Text 37. WHAT SHAPES A CREATURE'S LIFE COURSE AND BEHAVIOR?

More than 130 years ago, Charles Darwin laid out his elegant and then shocking theory that it was natural selection, the survival of individuals most suited to the environment, that molded species over evolutionary time. But until very recently, Darwin's ideas remained unproved. Now, for the first time, studies in the wild are rigorously demonstrating the particulars of how evolution works.

Last month, David Reznick, an evolutionary ecologist at the University of California at Riverside, published the results of an 11-year experiment involving guppies living in the Aripo River of Trinidad. The experiment proved that predators are among the principal forces driving the evolution of species, just as predicted by a mathematical model that modern biologists had formulated to refine Darwin's theme.

According to the model, animals that are preyed on as adults will evolve to produce as many babies as they can, as early in life as possible. «If your chances of dying young are good, then having babies yearly is important», says Reznick. However, the earlier a species reproduces, the sooner it burns out, so to speak, and the shorter its life span. Conversely, whose juveniles bear the brunt of predatory attacks tend to have their young later in life, in effect choosing to bear hardier offspring over a longer adult life.

The experiment that Heznick devised, along with his colleagues, was delightfully uncomplicated. The researchers tested the mathematical model simply moving 200 guppies from the base of a 20-foot waterfall in the Aripo River, where predatory fish eat only adult guppies, to the top of the waterfall. There, the single predator is a killfish, a species that devours only young guppies.

After 60 generations, the experimental guppies had evolved in their new environment just as the model predicted. The fish now reach sexual maturity nine days later when they first give birth. What's more, they have fewer offspring in their first brood, apparently saving themselves for future breeding opportunities.

Reznick's is only one of several recent studies verifying Darwin's grand theory in nature. In others, for instance, parasites are proving to be an even more potent agent of evolutionary change than predators. Marlene Zuk, a colleague of Reznick's at Riverside, recently completed a study demonstrating that parasites affect how female jungle fowl, the wild ancestors of, barnyard chickens, choose their mates. She found that rematode parasite, which lives in the gut, renders the eye and comb of the male fowl dull and consequently unattractive to females. The females prefer instead cocks bright of eye and comb, who will bestow their offspring with genes that will make them resistant to parasites.

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