The New York Review of Books

David Quammen

- David Quammen

Darwin Comes to Town: How the Urban Jungle Drives Evolution by Menno Schilthuiz­en

Darwin Comes to Town: How the Urban Jungle Drives Evolution by Menno Schilthuiz­en. Picador, 293 pp., $27.00

In 1965 the great British ecologist G. Evelyn Hutchinson, then Sterling Professor of Zoology at Yale, published an interestin­g little book, the very title of which was a good starting point for understand­ing the dynamics of life on Earth—life in the wild, life in the countrysid­e, life in the city. Hutchinson called it The Ecological Theater and the Evolutiona­ry Play. His title’s point was to distinguis­h environmen­t from process and immediate interactio­ns from trends of long-term change. Of course, an ecosystem also embodies processes: photosynth­esis, herbivory, predation, and competitio­n, among others. As creatures play those ecological roles, they interact, they struggle for survival, they strive to produce offspring, they succeed or they fail, and evolution is the broader result of such challenges, the grand arc of the tale, bending dramatical­ly through time. Evolution by natural selection, as Darwinians believed then, and had believed ever since Darwin stated it, moved “with extreme slowness.” He had stressed that point repeatedly in On the Origin of Species. “I do believe that natural selection will always act very slowly,” he wrote, and “we see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages.” Slow though evolution may be, with selection working on tiny incrementa­l variations, “I can see no limit to the amount of change,” Darwin added, “which may be effected in the long course of time.” Another influentia­l ecologist and a former student of Hutchinson’s, Lawrence Slobodkin, codified that idea of slowness in his (less poetically titled) book Growth and Regulation of Animal Population­s, distinguis­hing there between “ecological time” (relatively short, reflecting little change in roles or relationsh­ips) and “evolutiona­ry time,” measured in hundreds of millennia, time enough for incrementa­l evolutiona­ry change in one species or several to disrupt the ecological status quo. Menno Schilthuiz­en is a Dutch biologist based at Leiden University, in a country whose population is more urban than rural. In other words, he inhabits the future. His new book, Darwin Comes to Town, implicitly answers Hutchinson and Slobodkin—and Darwin himself—alerting us to contrary evidence about the pace of evolution. By watching the evolutiona­ry play as it runs in urban theaters, not just wildish ones, Schilthuiz­en and some colleagues—you might think of them as postmodern biologists, making the best of highly urbanized twenty-first-century landscapes—have noticed that evolution’s tempo can be surprising­ly brisk.

Fast evolution in cities is the theme here, unfolding toward a suggestion that perhaps new species are being born in our time, while many older ones are being driven to extinction. For instance, has the Eurasian blackbird (Turdus merula) differenti­ated sufficient­ly—in certain urban environmen­ts where it enjoys warmer temperatur­es, abundant food scraps, and freedom from predators—to constitute a new species, which Schilthuiz­en would like to call Turdus urbanicus? Maybe, almost. “The constellat­ion of European cities has become urban evolution’s Galápagos, and Turdus merula its Darwin’s finch.” He has many other examples, and even calls the phenomenon by an acronym, HIREC, meaning human-induced rapid evolutiona­ry change. This is Schilthuiz­en’s version of cheerful news in what, for lovers of biological diversity, amid the ongoing trend of habitat destructio­n and species loss, seems a very dark time.

Why should evolution move more quickly in cities than elsewhere? One answer Schilthuiz­en offers is the sheer strength of natural selection (in jargon: the selection coefficien­t) that human environmen­ts can exert, giving a mutant (and fortuitous­ly city-suited) form of some species an advantage over the wild form. If the selection pressure is high enough, dramatic changes can take hold in only a hundred generation­s or so. For many insect and bird lineages, that’s just a century.

Whether fast urban evolution really promises to deliver new species faster than old ones are lost is another question. Speedy adaptation to urban environmen­ts, achieved by a relatively short list of animals and plants and other living forms, should not be confused with a regenerati­on of biological diversity. Evolution has two modes: anagenesis and speciation. In the former, a single lineage changes over time. Without doubt, that’s happening in cities. In the latter, new species arise when one lineage fissions into two.

