By Scott Norris
Illustration by ©Rafal Olbinski
Something has shifted recently in my image of future life on earth. It’s not that the dire predictions of global climate change and ecological collapse have changed all that much or seem any less convincing. But if I really imagine being there—the year 2306, say—I see something that my ecological education has not prepared me for.
Our vision of the biological future has long focused on the gaps, the tears, the ecological dislocations. But if we are confronted today with a world of species and ecosystems on the brink of destruction, our descendants will be at home in a world of biological survivors. And it’s here, I’m starting to believe, we may have been missing something important.
We know a great deal about how species go extinct. But look at these survivors of 2306, still present after centuries of global upheaval. How exactly did they persist? Some biologists now say that riding out the coming changes may require species themselves to change.
Conservationists have long talked about maintaining evolutionary potential but explicitly denied that evolution would be a major force in “rescuing” species from impending global change. Extinctions happen quickly; evolution moves at the pace of mountains.
But not always. The most far-reaching ecology news in recent years is that natural selection acting on wild populations can produce big changes in short amounts of time—not just minute differences captured through elaborate statistical techniques but physical changes visible to the naked eye. In plants. In insects. In vertebrates. On timescales of decades or even less.
The flood of recent observations of rapid, contemporary evolution is blurring the distinction between “ecological time” and “evolutionary time.” And that has opened the way for a small but growing group of researchers who are proposing something previously unthinkable—active intervention in evolutionary trajectories as a conservation strategy. A number of recent papers have begun to explore the possibilities, using terms such as “facilitated evolution” and “microevolutionary management.”
If this all seems very new, it may be good to remember that the idea of managing the evolution of other species has been around for a long time and has a familiar name: domestication. The word alone conveys what some may fear as the dark side of evolutionary management. Might it not lead to a shift in conservation focus away from habitat preservation and toward something entirely different—the creation of a reduced and quasi-domesticated biota capable of coping with the mess humans have made of the natural world? If there is a slippery slope to be avoided here, how far along the path of evolutionary tinkering can conservation biology safely tread?
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Although the answer is far from clear, the spillover from contemporary evolution studies into conservation biology has begun. And it is al-ready challenging some deeply held tenets of conservation. In the traditional worldview of conservation biology, species are treated as static entities, unchanging on timescales relevant to conservation. Because their habitat requirements are fixed, they are threatened by changes in their environment. The role of conservation has been to try to reduce or mitigate the impacts of such changes.
An influential 2003 paper by Michael Kinnison and two other fish biologists—Craig Stockwell and Andrew Hendry— in the journal Trends in Ecology and Evolution set out to change that worldview.(1) All the major drivers of extinction today, the authors noted, are also drivers of evolutionary change.
The best-known examples are the Atlantic cod and other fish that have evolved toward smaller body size and younger age at maturity due to decades of heavy harvesting which only the small fish survived. However, rapid responses to human-influenced, seminatural selection are widespread. Many plant populations show adaptive resistance to a variety of soil and atmospheric contaminants. Invasive species quickly evolve to better fit their new environments, and natives adapt in response to new pressures brought by the invaders. From frogs to fish to fowl, populations separated by recent habitat fragmentation show differences in physical characteristics and behavior.
In essence, the authors challenged conservationists to see not just a world of threats but also a world of selective forces to which species actively respond. In such a world, endangerment is a function of both environmental change and adaptive response—or lack thereof. Shielding species from threats still makes sense. But a new conservation strategy is within reach—managing for evolutionary change.
Indeed, we may have little choice (see further reading). The management record in dealing with new and seemingly long-term stressors such as invasive species and emerging diseases has not been terribly successful, and prospects for “managing” the impacts of climate change are downright dismal. For threats that aren’t going to disappear anytime soon, conservation approaches based solely on impact reduction or mitigation are break-even propositions at best. And, as critics of the U.S. Endangered Species Act like to point out, they often entail the need for perpetual management intervention.
Moreover, adaptive changes already under way make the object of traditional conservation a moving target. One of the most jarring lessons of contemporary evolution is how often conservation itself is a significant selective force, altering evolutionary trajectories even as it tries to maintain the status quo. “We implement selection on species just by attempting to save them,” says Michael Kinnison. “Maybe actively directed selection is better than unintentional selection we don’t know about.”
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If the Hawai’i creeper and the ’akepa still inhabit the moist forests of the Hawaiian Islands in 2306, they will almost certainly have evolved a trait they lack today: resistance to avian malaria. Along with habitat degradation and exotic predators, the disease has had a ruinous impact on the Hawaiian avifauna—53 of 71 native species are either threatened or extinct—and forced survivors into high-elevation holdouts where cooler temperatures maintain a malaria-free zone. Conservation efforts have focused on protecting and restoring these malaria-free areas where populations remain relatively healthy. Just one problem, says A. Marm Kilpatrick, a senior scientist with the Consortium for Conservation Medicine: the malaria-free zone is shrinking. With another century of global warming, it may cease to exist.
