Science, May 9, 2003
Natural selection, once seen as a stately and imperceptible process, can be speeded up to resemble a case of hyperactive jiggles. Over the past 20 years, as evolutionary biologists have begun to study natural selection in the wild, they have documented record-breaking changes in some populations of animals and plants that occur in years—not centuries or millennia.
Now conservation biologists are beginning to take note. “The last year or two have been the first time that people have really been hammering on this issue,” says Andrew Hendry of McGill University in Montreal, Canada.
Hendry is the co-author of a recent paper in Trends in Ecology and Evolution that exhorts his colleagues to think about the effects of rapid evolution when they draw up plans to protect and manage species. Conservation biologists “have actually been studying species that change right under their noses,” adds Joel Brown of the University of Illinois, Chicago. They ignore it at their peril, he wrote in a recent paper in Biological Conservation, because conservation efforts can drive evolution in unexpected ways, sometimes making a protected species maladapted to its environment in just a few generations. But there’s a plus as well: A better understanding of rapid evolution may let conservation biologists harness its powers to save species from extinction.
The clearest cases of rapid evolution are triggered by sudden changes, either natural or anthropogenic, in a species’ environment. On the Galápagos Islands, for example, Darwin’s finches evolve larger or smaller beaks as their food supplies fluctuate with the climate. In Trinidad several years ago, scientists triggered a burst of evolution by simply moving guppies from a pond with predators to one without. After 11 years, evolution’s mark was apparent: The guppies took 10% longer to reach sexual maturity and as adults weighed 10% more (Science, 28 March 1997, p. 1934).
A number of biologists now suspect that fisheries managers have been inadvertently triggering similar bouts of rapid evolution. To keep stocks from collapsing, managers often put a minimum size limit on catch, giving younger fish a chance to breed before they are killed. Despite these efforts, the average size of caught fish has been falling in recent decades in many fisheries.
Studies in Europe and the United States strongly suggest that the strategy selects for smaller individuals. The evolutionary advantages are clear: If fish can become sexually mature while still small, they have more chance to reproduce and are likely to pass down more of their genes. As a result, the population on the whole gets smaller. Biologists don’t yet know whether this trend threatens the survival of the fish stocks, but fishers already know what it means to their pocketbooks.
In Norway, grayling in mountain lakes have long been protected by size limits. Thrond Haugen of the University of Oslo and colleagues have found that after 6 decades, the fish reached adulthood when they were 25% smaller. Researchers at the State University of New York, Stony Brook, have recreated such fishing pressures in the lab. David Conover and colleagues raised thousands of Atlantic silversides and harvested the biggest from each generation. In four generations, the fish became genetically programmed to grow only half as large (Science, 5 July 2002, p. 94).
Conover says these results suggest that fisheries will suffer from low yields for a long time even if managers remove catch sizes. Because the small size of the fish is genetically programmed, only an intense evolutionary pressure can reverse the trend. But natural selection in favor of larger sizes is far milder than the intense pressure created by commercial fishing. “That could take thousands of generations,” says Conover. “There’s no force that directs evolution in the opposite direction that fishing does.”
Some conservation biologists believe that captive breeding programs can also backfire in the face of unanticipated evolution. In studies on wild and captive chinook salmon, Daniel Heath of the University of Windsor in Ontario and colleagues documented that females face an evolutionary tradeoff. On one hand, it pays for a salmon to lay big eggs, because the extra energy she packs into them helps the offspring survive after they hatch. But the bigger the eggs, the fewer a salmon can lay. Captive breeding programs change this tradeoff, because in the less stressful environment of a hatchery, salmon eggs can survive even if they’re small, and females that lay a lot of eggs are at an evolutionary advantage. In studies at a British Columbia fish farm, Heath has found that captive salmon have indeed become more prolific egg-layers; over just four generations the eggs have become 25% smaller (Science, 14 March, p. 1738). “You’re looking at a phenomenal response,” says Heath—one of the fastest rates of evolution ever recorded outside a lab. If these fish were to be put back in the wild, Heath warns, their small eggs would be less likely to survive to adult fish.
He believes the lessons from salmon apply to many other endangered species. “If you grab the last few animals and you put them in a zoo to make sure they don’t die, you could potentially drive evolution of some trait that you don’t expect,” says Heath. If the animals were eventually released into the wild, “a loss of fitness might mean the difference of survival and extinction. That’s the scary part.”
Scary, but not hopeless, adds Hendry. He thinks captive breeding programs should try to breed animals and plants in the same way farmers breed crops—selectively, for certain traits. One way to do this, Hendry suggests, is to regularly release a few captive-bred individuals—“selection probes,” as some researchers calls them—and see which survive. The biologists could then breed the relatives of the survivors but not the less fit individuals. “It would be a drastic shift in the way people thought about these things,” says Hendry. But it might save some species in the process.
Copyright 2003 American Association for the Advancement of Science. Reprinted with permission.