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Say evolution and you probably think of changes that take thousands or even millions of years, dinosaurs evolving feathers or some ape learning to walk on two legs. But it turns out that evolution can sometime happen much faster, almost before our very eyes. And that could have big implications for anyone who likes to eat fish.

NPR's David Malakoff explains.

DAVID MALAKOFF reporting:

A couple of times a year biologist David Conover leaves his office at Stony Brook University and goes fishing, in his own private ocean.

(Soundbite of running water)

Mr. DAVID CONOVER (Dean and Director Stony Brook University): How many fish do we have?

Unidentified Man #1: Sixty-eight.

Mr. CONOVER: Good. Those fish are pretty small.

MALAKOFF: Conover's ocean is actually a dozen huge barrels of salt water. They sit on a concrete floor, surrounded by pumps and pipes, in the university's marine lab on Long Island in New York. Swimming around in the tubs are little fish called Atlantic silversides. They're about the size of a French fries.

Mr. CONOVER: In each one of these tanks, we have around 500 silversides and they are very torpedo-shaped. They are a schooling fish that is built for speed, not comfort.

MALAKOFF: Now, Conover doesn't fish for silversides because he likes to eat them. Rather, the fish are part of an experiment. It's aimed at solving a mystery out in the real ocean. The mystery does involve some popular table fish, like the northern cod off Canada. Northern cod were once so in demand that they were badly over-fished. Populations plummeted.

So about a decade ago, the Canadian government basically banned fishing for northern cod. The idea was to give the fish some time to have babies and get their numbers back up. Conover says the trouble is that the cod, and some other over-fished stocks, haven't bounced back.

Mr. CONOVER: That's not supposed to happen. According to theory, they're supposed to find plenty of food and be able to grow fast and produce lots of offspring. We depends on that rebound happening. And in many stocks it's not happening. So something else is going on.

MALAKOFF: Conover wondered if that something else could be evolution. That may sound strange, but here's how biologists think about it. Start with the idea that modern fishing fleets are really, really good at what they do. Decade after decade, they fill up their nets with the biggest, most valuable fish. That means those fish don't get much opportunity to reproduce. And in a relatively short time, there are fewer and fewer fish left that carry those genes for bigness.

So what remains in the ocean? Well, a bunch of genetic runts; little, scrawny fish that have gained an evolutionary advantage because they can slip through the nets and survive to have babies.

The problem is runty fish often don't have very many babies. And of course they are more likely to pass on the genes for runtiness. And so the babies are less likely to carry the genes for things that fishermen want, like fast growth and size.

Mr. CONOVER: Don't forget to watch for deformity.

MALAKOFF: Now, to see if catching all the big ones really could make that happen, really could force evolution to favor little fish, Conover built his ocean eight years ago and then started fishing.

Unidentified Man #2: Seventy-one.

Unidentified Woman #1: Sixty-two.

Unidentified Woman #2: Sixty-nine.

Unidentified Woman #3: Seventy-five.

MALAKOFF: Armed with long-handled nets, his team of researchers play the role of the fishing boats.

Unidentified Man #2: Sixty-seven.

Unidentified Woman #1: Sixty.

MALAKOFF: They scoop up silversides and then carefully measure each one.

Mr. CONOVER: I think everybody tonight will be dreaming 75, 54, 63.

(Soundbite of laughter)

Mr. CONOVER: And no one ever says hike.

MALAKOFF: Then, they throw back some of the fish. These are the lucky ones. They get to pass along their genes.

Mr. CONOVER: And the ones that are going in here are the survivors who will be the parents of the next generation. So out of the 500 or so fish we're harvesting today, about 55 or so will be the parents. The other 90 percent of the population is being harvested.

MALAKOFF: Conover has now fished his way through eight generations of silversides. And he's shown that, as predicted, if you take the big fish out of a school and leave the little ones, you'll end up with a population of runts. Moreover, it can happen very fast; in just four years, or four generations. And Conover says those results suggest that fish out in the ocean can undergo the same kind of rapid evolution that we see all the time on land.

Mr. CONOVER (Stony Brook University): We know that pests evolve rapidly in response to pesticides. We know that disease can quickly evolve responses to the antibiotics that we develop. All we're saying with fisheries is that the same process happens here, so we are undermining the capacity of the population to rebound from fishing.

MALAKOFF: Conover says that's a message that should worry officials responsible for managing fisheries.

Mr. CONOVER: Most fishery scientists had not accepted the premise that evolution was something that happened fast enough that they had to worry about it. Evolution happens slow. It takes thousands of years, so we can effectively ignore it.

MALAKOFF: Not anymore, says Steven Berkeley. He's a biologist at the University of California, Santa Cruz. The danger, he says is that fishing is taking all the good genes, the ones we want, out of fish stocks.

Mr. STEVEN BERKELEY (University of California, Santa Cruz): If a farmer did this, he'd be out of business pretty quickly. He would, in fact, be selecting for the poorest egg-laying chickens or the slowest-growing cattle or the ones that convert food the most inefficiently, and so it's exactly the opposite of what we would be doing if we were farming fish.

MALAKOFF: Berkeley says one way to save the good genes is to set up more marine reserves where fish are protected from nets, and he says that if there are some good news from David Conover's experiment, it is that it's still not clear if fishing can cause permanent evolutionary change. If given more time, he says over-fished species may still bounce back. David Malakoff, NPR News.

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