As Kilpatrick studied the predicament facing Hawaii’s native birds, he quickly reached a surprising conclusion: the only long-term solution is evolution.
That may sound like long odds, but Kilpatrick found reason to be hopeful. Not all native bird species have been unable to coexist with mosquitos and the malaria parasite they transmit. The Hawai’I ’amakihi, for example, has persisted in disease-invested habitat and even increased in number over the last decade. Researchers have also found malaria antibodies in individuals of other species, suggesting the presence of protective genes. The problem, Kilpatrick realized, is that those genes might not be distributed fast enough. Other factors such as predation are limiting the survival and reproduction of even disease-resistant individuals. Adaptation is slowing to a crawl while populations continue to decline.
So Kilpatrick began building mathematical models to explore how different management strategies might speed the evolution of disease resistance in Hawaiian birds. The work became one of the first carefully modeled case studies of managed evolution. Kilpatrick’s models included all the relevant biological information available for each species and simulated generations of population growth or decline under various conditions. The results, published in the April 2006 issue of Biological Conservation, were chastening.(2) Kilpatrick’s study showed that a traditional approach, focused on building populations in areas of superior habitat and little or no disease, is destined to fail. Such management may be highly successful in the short term but works at cross-purposes to adaptive evolution.
So Kilpatrick proposes something very different. Rather than avoid malaria-ridden areas where species are barely hanging on, we should focus intensive management precisely on those areas. The key is to give bird populations in the malaria zone a demographic boost and thus a chance to persist —and even grow—in a strong, selective environment for disease resistance. Kilpatrick recommends rodent control. On the island of Oahu, intensive rodent control greatly increased survival and reproduction of the threatened ’elepaio. The models show that if the same could be done elsewhere, present declines in some species might be reversed, leading to malaria-resistant populations in 50 to 100 years.
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Kilpatrick’s strategy of buying evolutionary time is risky. Two elements are required for such a tactic to work: the desired traits must be heritable and selective pressures must be strong enough to act on that trait without wiping out the population. Trying to ensure the latter, however, may conflict with traditional conservation management.
Consider the case of cowbirds. These so-called nest parasites lay their eggs in the nests of other songbirds. The larger and more dominant cowbird chicks often survive at the expense of native species, driving declines in songbirds across North America.
Some species, however, have evolved defenses. For example, songbird species with a longer history of exposure to cowbirds are more likely to recognize and reject cowbird eggs. This has led Kilpatrick and others to ask whether in some cases management efforts may be getting in the way of evolutionary progress. Intensive cowbird control has been credited with helping restore populations of some species, including the rare Kirtland’s warbler. But by reducing the threat of nest parasitism, managers also reduce the selective pressure for an adaptive response.
“In a sense, the [evolutionary] approach argues for letting go of the species a little bit so it can respond to the stressor and selective pressure,” Kilpatrick says. “For Kirtland’s warbler, it would mean letting some cowbirds parasitize warbler nests—which would result in some nest failures.” That may be too great a risk, Kilpatrick concedes, if the population size is already very small. But there is another kind of risk—management dependency that has no stopping point. “In my mind the advantage of facilitated evolution is that it offers a way to let the species take care of itself in the long run,” Kilpatrick says.
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Martin Schlaepfer and two co-authors took up this theme in the journal Ecology Letters (3). Schlaepfer, a conservation research fellow at the University of Texas, Austin, had been inspired by the Stockwell et al. paper in his thinking about “evolutionary traps”—situations in which behavioral strategies honed by natural selection become maladaptive due to environmental change.
This often occurs when foreign species invade new habitats. For native snakes in Australia, Schlaepfer explains, the strategy of viewing any frog-like creature as potential prey became a liability. Toxic cane toads, introduced to Australia in the 1930s, are poisonous enough to kill many snake species in a single meal. In fact, researchers have estimated that cane toads threaten up to 30 percent of terrestrial Australian snake species. Similarly, monarch butterflies incorrectly perceive the nonnative black swallowwort as a suitable host plant for egg-laying, although toxins in the plant prevent the monarch larvae from developing.
It is often impossible to undo changes that cause species to be caught in evolutionary traps. But such species may be particularly good candidates for evolutionary rescue. A simple shift in behavioral response or an incremental improvement in defenses may solve the problem.
Schlaepfer believes there are many situations in which evolutionary solutions are within reach—we’re just not used to looking for them. One approach he suggests is similar to Kilpatrick’s: subsidize survival long enough for adaptation to occur. Modeling studies suggest that adaptive shifts are more likely when selective pressures vary from strong to mild in different locations. By manipulating habitat, Schlaepfer says, managers might create just such an evolutionarily productive mix of refuge zones and areas of strong selection.
