Humans take control of evolution
New Scientist Print Edition, Peter Aldhous, Los Angeles, February 17, 2007
CALL it unnatural selection: human activities are driving the evolution of other species in dramatic and often unexpected ways. In effect, we have turned the Earth into a vast and uncontrolled evolutionary laboratory.
That was the warning issued last week in Los Angeles, as biologists and conservationists gathered to consider the problem at an unprecedented international summit. There was no shortage of examples of species that seem to be evolving in response to human interference, but biologists are still struggling to determine exactly what is going on, and to advise on how best to protect our planet's threatened evolutionary heritage. "We're in a nascent stage in understanding human effects on evolution," admits summit co-organiser Tom Smith, an evolutionary biologist at the University of California, Los Angeles. And above it all looms the threat of climate change.
One of the problems is that unnatural selection affects species in unpredictable ways. For example, yearling Pacific Chinook salmon in Snake river, Idaho, now measure 70 millimetres long, down from 90 millimetres just 50 years ago, seemingly a result of extensive dam construction. Rather than migrating straight out to sea, many salmon slow their growth, preferring to overwinter in reservoirs behind the dams and make the trip the following year. If the dams were removed, as some environmentalists would like, it is unclear whether the slow-growth salmon could survive. "That is scary," says Robin Waples of the Northwest Fisheries Science Center in Seattle.
Farmed animals escaping from captivity, meanwhile, can breed with wild relatives - causing unexpected changes even within the same species. At Laval University in Quebec, Canada, Louis Bernatchez and Christian Roberge have compared gene activity in wild and farmed Atlantic salmon, and second-generation hybrids between the two. Surprisingly, differences in gene expression between the hybrids and wild fish were greater than between pure farmed and wild salmon. This suggests that the effects of hybridisation will not quickly be diluted out.
In some of the most striking examples of unnatural selection, human disturbance has turned back the evolutionary clock. Last year, for instance, biologists showed that two diverging populations of Darwin's famous Galapagos finches are collapsing back into one as human settlements grow (New Scientist, 20 May 2006, p 18). The locals' habit of filling bird feeders with rice may mean that the birds don't need different-sized beaks adapted to specific diets.
The new focus on the evolutionary aspects of conservation raises questions about current practices, including captive breeding. At present almost 250 vertebrate species are being maintained in this way, with the hope of eventually restoring them to the wild. But the more generations a species is held in captivity, the more it adapts to confinement. "Overwhelmingly, these changes are deleterious in the wild," says Richard Frankham, a geneticist at Macquarie University in Sydney, Australia.
Once animals disappear from a habitat, species that coexisted with them also undergo evolutionary shifts. This argues against the idea of "re-wilding" ecosystems by introducing long-lost species, or substituting them with others that can perform the same function (New Scientist, 26 August 2006, p 10).
Craig Benkman, a biologist at the University of Wyoming in Laramie, is alarmed at such proposals, given his studies of black spruce and red crossbills, a type of finch. Red squirrels, which eat spruce cones on the Canadian mainland, were absent from Newfoundland for 9000 years. As a result, the trees lost their defence against squirrels - cones with fewer seeds - and entered an arms race with crossbills, evolving cones with thicker scales as the birds developed larger beaks.
After red squirrels were introduced into Newfoundland in 1963, the local subspecies of red crossbills went into dramatic decline, and is now down to as few as 500 individuals. Benkman believes the problem is that finches had evolved to specialise on spruce cones but were outcompeted when the squirrels arrived.
At last week's summit, while some biologists were concerned about habitat homogenisation and its effect on biodiversity, one concern rose above all others: global warming. Yet we remain surprisingly ignorant about the evolutionary consequences of climate change. Researchers have documented shifts in numerous species linked to rising temperatures, such as changes in the timing of breeding. But in few cases is there any genetic information to prove that these changes are down to evolution, rather than simple shifts in growth or behaviour that give species some wiggle room to deal with changing environments - a phenomenon known as "phenotypic plasticity".
One exception is the work of William Bradshaw and Christina Holzapfel at the University of Oregon, Eugene. They have studied a mosquito whose larvae develop in the water-filled leaves of pitcher plants. To survive the winter, the larvae must enter a dormant phase, and the cue they use is shortening day length. By 1996, because of milder winters, larvae from near the US-Canadian border had evolved to delay their dormancy until the period of daylight was about half an hour less than the critical day length that triggered shutdown in 1972.
That is an encouraging sign that species might be able to adapt to the changing conditions imposed by global warming, but not all species can evolve as fast as mosquitoes. If an environment changes too rapidly, so many individuals fail to reproduce that a population is wiped out, rather than evolving in response. "There will be a cost of selection and the cost, in many cases, will be extinction," says Loren Rieseberg of the University of British Columbia in Vancouver, Canada.
Rieseberg has looked at variations in traits including body size - or height in plants - and the timing of reproduction in 126 plant and animal species living mainly in northern temperate regions. Such traits often change with increasing latitude, which correlates with temperature differences. From the changes predicted by climate modellers, he has calculated the rate of adaptation needed to keep track of global warming. Alarmingly, his analyses suggest that for species with generation times longer than a couple of years, the pace required exceeds the theoretical maximum.
"These are crude, back-of-the-envelope calculations," Rieseberg stresses. Indeed, some biologists argue that we just don't know how to calculate a theoretical maximum for the rate of adaptive evolution. Nevertheless, if plants and animals are to adapt, biologists agree that efforts must be made to preserve hotspots of adaptation and genetic diversity, while allowing species to migrate into neighbouring habitats as conditions change. So far, few practical conservation efforts have taken these principles on board, but California is now poised to become a testing ground for evolution-based conservation.
Last November, the state's voters approved Proposition 84, a plan to raise $5.4 billion to invest in environmental improvements - including the purchase of new reserves for conservation. And the California Department of Parks and Recreation has asked a team led by Craig Moritz of the University of California, Berkeley, to advise on how to preserve the state's evolutionary heritage.
Moritz and his colleagues are producing a series of maps documenting hotspots of recent evolutionary change for most groups of vertebrates, and some plants. "If you can't put it on a map, you can't protect it," he explains. This should help California preserve the genetic diversity that species will need to draw on if they are to respond to global warming. Moritz also suggests that future reserves should be connected by corridors of habitat and should enable species to migrate to higher elevations as temperatures rise.
Endangered on the farm
Across the globe, some 1.4 billion domestic cattle are contentedly chewing the cud - and no one would describe them as endangered. But according to Pierre Taberlet of Joseph Fourier University in Grenoble, France, cattle are becoming so genetically impoverished that they are in serious evolutionary trouble.
Cattle were domesticated around 8500 years ago, and after thousands of years of artificial selection they diversified into more than 1200 distinct breeds. Modern agricultural practices are wiping out this impressive diversity, Taberlet argues, and artificial insemination using sperm from just a few stud bulls means that intensively farmed breeds are becoming highly inbred.
Geneticists assess inbreeding using a measure called the "effective population size". The smaller the number, the worse the level of inbreeding. For Holsteins in Germany, this figure is just 52. Japanese black cattle are in even worse shape, down to an effective population of just 17 in their native country. The consequences of this inbreeding may already be appearing, Taberlet claims, through genetic diseases such as achondroplasia - a form of dwarfism.
The declining evolutionary quality of livestock is causing less public alarm than the threats facing endangered wild species. "But it's our food," Taberlet warns. "We are losing irreplaceable genetic resources."
Published: Saturday, February 17, 2007