Urban Evolution: A Brief Introduction
Curious to understand the basics of urban evolution? Start here!
If you're reading this, I'll bet you live in a city.
Urbanization is a worldwide phenomenon.
All around the world , more people are moving to urban areas (the places that we commonly refer to as "cities"). In turn, global landscapes are changing to accommodate the influx of soon-to-be urbanites. Check out the map below to compare how many people lived in urban areas a mere 60 years ago versus today. In 1960, some continents like Europe were populated with a relatively high proportion of city-dwellers while other continents like Africa and Asia were populated by relatively few city folk. Fast-forward 60 years and watch the trends leap off the map.
Urbanization is also occurring at breakneck speed.
To us, 200 years seems like a very long time. But in the context of our planet's ecological and evolutionary history, 200 years is nearly negligible (for example, comb jellyfish arose 730 million years ago... our species has been around for about 6 million years...) In only 200 years, the proportion of people living in urban areas rose from 7% to over 50% (the graphic below ends before 2007, when the number of urban-dwellers surpassed rural-dwellers).
And it doesn't stop there: By 2050, we expect that more than 2 out of 3 people (66%) of the world's population will live in an urban area. But that's just the global average; several countries already exceeded that number years ago. For example, in Brazil, an incredible 81.2% of its population (>4 out of every 5 people) lived in an urban area as of 2000. That's a nine-fold increase from 1800, when less than 1 in 10 people lived in a Brazilian city!
Clearly, urbanization is occurring on a worldwide scale at a rapid rate.
Okay, but what does "urban" mean?
Depends on who you ask. Here are just a few definitions from the urban ecology literature.
"Urbanization can be characterized as an increase in human habitation, coupled with increased per capita energy consumption and extensive modification of the landscape, creating a system that does not depend principally on local natural resources to persist."
Photo: New York, USA, 2019
"An urban environment is a heterogeneous mosaic of residential dwellings, commercial properties, parks, and other land-use types that provides an array of habitat types that can be used by [species]."
From McIntyre 2000
Photo: London, England, 2015
"Urbanisation is characterised by the presence of artificial structures, impervious surfaces, high densities of people, domesticated plants and animals, and altered flows of energy and nutrients..."
Photo: Ghent, Belgium, 2018
"Urbanization is a complex socio-economic process that transforms the built environment, converting formerly rural into urban settlements, while also shifting the spatial distribution of a population from rural to urban areas. It includes changes in dominant occupations, lifestyle, culture and behaviour, and thus alters the demographic and social structure of both urban and rural areas."
From the United Nation's World Urbanization Prospects 2018
Photo: Ushuaia, Argentina, 2018
You can also read this multi-page list of how each country defines "urban" , but instead of getting caught in semantics...
...let's dive into some typical characteristics of urban areas.
High impervious surface coverage
Exemplified with Tokyo's legendary Shibuya Crossing, cities often contain large amounts of impervious surface- i.e., material that does not allow rainwater to permeate into the ground. When rain hits the pavement, it cannot soak into the ground. Instead, it moves very quickly into urban watercourses like streams, carrying pollutants and eroded soil into these waterways. Examples of impervious surfaces include roads, parking lots, sidewalks, and compacted ground (such as an unpaved area that experiences high foot traffic).
The abundant impervious surface also represents a lack of green space.
High human population density
New York, USA contains some of the most densely populated areas in the world. The quantification of how many people live in a certain area typically reflects the degree to which humans "disturb" the local environment. For example, the human population density of a place is often associated with the amount of resources extracted from the local environment and the number of structures built in it.
High habitat fragmentation
Habitat fragmentation occurs when a large area of habitat is broken into smaller pieces that are somewhat isolated from one another. In this photo taken in Toronto, Canada, you can see an example of how anthropogenic structures like roads split green spaces into smaller patches.
Higher temperatures
Urban areas frequently exhibit higher daily and seasonal temperatures owing a phenomenon known as the "urban heat island effect". Cities tend to be relatively warm areas because the high impervious surface coverage (see #1: High impervious surface coverage) stores and re-radiates heat much more than other types of land cover such as forests. You can visualize this effect by looking at surface temperature images of Rome, Madrid, Paris, and Milan taken during a June 2019 heatwave. The centers of these cities had very high temperatures compared to the outlying suburban and rural areas.