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But the possibilities don’t end there. Schlaepfer, Kinnison, and others are also talking about taking evolutionary management a step further—not just helping species survive long enough to adapt, but actively manipulating their genetic composition to speed adaptive change. In theory, at least, the ability to influence the number and even the type of genetic variants present in a population would give researchers a leg up on nature in driving evolution.
Conservationists already manipulate gene flow when they connect isolated populations through habitat corridors or maintain barriers to prevent hybridization. But the goal of such efforts has been to preserve gene pools (i.e., the status quo) rather than to foster rapid evolution. In the latter context, the trick is to determine the proper dosage of genetic variation. Not enough, and selection may have little to work with because the chances of a population containing genes for disease-resistance, for example, go down. Too much, however, and adaptation may be thwarted.
To see how this might play out, consider the plight of a plant species threatened by global warming. Adaptations that might ultimately help the species survive are most likely to originate in the southern portion of its range, where conditions are already hotter and drier than they are farther north. But heavy gene flow from other areas can prevent such locally adaptive responses from ever taking hold.
“Usually gene flow is managed as either off or on,” says Kinnison. “There may be ways to manage it at intermediate levels, avoiding the danger of inbreeding but allowing for possible adaptation to occur.”
Quantity of gene flow is not the only consideration. Rather than try to accumulate genes like lottery tickets—the more you own, the more likely you are to have a winner—another strategy might be to seek out variants of proven value. Kinnison has proposed the idea of “selection probes” to prospectively figure out what genes aid survival. Salmon populations that are supplemented by hatchery-raised fish, for example, might be good candidates. Initially, managers would release hatchery fish of different genetic stocks while maintaining populations of each stock in reserve. When the released fish come back to spawn, managers could see which stocks were most successful. In time, they could skew each year’s releases to favor the relatives of last year’s proven survivors.
There are many variations on this theme. Perhaps we could help plants respond to global warming by moving southern, more drought-adapted individuals into northern populations. Perhaps we could bring back blight-decimated chestnut tree populations by cultivating and transplanting disease-resistant individuals. Perhaps we could “inoculate” populations not yet affected by an invasive species by introducing individuals from other areas where a long history of exposure to the invader has produced an adaptive response.
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This is clearly the conservation frontier. Although many strategies for evolutionary intervention have some theoretical weight behind them, few have been tested by rigorous modeling, let alone actual application.
One thing, however, seems clear. Proceeding down this path will require rethinking and, in some cases, letting go of cherished ideas and values—such as preserving the full range of genetic and phenotypic diversity that past evolution has produced. Stockwell et al. summed up in their 2003 article the contradictions raised by contemporary evolution: “Conservation goals of maintaining population abundance, promoting population persistence, conserving genetic variation, and maintaining adaptation might not be fully compatible.” (1)
Biodiversity protection efforts have long assumed a fundamental link between maintaining species and preserving nature in something close to its “original” state. But the view ahead suggests that this may be an overly confining template for the conservation enterprise.
Now when I imagine life in 2306, I see flora and fauna that have been through several centuries of selective pressures strong enough to drive mass extinction and disrupt ecological patterns on a global scale. To varying degrees, those survivors will be different biological entities from the species we know today. For many, the differences may have much to do with the fact of their survival.
Literature Cited
1. Stockwell, C.A., A.P. Hendry, and M.T. Kinnison. 2003. Contemporary evolution meets conservation biology. Trends in Ecology and Evolution 18(2):94-100.
2. Kilpatrick, A.M. 2006. Facilitating the evolution of resistance to avian malaria in Hawaiian birds. Biological Conservation 128(4):475-485.
3. Schlaepfer, M.A., P. W. Sherman, B. Blossey, and M.C. Runge. 2005. Introduced species as evolutionary traps. Ecology Letters 8(3):241-246.
Additional Reading
Ashley, M.V. et al. 2003. Evolutionarily enlightened management. Biological Conservation 111(2):115-123.
Strauss, S.Y., J.A. Lao, and S.P. Carroll. 2006. Evolutionary responses of natives to introduced species: What do introductions tell us about natural communities? Ecology Letters 9(3):
357-374.
Stockwell, C.A. and M.V. Ashley. 2004. Rapid adaptation and conservation. Conservation Biology 18(1):272-273.
Rice, K.J. and N.C. Emery. 2003. Managing microevolution: Restoration in the face of global change. Frontiers in Ecology and the Environment 1(9):469-478.
Holt, R.D. and R. Gomulkiewicz. 2004. Conservation implications of niche conservatism and evolution in heterogeneous environments. In: R. Ferrière, U. Dieckmann, and D. Couvet, eds. Evolutionary Conservation Biology. Cambridge University Press, Cambridge, U.K. s Ltd: Nature, 437(7058):473-474, ©2005.
About the Author
Scott Norris is a science writer based in Albuquerque, New Mexico.
