿有各式各样并且稳定的食物：各种昆虫和富含多种营养的水果。有了这种健康的、可靠的食物供应，雌性极乐鸟可以完全不依靠雄性而独立抚养后代。这样就使得雄性有闲暇时间去发展求爱行为，而且多样饮食中的化学物质能让他们的羽毛更鲜亮。
In contrast, the drabber birds have a limited diet of simple figs, which are available more sporadically. They're monogamous, and both parents need to spend most of their time foraging. This means less time for elaborate courtship behavior.
与此相反，拥有单调毛色的鸟儿的食谱就相对简单，仅限于零星分布的无花果。同时它们实行一夫一妻制，父母双方都需要花费大量的时间寻找食物，这就大大缩短了用于考虑花哨的求爱方式的时间。
So different diets lead to different behaviors, which results in the evolution of different plumage. These exotic birds show just how complex evolution can be!
所以，不同的食物产生不一样的生活方式，也是进化出不同毛色的原因。这些奇异的鸟儿向我们解释了进化有多么复杂！
