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Evolution: Blink and you'll miss it
09 July 2005
From New Scientist Print Edition.
Bob Holmes
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Evolving fastEVERY weekend angler knows to throw back the tiddlers. Likewise, commercial fishermen use large-meshed nets to spare smaller fish. Both are working on the principle that by reducing their haul this way, they can keep fish populations vigorous and healthy. But they could be making a terrible mistake. It is becoming increasingly clear that such well-meaning strategies may actually have the opposite effect to what the fishermen intend.
What they and most of the rest of us have overlooked is evolution - not the familiar glacier-slow process found in textbooks, which takes millennia to work its wonders, but a burbling freshet of evolutionary change that can occur in a matter of years or decades. By leaving the smaller fish, fishermen may be shifting the evolutionary goalposts, reshaping fish species as they go. In fact, biologists are starting to suspect that this phenomenon, which they have dubbed contemporary evolution, is happening all around us. Besides emptying fishing nets, rapid evolutionary change cripples the efforts of doctors and farmers, thwarts trophy hunters in search of the big prize, and frustrates conservation biologists trying to rescue endangered species.
What's more, in the decades to come, the pace of evolution may quicken still further, as human activities transform the Earth, forcing species to adapt or die. That makes our need to understand the forces at work even more compelling. If we know what's going on, we may be able to find ways to control evolution, and even shape it for our benefit and that of the world around us.
Evolutionary biologists have long known that the process can happen rapidly - Charles Darwin himself pointed out the observable changes wrought by pigeon fanciers and dog breeders. A century later biologists showed that peppered moths in England's industrial heartland had evolved darker colours to camouflage themselves against soot-blackened trees. And by the end of the 20th century everyone knew that bacteria, insects and weeds were able to evolve resistance to antibiotics and pesticides within a few years. But few thought such speedy evolution was more than just a special case.
"When I was a graduate student in the 1970s, the prevailing idea was that evolution was this gradual, slow process," says David Reznick of the University of California, Riverside. "We already knew there were instances of evolution that people had witnessed, but it was considered to be exceptional, not the usual pattern."
The experts had good reason to be sceptical that evolution could happen quickly. After all, evolution is driven by a mismatch between an organism's needs and its abilities to meet them. The prevailing wisdom was that most organisms were already well adapted to their circumstances. Although there would be genetic variation between individuals within a population, no combination of genes would be particularly better adapted than any other, so there would be little pressure for natural selection to favour the survival and reproduction of some individuals over others. In other words, selection would generally be low and evolution slow - except where humans used antibiotics or pesticides to wipe out all but the one-in-a-million resistant individuals, or allowed only the gaudiest pigeons to breed.
All change
But in the 1980s biologists began to realise that adaptation might be a more dynamic process than they had thought. For example, on one of the Galapagos Islands, Peter and Rosemary Grant of Princeton University discovered that among one species of finch, individuals with small beaks do best in wet years, when small-seeded plants thrive, while their larger-beaked nestmates have the edge in drier years, when larger-seeded plants predominate. As a result, beak size see-saws back and forth rapidly.
More recently, a team led by Barry Sinervo of the University of California, Santa Cruz, has found the same kind of rapid change in the side-blotched lizard in the south-western US. Male lizards pursue one of three different genetically determined mating strategies, each corresponding with a different throat colour. Orange-throated males are big and aggressive, and easily bully the more timid blue-throated males into ceding their females. Yellow-throated males, which sneak in disguised as females, can steal mating opportunities from the orange males while they are busy blustering, but fail to fool the blue males as these pay close attention to their precious mates. The result is a game of evolutionary rock-paper-scissors, with each strategy becoming dominant every four to five years.
No one knows how common this sort of contemporary evolution is, because it is hard to spot in the wild. The change happens so fast that biologists are likely to miss it unless they keep very detailed records of exactly the right characters - a complete reversal of the old view that evolution is too slow to see in real time. "There's no reason this couldn't be going on all the time in organisms all over the place," says Reznick.