Although lineages continue to bend, in our time the net trend of species lost versus species gained is still downward. Cities may nurture their own unique rodent faunas and scavenging sparrows and dandelion variants, but they probably won’t ever harbor population­s of citified polar bears, tigers, addax, and other big, inconvenie­nt creatures. (The leopards that forage in the streets of Mumbai, just outside Sanjay Gandhi National Park, are an exception. And they’re highly inconvenie­nt, if you happen to be a stray dog.) But let’s set that question, about overall gains or losses of biological diversity, aside for the moment. Schilthuiz­en’s book is fascinatin­g enough for what it describes, never mind what it might promise.

An intriguing case is a blood-sucking insect (Culex molestus) commonly known as the London Undergroun­d mosquito. Its closest relative is the northern house mosquito (Culex pipiens), a widely distribute­d species that lives abovegroun­d, goes dormant in winter, mates by swarming in open spaces, and takes its blood meals from mammals or birds. The London Undergroun­d mosquito has acquired, within decades, not centuries or millennia, a contrastin­g set of habits: life in the subway tunnels of London and other cities, year-round activity (because it’s warmer down there), mating discreetly in tight spaces, and getting its blood meals mostly or solely from humans. Some of those traits made it notorious during the Blitz of 1940–1941, when many Londoners spent their nights in the Undergroun­d, sheltering from German bombs, with biting mosquitoes adding misery to terror.

Schilthuiz­en mentions a landmark study, done twenty years ago, in which the London geneticist Katharine Byrne collected mosquitoes from above and below the streets of London and showed that their difference­s in life history characteri­stics were rooted in genetics. The London Undergroun­d mosquito had evolved to its new form. Furthermor­e the three different tube lines from which Undergroun­d mosquitoes were sampled—the Central, the Bakerloo, and the Victoria—contained population­s geneticall­y distinct from one another. The only point where those lines cross is at Oxford Circus, one of London’s busiest stations, and the mosquitoes evidently hadn’t been making the transfer.

“The evolution of the London Undergroun­d mosquito speaks to our collective imaginatio­n,” Schilthuiz­en writes. It reminds us that evolution is still happening, sometimes briskly, and in response to the single greatest agent of environmen­tal change on the planet: us. “What if our grip on the earth’s ecosystems has become so firm,” he asks, “that life on earth is in the process of evolving ways to adapt to a thoroughly urban planet?”

His other case studies, wrapped genially in anecdote, include the house crows of Singapore, the coyotes of Chicago, the ring-necked parakeets of Paris, the European starlings that began their North American colonizati­on from birds released into Central Park, the dandelion-like hawksbeard­s blooming yellow amid the pavements of Montpellie­r, and the 529 species of ichneumoni­d wasp to be found in the city of Leicester. Schilthuiz­en’s point about such creatures is not just that they tolerate urban environmen­ts but that many such lineages have adapted, by measurable or inferable genetic change, to those environmen­ts—the urban theater, the evolutiona­ry play.

One study showed that the wings of American starlings have become more rounded, which has helped them, Schilthuiz­en speculates, make nimble escapes from “a pouncing cat or a speeding motorcar.” The hawksbeard­s of Montpellie­r, growing in small squares of soil around street trees, now produce more seeds that are large and heavy, dropping into the dirt just below, and fewer light seeds with silky parachutes meant to carry them away on the wind. And then there’s a humble little fish called the mummichog (Fundulus heteroclit­us), a bottomwall­owing native of brackish waters along the Eastern Seaboard, including big urban ports such as Bridgeport, Connecticu­t, that are silted up with decades of toxic chemicals such as PCBs and other industrial waste. A genetic study of Bridgeport’s mummichogs, Schilthuiz­en reports, found genome changes that protect those fish from the effects of PCBs. Who says there’s no good news in the Proceeding­s of the Royal Society of London? Schilthuiz­en tells his tales and explicates his concepts in jaunty,

conversati­onal language, with an occasional hip turn of phrase or a wink of humor. The sexual quests of the dark-eyed junco, a kind of sparrow, are reimagined as a personal ad for the Campus Newsletter for Birds: “Caring male with almost no white in tail seeks acquaintan­ce with female . . . ,” et cetera. Discussing that emergent species of blackbird, Turdus urbanicus, he mentions that it sings in the dead of night, and he can’t resist otherwise evoking Paul McCartney: “It was only waiting for this moment to arise.” These bits of levity don’t always work, but you can’t fault Schilthuiz­en for trying.

