英语听力文摘 English Digest 427、神奇的进化（在线收听）

Maggot evolution

蝇蛆的进化

You may already know how the evolution of a new species happens. Two or more populations of one species have to be separated from each other,     such as is the case with Darwin’s finches in the Galapagos.

Various populations of the same species of bird divided onto different islands, and because of this isolation they eventually evolved apart into different species. What can the problem of maggots teaches me?

These maggots provided evidence that the separate evolution that is the forerunner of new species does not have to be geographical or physical. You see, apple maggots were originally hawthornmaggots.

Apples weren’t introduced into North America until around 250 years ago, and some time after that, some of the hawthornmaggots shifted from feeding on hawthorn to feeding on apples, separating the maggots into two races.

The hawthorn and apple trees occupy the same lands though, so the two kinds of maggots were never physically separated. In fact, they' re still the same species. By looking at them, you can’t tell them apart.

However, the structure of their proteins and the food they prefer makes them quite different, clearly on their way to becoming new species. Their food preferences are so strong that scientists cannot lure them to the wrong fruit

This goes for mating too, which occurs on the fruit. Because the two maggots are still the same species they are capable of mating together, but they don’t. It’s probably only a matter of time before the maggots do evolve into different species.

   也许你已经知道一个新物种是怎样进化的了。属于某个物种的两个或更多的生物族群之间不得不相互区分开来，达尔文研究的大龟群岛上的雀科鸣禽就是一个例子。
    同属鸟禽的不同族群分布在不同的岛上。因为这种隔离使得它们最终进化成不同的种类。
    分离进化是新物种得以产生的先驱。这些蝇蛆证明了这种分离并不一定得是地理或地域上的。你看，苹果蝇蛆原先还是山楂蝇蛆呢。
    直到250年以前苹果才被引进北美。不久之后，以山楂为生的蛆转而以吃苹果为生，使得蝇蛆分离为两个物种。 
    然而苹果蝇蛆和山楂蝇蛆同存于同一片土地，所以这两种蝇蛆从未在地理上区分开来。事实上，他们仍属于同一个物种。光是肉眼看，很难把它们区分开来。
    但是，它们蛋白质的结构和它们偏好的食物不同使得它们差异很大，尤其是它们在进化成一个新物种的过程中表现得很明显。它们对食物的偏好如此强烈以至于科学家们很难引诱它们去吃它们不爱吃的东西上。
    在交配上发生的例子和在水果上发生的一样。这两类蝇蛆都同属蛆类，是可以进行配对的，但它们却不会这样做。也许，它们会进化成不同物种，这只是时间的问题。