Nor is rapid evolution confined to the cycling of different versions of the same trait. Sometimes evolution drives steadily in one direction. This may be crucial to our understanding of the biology of invasive species. Biologists have often noted that introduced species, such as zebra mussels or garlic mustard in the US, can lurk inconspicuously in their new home for decades or even centuries before suddenly exploding into problem pests. One possible, though not yet well tested, explanation is that the invaders are at first poorly adapted to their new setting, and cannot take off until they evolve a better match. And once that happens, the result can be dramatic. "Many of these invasions may reflect a genetic shift in the invading population," says Donald Waller from the University of Wisconsin-Madison. "A lot of [organisms] are just a couple of percentage points above or below break-even, so it only takes a little change to make a big difference."
Human activity is changing some ecosystems faster, and more dramatically, than ever before, and strong directional selection may be especially common in these cases. "It's possible these human-induced changes are not just greater, but more consistent and more permanent. They may be resulting in evolutionary changes that are rapid, but may also be persistent as well," says Andrew McAdam from Michigan State University in East Lansing. For example, ivory hunting has favoured the evolution of tuskless elephants in parts of Africa and Asia.
One of the best places to see evolution in action is high in the Rocky Mountains of Alberta, Canada, home of the largest bighorn sheep in North America. Hunters can pay six-figure sums for the right to shoot a big ram, the massive, curling horns of which make it the continent's most highly prized hunting trophy. On one peak, aptly named Ram Mountain, hunting has been so intense that rams can expect to live only a year or two after their horns reach the almost-360-degree curl that makes them a legal target for hunters. Not surprisingly, this has led to intense selection in favour of males whose horns never grow to reach trophy status.
Sure enough, a study led by Dave Coltman, now at the University of Alberta in Edmonton, found that average horn size has declined by about 25 per cent over the past 30 years (Nature, vol 426, p 655). And the genetic erosion doesn't end there, because larger-horned rams tend to have better genes in general. "You start taking out the prime-quality rams and the next generation will be missing those genes, because their fathers will be lower quality," says Coltman. In other words, every time they pull the trigger, hunters are working against their own long-term interests. "It's a form of artificial selection where instead of getting more of what you want you're actually going to end up with less," he says.
The same thing happens at sea, where fishermen are typically only allowed to keep fish larger than a particular size. Three years ago, David Conover from Stony Brook University in New York showed just how counterproductive this might be. Conover and his colleague Stephan Munch simulated intense size-selective fishing on lab populations of a small commercial fish called the Atlantic silverside. After just four generations, fish from the "fished" populations - in which the largest 90 per cent of fish were removed before breeding - averaged barely half the size of fish in the "anti-fished" populations, in which the smallest 90 per cent were removed. As a result of the size difference, the total weight of fish removed (analogous to the fishery harvest) in the fifth generation of the fished population was barely half that of the anti-fished one (Science, vol 297, p 94).
Since then other researchers have shown that cod off the coast of Newfoundland, Canada, have also evolved toward maturing at smaller sizes - presumably as a result of the capture of the largest fish. As well as contributing to the crash of the area's fishery, this shift may also hinder the cod's ability to recover, since small fish produce many fewer eggs than large fish. This could help explain why cod populations have failed to bounce back on the Grand Banks, off south-east Newfoundland, despite closure of the fishery there for the past 13 years.
If contemporary evolution really is a dominant force in heavily fished populations, then fisheries managers may unwittingly be doing just the opposite of what they should to maintain healthy stocks. Instead of catching the biggest fish and letting the rest go, we need to treasure the big fish as bearers of the best genes. One solution, says Conover, would be to let fishers take only medium-sized fish. If we did that, he says, a fish's best strategy would then be to grow through that window as fast as possible. Such a scheme would select for fast growth rates - a big improvement over the present system, which selects for scrawny fish that never reach the minimum catch size.
