Darwinian shift: survival of the smallest

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.

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.

The reason: Trophy hunters focused on taking the largest rams with the largest horns. These rams typically were shot before they reached their peak reproductive years. So, with many of those animals gone, the gene pool narrowed to favor the smaller rams.

The same trends and mechanisms also appear to be affecting two other wild sheep populations subject to the same management regime, says David Coltman, a University of Sheffield evolutionary biologist who led the team conducting the study.

Several factors have led to a deeper appreciation of the role contemporary evolution can play in the wild.

By the mid-1990s, an increasing number of researchers were finding examples of contemporary evolution - ranging from Darwin's finches in the Galapagos Islands and guppies in Trinidad to bacteria that quickly developed resistance to antibiotics. Moreover, researchers had new tools - from increasingly sophisticated statistical models to DNA sequencing. This allowed them to ask questions and test ideas in ways that hadn't been possible before, Dr. Coltman says.

Finally, "wildlife managers and people interested in ecology came from different schools from those of us who worked in evolutionary biology. We traditionally worked on different questions," he says. These days, an increased emphasis on interdisciplinary science has brought these groups of researchers together, he adds.

In some respects, several conservation approaches already account for evolutionary effects - if inadvertently - in the populations they seek to protect.

Coltman says that in Europe, for example, the size of the animals that hunters are allowed to bag depends on the experience level of the hunter. The biggest, wiliest game are reserved for those who have been hunting the longest.

Thus, human predation on game is spread more randomly throughout the game animal's population, and hunters are allowed to take females. The regime more nearly mimics predation found in the wild, and so doesn't put undue evolutionary pressure on the animals that can provide the most robust breeding stock.

Such techniques are more problematic in fisheries, says David Conover, marine ecologist at the State University of New York, Stony Brook. Historically, management regimes have specifically protected smaller fish while allowing the largest to be caught, he notes. "Rather than use a harvest strategy that mimics nature, fishing reverses the tables 100 percent."

Dr. Conover has studied the evolutionary effects on laboratory populations, harvesting only the heftiest of the fish known as Atlantic silverside. He says that for fishing techniques such as trawling, net design would make it difficult to toss back the biggest. Yet gill-net and long-line fishing, in which more fish are handled by humans as they are brought in, could be regulated in ways that reduce their evolutionary effect on fisheries, he adds.

Another approach, already being applied along coastal stretches of the United States, is to establish protected areas where fishing is banned and stocks are allowed to rebuild with as much of their natural genetic variation as possible, he adds.

For endangered species, the implications of managing for as natural an evolutionary future as possible holds its own challenges. For example, selection pressures on a species can vary greatly along the length of a river and within its tributaries, says Michael Kinnison, a biologist at the University of Maine in Orono who has studied the intersection of ecology and contemporary evolution.

Each population adapts to its local conditions. Thus, when it comes to reintroducing species to portions of a river from which it vanished, one can't always pluck salmon from one tributary and use them to populate another.

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