Higher pollution
Urban areas tend to exhibit higher levels of pollution than more rural areas. Just a few of the many types of pollution you might find in a city such as Delhi, India, include air, sound, chemical, and light pollution which are generated by several sources including industrial plants, traffic, and artificial lights such as streetlamps.
Alright, so how does urbanization influence evolution?
When many of us hear " evolution ", our minds jump to examples of adaptation and evolution by natural selection (think Darwin's finches, animals camouflaging into their habitats, etc.). And while this evolutionary process is, without question, one very inspiring facet of evolutionary biology, it is one of four processes that drive evolution. In total, these four evolutionary processes include
If you're a little rusty on the history of evolutionary thought (and who isn't?), click the links associated with each evolutionary process for some fantastic explanations of these concepts.
Now let's think about evolution happening in our cities.
When it comes to introducing the idea of urban evolution, researchers Max Lambert and Colin Donihue (2020) say it best:
"...urban species are evolving. What does this mean? It means that species aren’t a single, static entity. Rather, individual populations change over time. Some of these changes are neither good nor bad for urban life. Other changes can be adaptive and help populations live better city lives. And some changes can end up being maladaptive, making some aspects of life a bit harder."
Now that we're on the same page, let's get to the good stuff.
Here are some examples of how urban environments influence evolution in the species that we pass by, every day, usually without a second thought. But make no mistake—
Species undergoing evolution in response to urbanization are much more than "weeds" and "vermin"; they epitomize rapid evolution that is observable within our lifespans and within our walks to the corner store.
Natural Selection
Across five large North American cities, a plant called Virginia pepperweed has been found to exhibit marked differences compared to rural plants . Specifically, the urban plants grow faster, are physically larger, flower earlier, and have higher fitness than rural populations. Higher fitness means that, compared to plants in rural areas, urban plants were more likely to produce offspring and pass on their genes to the next generation. Researchers also found that these differences were genetically encoded— a signature of evolutionary change.
Natural Selection
Eurasian blackbirds have successfully colonized several European cities and are less likely to migrate than their rural counterparts , possibly because blackbirds can tolerate the warmer year-round temperatures of cities. City-born blackbirds were also found to have a lower stress response than their rural relatives , presumably in response to the frequent anthropogenic disturbances of their urban habitats (e.g. traffic noises).
Mutation
As of 2020, we have uncovered few examples of urban evolution driven by mutation. However, we are accumulating evidence that the pollution associated with urbanization likely increases mutation rates in urban-dwelling species. For example, herring gulls in Hamilton, Ontario's heavily industrialized urban harbor exhibited higher mutation rates than gulls from rural areas.
Gene Flow
Urbanization can influence gene flow (the movement of genetic material among populations) by facilitating and restricting the movement of individuals among patches (or, for species like plants, urbanization can affect pollen and/or seed movement).
For example, in Southern Italy, urban populations of the Prince of Wales feather moss experience lower levels of gene flow compared to non-urban populations.
Gene Flow
Our next stop takes us nearly 10,000 km west for a rare glimpse into how urban evolution is taking place in the Old World tropics. Researchers found evidence suggesting high gene flow across Singapore's population of olive-winged bulbul birds. Keep in mind that Singapore is extremely urbanized: less than 5% of its original forest cover remains! This case study offers hope that some species will be able to survive the intense environmental changes associated with rapid urbanization.
Genetic Drift
Urban features like buildings, roads, and railways can act as barriers among urban populations and between urban and rural habitats. Populations isolated by these barriers might experience decreased genetic diversity due to heightened genetic drift (i.e. a mechanism that randomly changes the genetic composition of a population's gene pool due to random chance or "sampling error", and which frequently results in the loss of genetic diversity). In New York, NY, white-footed mice showed lower genetic diversity when their environments were more urban, which suggests that city-dwelling mice experienced stronger genetic drift than more rural mice.
Genetic Drift
Species that colonize urban areas may also experience a loss of genetic diversity owing to bottleneck and/or founder effects . Often this is a cause for concern because populations with low genetic diversity may have a harder time surviving environmental changes such as droughts or heat waves. This is especially true when these environmental changes are compounded by the effects of habitat loss, which can include isolation and/or restricted gene flow among populations.
Urban coyotes in Los Angeles and New York provide an example of one species' consistent genetic diversity loss following colonization of urban areas. This carnivorous species also represents an issue that is forcing urbanites to reflect on urban ecology and conservation: How can we coexist with native species displaced by urbanization and who are now colonizing cities ?
