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Darwinian shift: survival of the smallest

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It's a rule every weekend angler knows: Throw back the small fish. It helps the population survive long term. Right?

Wrong. Mounting evidence suggests that by harvesting only the biggest fish - or biggest mammals, for that matter - mankind is unwittingly forcing many species to evolve rapidly. This process, called "contemporary evolution," isn't taking place over centuries. It's on a fast track that can happen within a few decades.

At a minimum, these changes can reduce a species' economic value. At worst, they can help drive it to extinction. And while that may not be news to biologists, it's throwing a Darwinian challenge to those who manage wildlife, preserve habitats, deal with endangered species, and control invasive species.

"Like it or not, we're having massive effects on many other species, and we're changing their evolutionary context in radical ways - and rapidly," says Donald Waller, professor of botany and environmental studies at the University of Wisconsin in Madison. Contemporary evolution is being seen as "an important factor in conservation biology."

Once, such up-tempo evolution was thought to be the exception rather than the rule, researchers say. Now, it's seen as widespread, affecting organisms ranging from bacteria to bighorn sheep.

For example: One of the big puzzles for managers of fisheries involves the plunge in Atlantic cod populations around southern Labrador and Newfoundland's Grand Banks. Between the early 1960s and the early '90s, the number of cod there plummeted by 99.9 percent - one of the worst collapses of extant marine or land animals ever.

The cod that remained were smaller, matured at a younger age, spawned much earlier in their lives, and yielded weaker offspring than did their ancestors. In 1992, the Canadian government closed the fisheries. With the ban, fisheries managers expected the stocks to rebound. Yet today the populations remain at historic lows.

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