Turning evolution back from the "dark side" in fisheries can be done, but it won't be easy. "If you had a maximum size limit, under present trawl technology there wouldn't be a way to let the large ones go except by picking them out on your deck and throwing them back, and a lot of them wouldn't survive," says Conover. But, he notes, modern trawls often use large-mesh metal grates to shunt sea turtles away from the net mouth while allowing fish through, and these might be adapted to exclude large fish as well.
But there is another, more drastic way to put the brakes on evolution: introduce no-fishing zones before stocks become too depleted. Such protected areas provide a refuge where larger fish can survive and continue to produce disproportionate numbers of eggs, so diluting the selection that would otherwise drive fish toward smaller sizes. No-hunting zones for bighorn sheep - or regulations that allow hunters to take a small number of sheep, but of any size - would similarly blunt selection for smaller horns.
Contemporary evolution is not always a bad thing, though. It is already being used to fit microbes for useful work (see "Change for the better"). And with man-made climate change looming, plants and animals will need the ability to adapt quickly. Biologists have noticed that several species have already responded to the warmer temperatures, and hence earlier springs, of the past few years by migrating or breeding earlier. For example, Stan Boutin and his colleagues at the University of Alberta found that red squirrels in Canada's Yukon territory now give birth about 18 days earlier than they did just a decade ago. Using tissue samples to determine each squirrel's parentage, the researchers could see how much of the variation in birth date ran in families. From this they calculated that at least 13 per cent of the change - representing a shift of almost a full day per generation - was due to evolution and not behavioural flexibility.
Fast forward
Whether evolution can move fast enough to cope with the unprecedented rates of climate change expected over the next century remains to be seen. Clearly though, a species cannot evolve a new adaptation unless it has the right genes - and larger populations are more likely to possess this genetic capital than small ones. This means the losers in the climate-change shuffle are likely to be the species that are already rare. Conservationists might even need to consider abandoning some marginal populations and concentrating on those with the genetic resources to evolve successfully, says Boutin. "That means we maybe don't save every caribou herd in Alberta, but we focus on the ones with the highest probability of success."
And if rapid contemporary evolution really is as widespread as some researchers are beginning to suspect, it has one more unsettling implication: we may have to modify our notion of "preserving" rare species, because every effort to rescue a species through captive breeding, founding new wild populations, or modifying existing habitats may cause it to evolve away from its starting point.
"This brings up an interesting philosophical question," says McAdam. "What is it that we're hoping to conserve? Is it particular species, or is it something about those species? Would we be happy if we were able to maintain all the species we have today, but human-induced evolutionary changes were so great that they essentially became functionally domesticated? Would we be satisfied with that? I would say no, that's not satisfying - at least to me."
From issue 2507 of New Scientist magazine, 09 July 2005, page 28
Change for the better
When it comes to rapid evolution, microbes are the clear champions - just ask any physician struggling to treat an antibiotic-resistant infection. But that same evolutionary precociousness also makes microbes an unparalleled tool for cleaning up toxic messes.
Take weedkillers such as atrazine and 2,4-D, or nitrotoluenes such as TNT. Born in chemists' labs, these chemicals had never existed on Earth before. Yet just a few decades after their introduction, bacteria whose ancestors have been around for 3.5 billion years had evolved the enzymes needed to break them down for food. Their secret? They can pick up second-hand genes from their neighbours at what is essentially a vast, freewheeling flea market, and then tinker with them to alter their function.
Microbiologist Michael Sadowsky from the University of Minnesota, Twin Cities, has found evidence for this in the soil bacterium Pseudomonas. The four genes it uses to break down atrazine are scattered at random in its genome, suggesting that they were picked up one at a time. What's more, each is bracketed by transposons, bits of mobile DNA often implicated in genetic reshuffling.
Off-the-shelf genes, even in new combinations, will not always work against novel chemicals. But bacteria have other ways of opening the evolutionary throttle when they need to. One tactic is to up the mutation rate when times are tough. Some bacterial plasmids carry a gene encoding an error-prone DNA-copying enzyme called DNA polymerase V that is activated during times of stress. More mutations mean more tickets in the genetic lottery, which increases the chance that some lucky bacterium ends up with a useful new variant on an old enzyme.