托福考试阅读加试之达尔文进化论(原文)

Observing Natural Selection

Witnessing natural selection would not have seemed possible to Darwin because he assumed natural selection was too slow and gradual for our short-term minds to perceive. Yet later biologists have been able to witness flashes of evolutionary change. In the late 1980s, for example, biologist David Reznick began to use the guppies that swim in the streams of Trinidad forests in natural experiment. At lower elevations these guppies face the assault of predatory fishes, but the ones in higher waters live in peace because few of the predators can move upstream past the waterfalls and craggy rocks.

Like all animals, guppies have a timetable for their lives - how long they take to reach sexual maturity how fast they grow during that time, how long they live as adults. Theoretical biologists have predicted that the life history of animals can evolve if mutations that alter it bring the animals more reproductive success. Reznick put their predictions to the test.

In ponds with a lot of predators, guppies that grow fast should be more successful than slow-growing ones. With the threat of death hanging over a guppy, it will grow as quickly as possible so that it can start mating as soon as possible and have as many offspring as possible. Of course, the strategy comes with a heavy price. By growing so quickly, a guppy may shorten its own natural life span, and by quickly giving birth to babies, female guppy cannot take time to support her offspring with energy, which put them at risk of dying young. But Reznick reasoned, that the threat ,of an early death offset