Granted, perhaps the mange epidemic struck bobcats living north of the 101 because they were already weakened by genetic drift and inbreeding. But at the same time, the smallness of the population also allowed it to adapt very rapidly to a challenge that faced its specific locale. And that is something that may not have been possible in a larger population, because non-adapted genes would have come flowing in from all directions. Same with the white-footed mice: adapting to a specific Central Park environment is only possible if Central Park mice are sufficiently isolated from the mice in other parks. Even in Paris the northern parakeets have slightly different head and wing shapes from the southern ones. This could be because the birds that founded them were already different, but it could also mean that subtly different conditions in the different quarters of the city have caused the populations to adapt to the local situation.
For Munshi-South, this change from seeing genetic fragmentation as the bane of urban wildlife to viewing it as an opportunity for each fragment to adapt to the demands of the local neighborhood is a tantalizing paradigm shift. “It’s a really interesting question,” he says. “Right now, what I want to do is go look at a whole range of populations, several in the city and several along a gradient of suburban and rural, and see whether [local adaptation] is generalizable across all the populations across New York City. And once we have the technology we can look at different cities. I think that’s really where urban evolution needs to go. I think that’s an open question and a really important one.”
11
POISONING PIGEONS IN THE PARK
Douglas Adams, in So Long, and Thanks for All the Fish (the fourth book in his famously funny Hitchhiker’s trilogy) at one point describes how protagonist Ford Prefect has a dream about New York’s East River, so “extravagantly polluted” that new life forms are emerging from it and demanding welfare and voting rights. I like to think that the late Adams, also an amateur zoologist and conservationist, would have been tickled by how reality approaches his science fiction.
While many countries today are cleaning up their act and beginning to outlaw the wanton use and release of pollutants into their densely populated lands, we have to face the fact that pollution can never be completely avoided in human habitation. The sheer density and intensity of the smorgasbord of human activities simply means a continued release of substances at higher than background concentrations. And while the most noxious pollutants are replaced by less evil ones, and their use and dissemination is regulated and channeled as much as possible, wild animals and plants in urban environments will come across a different, denser, and more varied set of chemical compounds than their brethren in unspoiled places. And all these things can become spanners in the works of the smoothly functioning physiology of animals and plants.
So, coping with a varied and constantly changing landscape of pollutants is one of the many requirements for urban species to survive in the city. Over half a century ago, when Rachel Carson wrote the opening pages of Silent Spring, her famous 1962 assault on the pesticides that had become so commonplace so quickly, there was still no reason to be other than despondent about this. She writes:
Given time—time not in years but in millennia—life adjusts […]. For time is the essential ingredient; but in the modern world there is no time. […] The rapidity of change and the speed with which new situations are created follow the impetuous and heedless pace of man rather than the deliberate pace of nature. […] To adjust to these chemicals would require time on the scale that is nature’s; it would require not merely the years of a man’s life but the life of generations.
Today, we know that nature, when faced with pollutants, is not always as lame as Carson feared it to be. Potent pollution may cause species to evolve their way out of the noxious quagmire, and many animals and plants have been able to do what New York City’s white-footed mice have done. That is, tinker with their physiological works if pollution throws a spanner in it.
One animal that famously tinkered with its organismal cogs and wheels in the face of pollution is the mummichog, a fish with a name that could easily have sprouted from Douglas Adams’s mind—especially given that it was the first fish ever to fly in space. Known to ichthyologists as Fundulus heteroclitus, the mummichog is a sturdy brackish-water fish about the size of your index finger, with pretty silvery speckles on an olive-brown background. It lives along the North American east coast, where it can be found in estuaries and marshes from Florida all the way up to Nova Scotia. This vast range already betrays its tolerance and hardiness, which is one of the reasons why it has been a favorite subject in all manner of experiments since the late nineteenth century. In 1973, it was even chosen as a passenger on Skylab for experiments on balance and orientation in zero gravity.
