Darwin Comes to Town How the Urban Jungle Drives Evolution

We biologists are a moody bunch, aren’t we? Forever lamenting the loss of biodiversity and unspoiled wild nature around us as humanity transforms the planet. The Anthropocene, the sixth extinction – I dare say you could accuse us of a certain doom-mongering. We ought to present a united front to the many threats unscrupulous groups in the outside world throw at our precious wildlife. So, beware the biologist that breaks rank and suggests a different narrative – he or she can expect a healthy amount of criticism. So it was with Chris D. Thomas’s recent book Inheritors of the Earth: How Nature is Thriving in an Age of Extinction (read my interview with him here). And so it is with Menno Schilthuizen’s new book Darwin Comes to Town: How the Urban Jungle Drives Evolution. You leave it to us pragmatic Dutch to say out loud the things you don’t like to hear...

Now, before I continue, let’s make one thing clear here. Neither of aforementioned authors, nor myself, are suggesting that humanity at large is not inflicting large amounts of damage to our environment, nor suggesting that we are not causing the extinction of large numbers of species. However, it has become almost anathema to even take an objective look at these processes and dare to suggest that there is a flip side to it all. We have become so afraid that our findings will be hijacked by those who stand to gain from the destruction of our environment, that we are stifling what is healthy scientific discourse and debate. I am willing to stand up and say that we can have this conversation without it automatically implying that this lets humanity off the hook, as if we have no responsibilities for our actions, no moral obligations. And it’s heartening to see that I am not alone. Where Inheritors of the Earth highlighted that the current biodiversity crisis is simultaneously setting the stage for a new era of speciation (as every one of the past big five mass extinction has), Darwin Comes to Town takes a closer look at the evolutionary adaptations of flora and fauna to our urban environments. Next to being fascinating and well written, I think this book is an important contribution in having this conversation, which is another reason for me to highlight it.

Urban ecology is a thriving biological discipline that has spawned a large literature. As Schilthuizen points out, what we call "the urban environment" is in fact a rich collection of different micro-environments, exerting its own evolutionary pressures on its inhabitants. Some animals and plants, courtesy of the environments they used to live in, are already set up with traits and behaviours that can exploit these niches, allowing them to thrive in cities.

Despite what I mentioned above, from the 1960s onwards, biologists have ventured into our urban environment to observe how plants and animals are adapting to life here. A famous textbook example are the peppered moths (Biston betularia) that showed rapid evolutionary response to air pollution. They are the subject of a separate chapter due to the controversy that arose when doubts were cast on this story. Creationists had a field day, though subsequent research has vindicated this story. But Schilthuizen’s engaging narrative draws on a rich palette of findings. From swallows evolving shorter wings to better evade moving cars, lizards evolving toe-pad characteristics that better suit concrete and metal surfaces, fish in polluted harbours evolving tolerance to pollutants such as PCBs, or insects learning to avoid artificial lights while their predators evolve attraction to it. These are just a few examples of rapid evolutionary adaptation to urban environments, thanks to what is known as :"standing genetic variation": the sum of subtle gene variants you will find across all individuals in a species.

Other than adaptations to the physical components of the urban environment, species also engage each other (or us) in evolutionary competition and arms races. There is tantalizing evidence in the form of herbivores adapting to plants we have brought into cities, or birds using their problem-solving intelligence to uncover new food sources (e.g. tits learning to open milk bottles to access the cream on top, when milk bottle delivery was still in vogue). Sexual selection and mate choice, too, respond to the challenges posed by the urban environment. Having studied at Leiden University myself, I was quite familiar with the work of Hans Slabbekoorn who showed that tits sing at a higher pitch to rise above the dominant frequency of urban background noise, something that has subsequently been shown for many other species. Ultimately, this could and should lead to new species coming into existence, and there is tantalising evidence for such incipient speciation in for example blackbirds.

Finally, Schilthuizen shortly considers invasive species, the rapid spread of human ideas leading to uniform changes in the urban environment around the globe (e.g. developments in city lights), and how these findings could inform the booming discipline of green urban design and architecture.

Schilthuizen offers a whirlwind tour through a large number of studies on urban ecology and evolution that is very readable and interspersed with the irreverent wit of a biologist writing a popular science book. As he also remarks, not all species can adapt to the unprecedented changes brought about by the urban environment we have created, and species are going extinct, but it would be a mistake to think that the urban environment equals the wholesale annihilation of life forms. Nature is fighting back the way it always has and rapid evolution is happening right under our noses. Will Darwin Comes to Town make city dwellers appreciate how nature and humans remain interwoven, and open our eyes to the wonders around us? I certainly think this book has that potential.