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Darwinian shift: survival of the smallest

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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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