Nature has played its own experiments on the mummichog. Since its area of distribution also comprises some of North America’s biggest cities and busiest ports, this little fish has seen its fair share of polluted environments. It wallows in the muddy bottom of New Bedford Harbor in Massachusetts, and the port of Connecticut’s largest city, Bridgeport: filthy industrial mudholes that contain up to 20 milligrams of PCBs (polychlorinated biphenyls) per kilo of sediment—the result of decades of twentieth-century, unfettered release of industrial waste straight into the water. Twenty milligrams may not sound like much, but PCBs, once used ad libitum in cooling, lubrication, printing, and a whole range of other applications, are among the nastiest and most persistent compounds that the previous century has produced. Other compounds with melodious names but similarly malign outcomes that the fish’s urban habitat are rich in are polycyclic aromatic hydrocarbons (PAHs).
The reason that PCBs and PAHs are so troublesome is that they latch on to a type of protein called AHR, which stands for aryl hydrocarbon receptor. These proteins, in humans as well as in fish, act like switches that turn on and off programs of embryo development. When an animal is faced with high levels of PCBs and PAHs, these molecules are constantly tampering with AHR, so that programs are switched on too early or fail to switch off in time. This results in birth defects, especially problems in the development of the heart and blood vessels. Baby mummichogs exposed to PCBs often get hemorrhages in the tail, inflated or underdeveloped hearts, and usually die in mid-development. In fact, mummichogs are among the fish species that are the most sensitive to PCB and PAH pollution.
That is, average baby mummichogs. But the mummichogs puddling about in the poison-laden mires of Bridgeport, New Bedford Harbor (and at least two more heavily polluted portside cities on the North American east coast) aren’t average. They’ve evolved a way to cope with chemical foulness.
Andrew Whitehead, a biologist with the University of California in Davis, has been studying the fish’s evolutionary agility by comparing their properties in heavily polluted sites (so-called “Superfund” sites, earmarked for clean-up under the US federal program of that name) with those in nearby, pristine waters. Some 40 miles to the southwest of New Bedford Harbor, for example, lies the pleasantly unspoiled Block Island, with PCB levels almost below detection level, a full 8,000 times lower than in New Bedford. And 9 miles south of the toxic cocktails of urban Bridgeport, on the other side of Long Island Sound, mummichogs are happily flapping about in the beautiful lagoon of Flax Pond, devoid even of trace amounts of PCBs.
At these and two more pairs of polluted-versus-unpolluted places, Whitehead caught mummichogs and, using a general DNA test, checked if the fish from each pair were each other’s closest relatives. This panned out quite nicely: the fish from Flax Pond and nearby Bridgeport shared a common ancestor as did those from adjacent New Bedford and Block Island. But that’s where the similarities ended, because in many other respects, the mummichogs from each polluted site had evolved drastically away from their relatives in the nearby pristine environments. First of all, Whitehead showed in lab experiments that they were resistant to normally deadly levels of PCBs. At PCB concentrations that would have made the Block Island fish go bel
ly-up ten times over, the hardy New Bedford ones did not bat a gill-flap. Same for the Bridgeport–Flax Pond comparison and the other two pairs of poisoned–pristine sites.
In a paper published in Science in 2016, Whitehead and his team showed how the fish had managed to pull this off. From each of the eight locations, they read the genome (the entire length of all the chromosomes, letter by letter) for some fifty mummichog fish. As it turned out, the fish that hailed from the polluted sites all had mutations (rewritings and missing sections of the genetic code) in the genes that code for the AHR proteins, and some also had mutations in the genes of the proteins that AHR interacts with. What is interesting is that many of these mutations varied according to different polluted sites, meaning that evolution had repeatedly, independently, produced PCB tolerance.
