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Darwinian shift: survival of the smallest

By Peter N. SpottsStaff writer of The Christian Science Monitor / May 20, 2004



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

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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.

So Esben Olsen, a Norwegian marine ecologist, and a team of researchers decided to find out why. Were factors such as low food supplies or unusual ocean conditions responsible for the population's failure to rebound? Or did the fishing industry, by pulling up the larger fish, channel the populations' evolution toward smaller sizes, earlier maturity, and less reproductive success?

After analyzing nearly three decades' worth of data, the scientists concluded that evolution was indeed at work: Survival of the smallest. Dr. Olsen's team reported its results in the April 29 edition of the journal Nature.

"This shift toward early maturation could slow down the recovery of the population" because the fish can't produce offspring as robustly as the older fish could, Olsen says in a phone interview from his Oslo home.

The team made another key finding. The change showed up in the cod's population statistics before the collapse actually snowballed. He says this approach could be used as an early warning system for evolutionary trouble ahead.

Such a finding implies big changes for the way fisheries managers operate. If they are to take contemporary evolution into account, managers will have to cut back fishing of endangered populations earlier than ever - when the genetic changes are beginning to appear rather than when populations begin to collapse.

Another potential change: a more rigorous process for preserving genetic diversity. That would involve, scientists say, better screening to identify individuals to reintroduce; more detailed, persistent monitoring programs to find out how they're faring; and a focus on the genetic adaptability of distinct populations of a species, rather than on organisms thought to be most representative of a particular species.

Fast-track evolution affects more than fish. Last December, researchers in Alberta who closely tracked family histories within a group of mountain sheep at Ram Mountain reported that over a 30-year period, the rams in the population matured to smaller sizes and sported ever-smaller sets of horns.

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