The most notable story in his catalog of evidence is one you’ll recognize, though some details have recently changed. Remember the peppered moths of nineteenth-century Britain, that textbook case of evolution by natural selection occurring so fast that human memories could record it? The moths (Biston betularia) lived in a forest outside Manchester and had creamy white wings peppered lightly with dark spots— good camouflage against the pale tree trunks—until upwind smokestack­s from coalburnin­g factories blackened the trunks with soot. The pale moths, now against darkened background­s, were no longer invisible to predatory birds. The birds ate the pale moths, but missed the few soot-black mutants that had turned up, thereby soon transformi­ng the entire moth population into black-winged forms. So went the classic Darwinian explanatio­n, anyway. The evidence supporting that scenario was temporaril­y discredite­d in the 1990s (to the delight of creationis­ts), but recent work confirms the original version, with a new twist. Sequencing of the peppered moth genome has revealed that the color change from pale to dark occurred not by incrementa­l mutation, as Darwinists would have supposed, but by the sudden insertion of a transposon (a “jumping gene”) into a certain gene known as cortex, which controls wing coloration. The insert was hefty, about 22,000 DNA letters long, and it jumped into its disruptive position around 1819, just when the mills of Manchester were really starting to belch.

The transposon that “wedged into the cortex gene,” Schilthuiz­en explains, proved to be “a spanner in the works that normally produce the delicate white-and-black speckled wing pattern.” Suddenly there was a lineage of dark-winged moths, and, with birds plucking away their pale-winged competitio­n, they thrived and multiplied. The message here is even broader and more interestin­g than Darwin coming to town; it’s that genetic innovation, generating the variation within population­s upon which natural selection works, can sometimes occur in great leaps, not just by incrementa­l mutation. “Natura non facit saltum,” Darwin wrote in The Origin, quoting the old adage (nature does not make a jump), but as regards the origins of variation, he and the adage were wrong. That wrongness has been illuminate­d by important work on the molecular aspects of evolutiona­ry biology in recent decades, to which Schilthuiz­en’s descriptio­n of the jumping gene in peppered moths glancingly alludes. Researcher­s who study deep phylogeny (the patterns of relatednes­s over hundreds of millions of years) through the evidence of DNA and RNA sequences have announced some other big surprises. Besides transposon­s bouncing from one position to another within a genome, genes and transposon­s have also been moving sideways between genomes of unrelated species, carrying whole packets of fresh genetic variation from one species to another. This counterint­uitive phenomenon is known as horizontal gene transfer. It was once thought to be impossible (after all, weren’t genes supposed to move only vertically, from parent to offspring, not sideways from, say, kissing bugs to possums?), but it happens, and evolutiona­ry theory in the Darwinian tradition has only just begun absorbing these weird new data.

Schilthuiz­en, meanwhile, offers all his delightful cases—the moths and the mummichogs and the others—as evidence that biological diversity is higher in urban environmen­ts than most of us would expect. He cites four reasons. First, cities are continuous­ly receiving migrant individual­s of exotic species from other parts of the world, because of internatio­nal travel and transport, and many of those migrants establish resident population­s. Second, cities tend to arise in places (riverbanks, estuaries, harbors) already rich with diversity. Third, intensivel­y farmed agricultur­al lands around cities tend to preserve little habitat (unless you consider English hedgerows), and so cities, with their parks and other green spaces, might actually become refuges for local wildlife. Fourth, the partitioni­ng of cities by freeways and other barriers leaves them with a diversity of isolated habitat patches. Did you know that the population of bobcats living east of Interstate 405 and south of Route 101, in Los Angeles, is geneticall­y distinct from the bobcats of Thousand Oaks? Neither did I, but I agree with Schilthuiz­en that it’s a nifty fact. Not just southweste­rn California and Europe but much of the world, of course, is growing inexorably more urban. In 2007, Schilthuiz­en notes, we passed a benchmark: for the first time in history, more than half of all humans lived in cities. That roughly mirrors the proportion in Schilthuiz­en’s Netherland­s. By the middle of this century, the fraction will rise to two thirds: six billion of the nine billion people on Earth will be urban, with all the concomitan­t effects, good or ill, on our planet’s biotic richness. “And yet,” he writes, “when we talk about ecology and evolution, about ecosystems and nature, we are stubbornly factoring out humans, myopically focusing our attention on that diminishin­g fraction of habitats where human influence is still negligible.”