They then tested what the effect of these mutations were in living fish and discovered that by and large, the voice of AHR had been muted in pollution-tolerant mummichogs. When exposed to PCBs, AHR no longer switched on as eagerly as in fish from unpolluted environments. So, somehow, the fish had evolved ways to keep their organism developing and running, while essentially removing a few vulnerable cogs and wheels from it. Presumably this meant some other components had had to be inserted elsewhere or maybe the organism no longer ran as smoothly as it could, but the crucial point is that, thanks to rapid urban evolution, the mummichog is surviving in places where common sense says it shouldn’t. “Isn’t that remarkable?” Whitehead asks rhetorically. “And that evolved by natural selection over just a few dozen generations!”
PCBs are just one ingredient of the chemical cocktail that we bathe our cities in. Think of road salting. Applying generous quantities of sodium chloride to roads to keep them ice-free in winter is common practice in the colder parts of the world. In the USA alone, a staggering 55 billion pounds of salt are sprayed over its roads each winter. That’s a block of table salt about 353 million cubic feet in size per year. No wonder the stuff is pervasive in the environment: salt has been picked up more than a mile away from (and sixty floors above) the roads it is intended to de-ice. Because of this, the water of canals and streams in big cities can be decidedly brackish throughout winter.
For most life forms, all this salinity can be problematic. As we’ve all learned at school, osmosis causes water to move in the direction of higher salt concentration. That’s why, in a salty environment, the cells in the bodies of animals and plants have to work harder to keep pumping the escaping water back in and stop themselves from drying out. And there is another reason why salt isn’t good. Chemically, sodium is very similar to potassium. But while potassium is essential for many processes in the cells of animals and plants, the same processes won’t work if sodium replaces potassium. And when there’s more salt in the environment, sodium insinuating itself into the cell’s potassium-powered processes can become a big problem.
Organisms that manage to cope with saline situations have usually evolved mechanisms to counteract the salty onslaught on their cells. And when the environment regularly gets covered in road salt, those same species snap up the places vacated by species that lack such mechanisms. As I mentioned earlier, this is why salt-tolerant beach plants colonize the hard shoulders of major inland roads, pushing out the regular verge verdure. But chances are that the animals and plants that are already there also evolve salt tolerance thanks to road salting.
To test this latter idea, PhD student Kayla Coldsnow and her colleagues of the Rensselaer Polytechnic Institute in Troy, New York, did a laboratory experiment with water fleas (Daphnia pulex). Using the same batch of animals, they introduced these tiny freshwater crustaceans into so-called mesocosms (basically large tanks with a real ecosystem in them composed of plankton, plants, clams, snails, and crustaceans). Some of the mesocosms were freshwater, whereas others were brackish (about one-third the salinity of sea water). Yet other ones had intermediate salt concentrations. They left the water fleas to live there for ten weeks (which, in the prolific Daphnia, translates to between five and ten generations). At the end, to make sure that any changes were actually genetic, and not some other effect of the saltwater, they took some of the descendants out and cultured those for three more generations in clean lab aquariums with unsalted freshwater. Then they tested each strain for their salt tolerance. As it turned out, the water fleas retained the evolutionary signature of having adapted to salty water. When placed in brackish water with 1.3 grams of salt per liter, the Daphnia strains that had lived under moderate salt concentrations in the mesocosm survived well (between 75 and 90 percent made it), whereas the ones previously naïve to salinity experienced only 46 percent survival.
Of course, this is only a laboratory experiment, but there is a good chance that the same kind of evolutionary adaptation to winter salt also takes place in wild animals and plants along major roads, and that we’re causing the city flora and fauna of the colder parts of the world to evolve into something akin to a seaside biome.
A seaside biome, yes, but perhaps also a mining town biome. Heavy metals (zinc, copper, lead, for example) are elements that normally are rare in nature. They occur in veins of ore in rocks and under natural circumstances only get into the environment where such a vein hits the surface and is slowly weathered away. All that has changed since humans have discovered the mixed blessings of metals. From the copper axes of yore via the leaded fuels of the twentieth century to today’s cobalt, silver, gold, manganese, yttrium, tin, antimony, and gallium that hide in your smartphone … Humans are the world’s super-accumulators of heavy metals. And much of that accumulation happens in and around cities.
