In schools, we are taught that evolution is an imperceptibly slow process, the long and drawn-out ascent from fish to reptiles, reptiles to birds and mammals, mammals to humans (that is, assuming you lived in a district lucky enough to have evolution in the curriculum at all).
But it turns out, when you take the time to look closely, evolution is taking place all around us, fast enough for us to see and measure. What’s more, evolution may even happen faster around us since we humans tend to create novel and often extreme selection environments that encroach into natural habitats (or entirely new habitats for those species that hitchhike into citeis with us). Johnson and Munshi-South (2017) reviewed the growing list of studies uncovering rapid evolution of wildlife in response to urbanization.
I showed the article to my partner (Baylaart.com) who used it as a theme for her most recent piece which is comprised of organisms subject to contemporary urban adaptations. Both the article and the painting are exceptional work that I’ll walk through below.
When faced with new environmental challenges, populations of organisms are faced with two options: move or adapt. (The third, extreme alternative is extinction). Urban wildlife populations are either residual populations that existed prior to urban development or new populations that colonized after a city emerged.
A classic example of urban adaptation is industrial melanism in the peppered moth (Biston bitularia) (Kettlewell 1955, 1958). As soot poured out of cities during the industrial revolution, the light colored polymorphism of the peppered moth offered poorer camouflage than the black, soot-colored morph. As a consequence, the dark morph came to dominate urban populations. As industry cleaned up its act, the trend reversed.
We can see the effects of urbanization in modern cities, today. White-footed mice (Peromycus leucopus), a North American native, in urban settings are marked by much less genetic variation, and therefore, lower evolutionary potential (Munshi-South et al. 2016). On the flip-side, this reduced variation could be the result of selection sweeping all unadapted alleles from the population, leaving a more genetically homogenous population as a result of the evolutionary process. Even animals that look unchanged may have evolved subtle adaptations. For instance, blackbirds (Turdus merula) in cities exhibit a molecular level difference in a gene associated with harm avoidance behavior compared to their natural brethren (Mueller et al. 2013).
Some species have adapted so well to urbanization, that they are almost synonymous with cities world-wide. For instance, the German cockroach (Blatella germanica), Rock dove (Columba livia), and Norway rat are veritable mascots of cities. Urban roaches and rats have evolved resistance to pesticides (Booth et al. 2011; Rost et al. 2009), and roaches in cities have even evolved an aversion to glucose in response to selection by sugar-baited traps (Wada-Katsumata et al. 2013). Rock doves in the cities have evolved defenses, not to human extermination attempts, but to predation by city-dwelling falcons (Palleroni et al. 2005).
Some populations precede city creation. Red-backed salamanders (Plethodon cinerus) in Montreal, Canada managed to persist as the city was constructed around them, but their populations have been isolated genetically, resulting in low variation (Noel & Lapointe, 2010). Across the Atlantic fire salamanders (Salamandra salamandra) also had a city (Oviedo, Spain) built atop their population starting over a millennia ago and managed to persist despite severe restriction in gene flow (Lourenco et al. 2017). Animals don’t need hundreds of years to adapt to new development, though. Water dragons (Intellagama lesueurii) inhabiting newly established city parks built as recently as 2001 in Brisbane, Australia have developed genetic difference in body shape and total size (Littleford-Colquhoun et al. 2017). Similarly, a small population of dark-eyed juncos (Junco hyemalis) established in the 1980s have been thoroughly studied (due largely to their location on the UC San Diego campus) and found to have evolved shorter wings, shorter tails, and alternate plumage pattern in just the past few decades (Rasner et al. 2004; Yeh 2007).
Other species, like the common wall lizard (Podarcus muralis) and striped mouse (Apodemus agrarius), seem to have been able to adapt to the development of Trier city in Germany (for lizards (Beninde et al. 2016)) and Warsaw, Polans (for mice (Gortat et al. 2014)) and now disperse through the city architecture in a similar way to their natural environment. Cityscapes can offer very similar (although much more angular) habitat to an organism’s natural habitat. For instance, Anoles (Anolis cristatellus) perch on the vertical trunks of trees in the forest. The flat walls of building in Puerto Rico offer a similar niche; however, artificial walls tend to provide less grips for lizards. In response, urban Anoles have evolved longer limbs and stickier toepads to cling to homes and businesses (Winchell et al. 2016).
Small animals are not the only wildlife subject to urban impacts. The movement of many large mammals, such as bobcats (Lynx rufus) (Serieys et al. 2014), are restricted by roadways, despite our best efforts to promote connectivity. In some cases, large and mobile animals are able to break into the new habitats afforded by cities. Red foxes (Vulpes vulpes) colonized the city of Zurich, Switzerland less than two decades ago and in that time their urban populations have exploded (Wandeler et al. 2003). While the original urban populations were likely established by just a few intrepid foxes, now that the urbanite populations are large enough, they have established genetic connectivity with their rural counterparts, essentially extending the larger population’s range to include the city.
In addition to landscape alterations that reduce gene flow and incur selection, urban settings can actually increase genetic mutation rates (which is ultimately the raw substrate for evolutionary adaptation). As an example, the rate of mutation in herring gulls (Larus argentatus) that nest in a heavily industrialized site in Ontario is double their less urban counterparts likely due to exposure to toxic chemicals in the environment (Yauk & Quinn 1996).
Let’s not forget that animals are not alone in their evolutionary response to urbanization—plants have also demonstrated adaptations to cities. Clover (Trifolium repens) in urban populations have evolved a reduction in cyanogenesis, a process that makes the plant less palatable to herbivores but in trade-off leaves the plant less tolerant of freezing temperatures (Thompson et al. 2016), which makes sense since there aren’t many large grazers wandering city streets and urban settings tend to act as heat islands.
In order to reduce seeds falling on infertile concrete streets and sidewalks, Holy hawksbeard (Crepis sancta) a weed in Montpellier, France have evolved to produce less dispersing seeds (Cheptou et al. 2008). In addition, the urban plants evolved an increase in photosynthesis and larger flowers (Lambrecht et al. 2016). Virginia pepperweed (Lepidium virginicum), is a common weed in many U.S. cities. A study of genetic divergence between urban and rural settings found that urban plants were more closely related than the more geographically proximal rural populations (Yakub & Tiffin 2016). In addition, the city pepperweed had developed a different shape and growing season to rural plants.
It’s clear that almost anywhere you look in cities, wildlife is evolving in response to our presence. This bestows us with a massive responsibility and indebts us to an ethic of conservation, not of species themselves, but to preserve as much unencumbered wild habitat as possible (for instance, as designated Wilderness). Where saving wild space is impossible, we must work on mitigating the effects of our urban infrastructure (for instance).
Literature cited:
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