Okay, so we have evidence that urbanization can influence multiple microevolutionary processes (i.e. natural selection, mutation, gene flow, genetic drift)... mostly in animals... mostly in North America and Europe. What about all the other species on the tree of life and other parts of the world?
The majority of urban ecology and evolution studies have been performed on animals in temperate climates.
It is critical that we expand our scope to a wider variety of species, biomes, and geographic areas so that we can build a comprehensive understanding of how urbanization is impacting evolution.
So, what am I studying?
Common milkweed growing in downtown Toronto. Each of the pods holds hundreds of seeds that take flight and are carried by the wind after they are released from the pods in the fall. This plant was particularly fertile- it had over a dozen pods!
I research how urbanization influences the evolution of a plant called common milkweed (Asclepias syriaca), which also happens to be a favorite host of the Monarch butterfly. Specifically, I'm investigating how urban environments influence natural selection, gene flow, and genetic drift within plant populations dispersed throughout the urban, suburban, and rural parts of the Greater Toronto Area and other metropolitan areas.
I'm also studying how urbanization affects the reproductive success of common milkweed, which will help us understand how likely it is that plants living in the city will contribute to future generations of the species.
Why is my research important?
There are several reasons why it's important to study urban evolutionary biology. This young field holds relevance for urban planners, policymakers, conservation practitioners, public health officials, and the entire public sphere. Here's just one example of how we can leverage this research to benefit human and non-human urbanites.
⇒ This knowledge will help us build wildlife-friendly cities.
Praying mantises are eager to help out with research when it comes to urban evolution.
Is a city an ecosystem ? Certainly!
Urban areas are places where we find complex relationships between biotic factors (living components, like plants and fungi), and abiotic factors (non-living components, like rocks and wind). Just like in a more "pristine" location like a grassland, we can find intricate networks composed of urban plants and pollinators, predators and prey. In fact, many of these species existed in what are now "urban" areas before they were "urbanized".
The first step to building wildlife-friendly cities is to understand how city-dwelling species are responding to urban pressures. From here, we can find out what is helping and what is hurting their chances of survival. For example, many native plants struggle to thrive in urban areas owing to factors including habitat fragmentation and low pollinator visitation. As a result, these plants can experience limited seed and pollen dispersal, which can ultimately jeopardize their long-term persistence in the city. One proposed solution to support these plants is to build green roofs (i.e. vegetated areas installed on building roofs) to serve as habitats for the plants and hubs for their pollinators. Early research shows that green roofs may indeed support gene flow between urban plant populations, providing the means for data-driven decision-making by policymakers. After we accumulate these "report cards" for different species in a variety of cities, we can start working to construct cities and retrofit our current cities to support urban biodiversity and the ecosystems in which they live.
Biodiversity-rich urban areas are beneficial for a variety of reasons. To name just a few, functioning urban ecosystems can boost human health and wellbeing by preventing floods, reducing the spread of infectious disease, and improving mental health.
I still have questions!
Great! Feel free to contact me . If I can't answer your question, I'll find someone who can!
In the meantime, check out these awesome popular science articles to read about how all kinds of urban-dwelling species are evolving in cities.
- Life in the City : Blog about urban evolution for specialist and non-specialist readers
- " How Cities Reshape the Evolutionary Path of Urban Wildlife ": WIRED magazine
- " White Clover Can Be an Annoying Weed. It May Also Hold Secrets to Urban Evolution. ": New York Times
- " In Cities, Wildlife Evolves Astonishingly Fast ": National Geographic
- " Cities Can Serve as Cauldrons of Evolution ": The Scientist
- UC Berkeley's terrific "Understanding Evolution" segment on urban evolution , complete with teaching resources and discussion questions.
For an experiential learning opportunity, take a walk around your neighborhood and use the app iNaturalist to help you identify the incredible diversity of urban-dwelling species. Speaking from experience, be aware that the weeds in your sidewalk might suddenly start to look cool and/or distinguishable.
If you'd like to use your enthusiasm and curiosity to help scientists solve age-old scientific questions, there are hundreds of "community science" (a more inclusive term for what we used to call "citizen science") opportunities that allow everyone to play a part in advancing our understanding of the natural world. Take a look at all these opportunities ! If you came here for the milkweed/monarch angle, check out these programs .
Do you (or your kids) best absorb information when it's conveyed as a musical? Look no further.