Heavy metals are often toxic, because the molecules tend to attach themselves to enzymes and other proteins, and to DNA, which interferes with the organism’s normal functioning. With heavy metals being so rare under natural conditions, most animals and plants have never had a chance to adapt to them, and therefore do not tolerate them well. Enter Homo sapiens and its copper slag heaps, leaded petrol, and zinc-coated lampposts and electricity pylons. Suddenly, heavy metals are everywhere. Once again, species have needed to either adapt or disappear.
One species that has adapted is the yellow monkey flower (Mimulus guttatus). At the deserted and delightfully named Copperopolis copper mine in California, this species, a widespread wildflower throughout western North America, has evolved ways to deal with high concentrations of copper in the soil thanks to a mutant version of the gene Multicopper Oxidase. This mutation, which probably helps to flush copper atoms out of the cell, has become a fixed feature of all the monkey flowers growing on the mine’s spoil tips, where they have persisted ever since mining was started 150 years ago.
Something similar appears to have happened to the grasses growing underneath zinc-coated electricity pylons in the UK, where the zinc flaking off the iron structures causes zinc levels in the soil to be up to fifty times higher than normal. In 1988, Sedik Al-Hiyali and colleagues from Liverpool University picked five species of grass from among the feet of pylons that had been erected eighteen to thirty-three years previously, and also from grassland farther away. Then, to gauge the zinc tolerance of these plants they grew them in the lab on zinc-containing soil and measured their root lengths. As it turned out, all five species of grasses growing underneath the pylons grew splendidly, producing roots up to five times longer than the grasses that had been picked from other places.
Not only plants, but even animals in cities have found ways to deal with heavy metals. Between 2000 and 2004, the Russian geneticist N. Yu. Obhukova took it upon himself to travel the length and breadth of Europe and jot down the physical features of almost 9,000 city pigeons. For each pigeon he recorded whether the bird was pale or dark sooty gray—a variability that, in pigeons, is down to genetics. His exercise in pigeonholing revealed that the dark “melanistic” birds, which have much more of the dark pigment melanin in their feathers, were more common in big cities than in less urbanized areas, leaving him to wonder whether this was just the resu
lt of genetic mingling with pigeon fanciers’ birds, or had another meaning.
In Paris, at the Sorbonne, Marion Chatelain is using the pigeons that so famously blend in with the Parisian zinc-clad cityscape to pick up on Obhukova’s hunch. Knowing that melanin binds to metal atoms, she figured that perhaps darker pigeons do better in cities because they are better able to purge their bodies from heavy metal pollutants, such as zinc—simply by transferring the stuff to their feathers. So, she got one of her team members, Lisa Jacquin, to catch some one hundred Paris city pigeons; Chate-lain then measured the darkness of their plumage, and housed them under zinc-free conditions in aviaries at her lab, providing each with a leg ring for identification. Then, she plucked two wing feathers from each bird. After one year in the aviary, the feathers had regrown. Chatelain replucked these and did a chemical analysis to see how well the birds had been able to purge their bodies of zinc by storing it in their feathers in the year that they had lived under clean conditions. As it turned out, darker birds had managed to put about 25 percent more zinc in their feathers.
In a follow-up study, Chatelain once again wrangled about a hundred Parisian pigeons to the ground and placed them in her aviaries. Again, she went through her banding and feather-plucking routines, but this time she did not keep the aviaries free of heavy metals. Instead, she divided the birds over aviaries that contained small amounts of lead, zinc, or both, in the drinking water, with two aviaries kept free from pollution as a control. Again, her studies proved that the darker birds stored more zinc and lead in their feathers than paler birds, but also showed that surviving chicks who themselves, as well as their parents, had been exposed to lead, exhibit a darker plumage than the ones who had grown up under lead-free conditions. This suggests that paler juveniles had died during the early stage of their life—a sign that there is a real evolutionary advantage to having darker feathers in a polluted environment.
Darwin Comes to Town Page 11