In his new book Darwin Comes to Town: How the Urban Jungle Drives Evolution (2018), Dutch evolutionary biologist Menno Schilthuizen unveils the evolutionary drama that is occurring in the urban environment.
The core message of the book is that, in this urban landscape, rapid evolutionary and adaptive change is taking place among the flora and fauna that inhabit it. Scientists, he notes, too often look for evidence of evolution in "unspoiled" environments, wild places far from the city. Rather, he counsels, they might profitably look right at home, in their own backyards, or down the street in small parks isolated amid buildings. He leaves the reader with no doubt that right here and now evolution is modifying and changing organisms, from distinct mosquito species evolving in different tunnels in the London Underground, to White-Footed Mice in New York City parks (more on them below), to parakeets in France, to grasshoppers, to Hawksbeard plants, to snails, . . . the list of organisms being changed by city living that Schilthuizen describes for us is long. For this tour of evolution in the city, one could not ask for a more genial and knowledgeable guide; he couples his descriptions of evolving flora and fauna with an introduction to the work of many scientists studying these organisms. In this post, I can only touch on a few examples of the riches Schilthuizen marshals for the book.
One issue needs to be addressed at outset. Schilthuizen is not arguing that, in the face of spreading urbanization, evolution will take care of things. Yes, he offers many convincing examples of plants and animals that have taken to urban living with relish, finding what they need in these human-made environments and often evolving in response. But this is not good for nature writ large; all is not well. He succinctly lays out why:
Natural selection here [in the city] is so strong that urban life forms evolve rapidly. But we must also remember that all the examples of urban evolution in this book form a biased sample of those life forms that were pre-adapted, variable, or simply lucky enough to evolve and survive. For each successful urban species there are dozens of other species that could not adapt to city life and disappeared. Besides being evolutionary powerhouses cities are also places where great loss of biodiversity takes place No matter how interesting they are biologically, we cannot rely on them for the preservation of the bulk of the world’s species For that, we must preserve, appreciate, and explore what remains of pristine, unspoiled wilderness. (p. 244-245)Cities are particularly challenging places to live. As the landscape is built up and paved over, cities turn into heat islands where temperatures are dramatically higher than in surrounding, non-urbanized areas. This rise in temperature is joined by myriad other, potentially negative attributes, including ubiquitous lighting that can wreak havoc with life cycles and elevated noise levels.
Previously, I hadn’t given a thought about the impact of urban noise on wildlife. Such noise acts as a screen, keeping some animals from living in the city. For the animals that sing or chirp or call in order to socialize, warn, and, perhaps most importantly, find mates (think birds, frogs, grasshoppers, etc.), background noise levels can be critical. Through many clever experiments that Schilthuizen describes, scientists have determined that, in response to urban noise, which is mostly at low frequencies, those animals able to make a go of it in the city have raised their voices in order to be heard. In some species this ability to call at a higher pitch is innate, present whether they live in the city or country (where there can also be lots of noise). But there is evidence that urban life is affecting singing (songs are thought to be more hardwired than just calls) in ways that suggest some citified species are evolving away from their non-urban counterparts.
The lure of the city has proven irresistible to some plants and animals, so much so that they no longer inhabit wild places, now spending all of their time in urban or semi-urban places. Many already had traits predisposing them to life in an urban environment. A prime example is the House Sparrow (Passer domesticus) which, worldwide, truly lives up to its species name. The Animal Diversity Web (University of Michigan’s Museum of Zoology) describes its habitat as follows:
House Sparrows like areas that have been modified by humans, including farms, residential, and urban areas. They are absent from uninhabited woodlands, deserts, forests, and grasslands.Schilthuizen describes how in his hometown of Leiden, these sparrows have taken to living in areas by train stations set aside the parking of bicycles, a favored mode of transportation in the Netherlands. The sparrows seem particularly attuned to living among bicycle wheel spokes, seats, and handle bars. Yet, as he notes, this “is not a habitat the species ever evolved to occupy.” (p. 63) They were, he posits, “preadapted” to this environment because it mimics the environment they did evolve to live in. For instance, bicycle spokes are certainly similar to, and possibly as effective as, spiny thickets in affording protection from predators.