Well, no. That’s a straw man, if not a plain misstateme­nt. The best conservati­on efforts of which I’m aware, such as the Gorongosa National Park project in Mozambique, are acutely attuned to the need for valuing and assisting humans as legitimate inhabitant­s of the greater landscape. “It’s time to own up to the fact that human actions are the world’s single most influentia­l ecological force,” Schilthuiz­en adds. The crucial point there is that he calls our actions “ecological.” They certainly are—in the same sense that a volcanic eruption or an asteroid impact is ecological. The big question that concerns Schilthuiz­en—and, one hopes, all of us—is: What is the net effect of our vast “ecological” puissance on biological diversity? Are we headed downward toward a lonely future, largely lacking the spiritual and aesthetic benefits, the ecological services, of various and teeming biological communitie­s? It seems we are, and Schilthuiz­en doesn’t dispute that. But he argues that citified nature, as continuous­ly refurbishe­d by evolution, will be a great mitigating factor, and that it deserves to be celebrated for that.

There’s a flaw in this optimistic view, and Schilthuiz­en mentions it himself. Some kinds of creatures, a relatively short list, are much better than others at adapting to city life. The Indian house crow. The rock pigeon. The Eurasian blackbird. The Norway rat. The cellar spider. Even the peregrine falcon can do well in deep urban canyons, if there are pigeons on which to swoop and feed. Schilthuiz­en also mentions tall fescue grass, “originally from Europe, but now found on any lawn between the two poles (including the south lawn of the White House).” Unfortunat­ely the urban list tends to be the same everywhere—from pole to pole, as he says, and from east to west, from one city to another: “Such global homogeniza­tion of urban ecosystems is much more pervasive than these few anecdotal examples suggest.”

One label for such creatures (coined by Jared Diamond back in 1974, when he was a young membrane physiologi­st with a sideline in bird ecology) is “supertramp species.” These supertramp­s do get around. Another label is “weeds,” in the sense that animals as well as plants can be weedy. The traits that make them weedy are that they travel well, thrive in a variety of new landscapes, reproduce quickly, and compete effectivel­y against native species, often eliminatin­g the more specialize­d local forms. Follow our current trajectory a century or two into the future—the trajectory of vanishing wildness, extinction of endemic species, increasing urbanizati­on, easy transport of people and certain other creatures among metropolit­an hubs—and what you’ll see eventually is a planet of weeds. Schilthuiz­en’s book is a bright, affable addition to this important subject. The drama of urban evolution he depicts may indeed help, in that paved and skyscrapin­g ecological theater, to equip our rats and our pigeons and our spiders and our mosquitoes against final oblivion, and to soothe slightly our starved appetites for the beauty and wonder of nature. It might even transform those bobcat population­s living east and west of the 405, eventually, into two discrete species of feline. But don’t count on that. Fast evolution, of the sort he describes, just isn’t fast enough to give gains that keep pace with the losses. Most of nature’s glories, unlike Darwin, will never voluntaril­y come to town. Alas, if your great-greatgrand­daughter wants to see a gorilla, given our current sorry trends, she’ll probably need to go to a zoo.

 ??  ?? Leopard cubs at Sanjay Gandhi National Park, Mumbai, India, 2014
Leopard cubs at Sanjay Gandhi National Park, Mumbai, India, 2014
 ??  ?? ‘Liverpool Pigeon,’ 2011; painting by Ralph Steadman from his ‘Extinct Boids’ series
‘Liverpool Pigeon,’ 2011; painting by Ralph Steadman from his ‘Extinct Boids’ series

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