Urban Evolution: How Natural Life Adapts to Human Cities
We are currently living in the Anthropocene era, named for the massive influence humans have exerted on the world since we came into being. But only recently, we are beginning to understand the sheer scope of that reach as scientists begin to explore how our influence is rapidly reaching down into the genes of plants and animals.
What we are beginning to see is that through fast-paced natural selection, creatures in cities and suburbs are genetically evolving to deal with the omnipresence of humans. In fact, they are adapting to city life much faster than we are in many ways. Urban landscapes present new evolutionary pressures. Street lights confuse and massacre moths and cause songbirds insomnia. Metal concentrations can be toxic. Noise drowns out birdsong. But instead of halting adaptation, however, these novelties bring out the ingenuity in evolution.
Famously peregrine falcons have taken to cities as opposed to their normal cliffside habitats because larger cities have plenty of high perches and an abundance of pigeons for them to prey on. But for the moment, this is an example of a behavioral adaptation to a new environment and they aren’t yet a separate species from their rural cousins. Instead, what scientists are coming to see now that is really interesting is that many urban-dwelling species are actually experiencing heritable, genetic adaptation to this new, human environment.
Let’s get into the science!
Playing chicken—or, well… Swallow?
One of the biggest dangers facing animals in the urban world is the car. Millions of animals are struck by vehicles on the road every year, but scientists are starting to see that the evolutionary pressure this puts on animals is actually starting to have an effect. For example, take the American cliff swallow. These birds often find themselves foraging on roads because seeds there are easier to find, but that also meant that they were having more and more encounters with speeding cars.
As a result, researchers have found that the species wings as a whole have actually become shorter over the last couple decades, which allows them to take off faster vertically in the face of oncoming cars. Why they developed this adaptation instead of, I don’t know, finding seeds somewhere other than a busy road—Who knows. Maybe they just like the thrill of it.
Like moths to the flame.
One of the most prevalent issues a city exerts on the natural world around it is a constant stream of light pollution. But because of this unrelenting stimulus, many nocturnal urban species are developing a resistance to the light’s effects, it may even be fundamentally changing our oldest idioms.
For moths, the lure of a light bulb can be fatal, whether they scorch themselves on it or end up in a waiting spider’s web. But many entomologists believed that the attraction to light is so hard-wired into an insect’s brain that it can’t possibly be removed.
At least they did until a Swiss scientist collected hundreds of ermine moth caterpillars from the city center and the forest then reared them to adulthood together in a lab before releasing them into a dark cage with a single light at one end. Lo and behold, the rural moths swarmed the lamp while the urban moths mostly ignored the light and settled evenly throughout the cage. Their hardwired, genetic instincts had adapted!
I’m blown away (keep reading to get the pun).
One of the simple joys in life is finding a dandelion in full fluff-balloon mode that you can pick up and make a wish on. (Or just destroy if you’re not into all that wishing stuff.) But in cities, that experience may be on the way out.
Under normal conditions, dandelion seeds are supposed to drift in the wind and land far away to germinate anew, but in cities, that strategy doesn’t often work. Usually, the tiny patch of dirt that the urban dandelion is growing out of is the only fertile land for a long distance. So, they have started to develop larger, heavier seeds and smaller, ineffective parachutes so that the seeds have a better chance of falling straight down into the same patch of dirt the old weed occupied. I say that’s probably a good thing. Keep your city dandelions where they are, I’ll keep my simple joys to the suburbs.
Like deep caves or sulfur springs, cities are what many ecologists refer to as an “extreme environment”, and that’s important because while the risks of living in an extreme environment are high, so too are the potential rewards… If you can take advantage of them. That’s why the evolution of species also occurs faster in these places; because new mutations, which give a species the ability to survive in that extreme environment, will spread very rapidly.
This is what we call HIREC, or human-induced rapid evolutionary change. We see that in cities and also in other environments where humans create a new habitat or ecological situation. In those places, you see very, very fast evolutionary adaptations, which can take place in the space of decades or even years.
This would seem to contradict a core principle of Darwin’s theory of evolution. He famously wrote, “We see nothing of these slow changes of progress until the hand of time has marked the long lapse of ages.” But in fact it isn’t a contradiction, it’s simply that humans have more of an impact of the environment than we sometimes like to admit. And under significant enough natural selection pressure, evolution can proceed much more quickly than Darwin ever thought possible. Especially with organisms that are quick to reproduce.
This acceleration of evolution was investigated by a meta-analysis of over 1600 studies led by researchers at the University of Washington and found that urbanization has most certainly sped up evolution, in some places going so far as to double it! It also came up with a few other interesting findings.
One of the often overlooked elements of change this research pointed to was the introduction of exotic species from multiple different environments into a single habitat. These species can be brought in accidentally or intentionally: agriculture crops, pets, ornamental plants, as well as the seeds and insects people unwittingly carry in on their clothes and cars.
When these organisms all come together in densely packed cities they form an ecosystem of species that interact with each other often without ever having an opportunity to adapt to one another. This can set the stage for the rapid mutual evolution of new attack and defense strategies. For example, exotic parrots introduced to a city might adapt their beaks to feed on native city seeds, whereas city pigeons could evolve immunities to exotic parasites.
The fragility of it all.
So life is adapting to the changes humans make to the environment faster than we thought. That’s a good thing right? We can just keep letting cities dominate the Earth and stop worrying about it because everything will be fine!
The fact is that for each success story, there are likely dozens of cases of urban extinction. Only certain species will be able to colonize, survive in, and ultimately thrive in human cities because though it may not seem that way for us, as far as nature is concerned they are extreme environments.
And never before has an extreme habitat had such a global presence, so much so that the same adaptations are happening in parallel in cities thousands of miles away from each other. It may be that human kind’s biggest influence on the environment in the coming years could be this strange kind of homogenization across the global cityscapes. Something that sets urban evolution in stark contrast to natural evolution.
Of course, because such a development is totally unprecedented, we can really only guess what dangers and obstacles the future will hold. And at least for now we need to remember that many species will continue to need the reserves, protected areas, laws, and other safeguards that allow their fragile natural habitats to continue to exist.
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