For me, the animal stars of the book, hands down, are Blue Tits (Cyanistes caeruleus) and Great Tits (Parus major). These oh-so-smart birds demonstrate quite convincingly the traits that, according to Schilthuizen, the city selects for in animals: the ability to problem solve, neophilia or interest in new things, and a lessened fear of humans. Consider the epic battle between tits and milkmen that apparently had its start in England in the late 19th century, soon spreading elsewhere. Then as now, tits are lactose intolerant, but they adored the cream that rose to the top of bottles of unhomogenized milk being delivered to households. The cream was a rich source of fat and had little lactose in it. So they went for the bottles of milk with gusto because, at the outset, milkmen delivered the bottles with no tops. Tits would swoop in, perch on the lip of the bottle opening, and drink upwards of an inch of cream. To thwart this behavior, the milk industry logically turned to bottle tops. But the introduction of cardboard lids did little to deter the tits who quickly learned how to remove the lid entirely or how peel off the cardboard layer by layer until a hole could be poke through it. When the war escalated with the appearance of aluminum caps, the tits figured out how to strip off the cap piece by piece, drinking the now exposed cream in the bottle. They also learned how to remove the cap whole, then they would fly off with it to savor the cream that stuck to it. A collection of caps could often be found under the trees to which the tits flew.
To get a flavor of the battle, Schilthuizen draws from a fascinating study in the 1940s of this “bottle-opening skill” that was based on data provided by questionnaires filled out throughout Great Britain and later Europe. He writes,
People were exasperated at how quickly the tits were at their milk bottles, often within minutes of the milkman placing them there. As if the birds were waiting for it! (They probably were, since one milkman complained that some tits did not even wait for him to deliver the bottles to a house, but rather raided his cart while he was out placing bottles on somebody’s doorstep. And then as he ran back to his cart, other tits would befall on the bottles just delivered.) (p. 169)The importance of this study lies in what it showed about the spread of this bottle-opening skill – it popped up suddenly and independently in towns well beyond the normal flying range of tits. Schilthuizen posits, “So, it is more likely that the behavior was invented independently by multiple, particularly clever birds that then were imitated by others.” (p. 170) Development of this skill isn’t an example of evolution at play, but, rather, evidence of how the tits bring to bear those traits prized by the city of problem-solving, curiosity, and diminished fear of humans to develop learned and taught abilities. It's also a neat example of the sharing of learned behavior.
One critical feature of urban life for flora and fauna is that hospitable areas are fragmented, that is, they often constitute islands, typically small ones, within a sea of concrete. In general, islands of whatever kind, and particularly small islands militate against diversity. As Schilthuizen notes, to many conservationists, this kind of isolation in cities is anathema to the health of living organisms because the gene pool they draw from is small, causing a loss of variability and, with that, a decreased ability to respond to environmental challenges. Indeed, this is bad news for some species, but not for others. He offers multiple examples of the evolution of genetic differences in taxa living and apparently thriving in different, discrete urban habitats.
Take, for example, White-Footed Mice that inhabit New York City parks. They are doing well and the genetic composition of each population of mice differs from park to park. Arguably, they have balanced the disadvantages of a small, increasingly uniform gene pool in each park, with evolution that is equipping each isolated population to deal with park-specific challenges. One analysis that compared the genes of mice from different New York City parks with those from rural areas found that the mice in Central Park had “a distinctly aberrant AKR7 gene . . . [that] takes care of neutralizing aflatoxin, a toxic and cancer-promoting substance produced by a fungus that often grows on moldy nuts and seeds.” Not hard to imagine why the Central Park mice would have evolved to handle this.
There is so much more to be found in this singularly accessible volume, much more to be savored and enjoyed. That said, I will close with a philosophical quibble I have with Schilthuizen’s response to biologists, conservationists, and others considering potential changes to the urban landscape. Clearly he embraces the ecosystems that have been created in these fragmented habitats; the book is about the flora and fauna making up those ecosystems. They represent adaptive and evolutionary responses to these niches. So, it isn’t surprising that at the end of the book, he takes some to task for seeking to join isolated urban habitats in order to broaden isolated gene pools. I find his objection to this adjustment of the urban environment, which, of course, would have significant consequences for urban ecosystems, somewhat problematic. Is he calling for the urban landscape to be locked in place as it is at this precise moment to protect the present ecosystems? That seems, to me, to ignore the reality of the urban environment – it is always changing and raising new environmental challenges in the process. Arguably, the plants and animals that make it here are those equipped to respond to that kind of incessant change.
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