David Quam­men

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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

‘Liver­pool Pi­geon,’ 2011; paint­ing by Ralph Stead­man from his ‘Ex­tinct Boids’ series

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